Case 4 Flashcards
what are the functions of the colon?
- Absorption of water and electrolytes from the chyme to form solid faeces.
- Storage of faecal matter until it can be expelled.
what is the proximal half of the colon primarily concerned with?
absorption
what is the distal half of the colon primarily concerned with?
storage
what are movements of the colon usually like? what is the function of these movements?
Intense colon wall movements aren’t required for the functions of the colon and so the movements of the colon are normally very sluggish.
• These movements still play similar roles as the movement in the small intestine:
Mixing movements
Propulsive movements
mixing movement
- another word for this
- describe the movement
- what produces it
- where
- what happens
Mixing Movements (“Haustrations”) - Segmentation • In the same manner that segmentation movements occur in the small intestine, large circular constrictions occur in the large intestine. At each of these constrictions, about 2.5cm of the circular muscle contracts, sometimes constricting the lumen of the colon almost to occlusion. • At the same time, the longitudinal muscle of the colon, which is aggregated into three longitudinal strips called the teniae coli, contracts. • These combined contractions of the circular and longitudinal strips of muscle cause the unstimulated portion of the large intestine to bulge outward into bag-like sacs called haustrations. Each haustration usually reaches peak intensity in about 30 seconds and then disappears during the next 60 seconds. • They also at times move slowly toward the anus during contraction, especially in the cecum and ascending colon, and thereby provide a minor amount of forward propulsion of the colonic contents. • After another few minutes, new haustral contractions occur in other areas nearby. • Therefore, the faecal material in the large intestine is slowly dug into and rolled over. • In this way, all the faecal material is gradually exposed to the mucosal surface of the large intestine, and fluid and dissolved substances are progressively absorbed.
how much faeces expelled each day?
Between 80-200ml
propulsive movements
- what produces this
- how long does it take
- what can take over the propulsive role
- when does this occur
- Propulsion in the cecum and ascending colon results from the slow but persistent haustral contractions, requiring as many as 8 to 15 hours to move the chyme from the ileocecal valve through the colon.
- From the cecum to the sigmoid, mass movements can, for many minutes at a time, take over the propulsive role.
- These movements usually occur only one to three times each day, in many people especially for about 15 minutes during the first hour after eating breakfast.
mass movement
- what is it
- what happens
• A mass movement is a modified type of peristalsis characterized by the following sequence of events:
1. First, a constrictive ring occurs in response to a distended or irritated point in the colon, usually in the transverse colon.
2. Then, rapidly, the 20 or more centimetres of colon distal to the constrictive ring lose their haustrations and instead contract as a unit, propelling the faecal material in this segment en masse further down the colon.
3. The contraction develops progressively more force for about 30 seconds, and relaxation occurs during the next 2 to 3 minutes.
4. Then, another mass movement occurs, this time perhaps farther along the colon.
A series of mass movements usually persists for 10 to 30 minutes.
Then they cease but return perhaps a half day later.
5. When they have forced a mass of faeces into the rectum, the desire for defecation is felt.
what are the small intestinal reflexes?
- Ileogastric reflexes: distention of ileum leased to decreased gastric motility.
- Gastro-ileal reflexes: increased gastric distention leads to increased ileal motility and ileocaecal valve relaxes.
- The ileocaecal valve is normally closed. It opens (gastroileal reflex) when a peristaltic wave reaches it.
how are peristaltic contractions regulated? explain this
- “Slow waves” determine the frequency of contraction. There is a basic electrical rhythm (B.E.R).
- The slow waves have a resting potential of -40 to -60 mV.
- The slow waves are superimposed on the resting potential.
- The size of the slow wave is modulated by nervous and hormonal inputs.
- Contraction of the smooth and striated muscles in the intestinal wall will only occur if the potential of a slow wave exceeds the threshold potential.
- Vm positive of threshold voltage-gated Ca2+ channels [Ca2+]i =contraction.
what are the pacemaker cells of the GI tract? what do they do?
- Interstitial cells of Cajal
- These create slow wave potentials that leads to the contraction of the smooth muscle.
how are ‘slow waves’ modulated?
Food stimulates nerve and hormonal activity:
Increase or decrease size of the maximum depolarisation.
Nerves (intrinsic & extrinsic):
ACh, Substance P depolarisation (= contraction)
NO, VIP, opioids hyperpolarisation (= ↓ contraction)
Noradrenaline hyperpolarisation (= ↓ contraction)
Hormones:
Motilin depolarisation (= contraction)
Secretin, G.I.P. hyperpolarisation (= ↓ contraction)
Adrenaline hyperpolarisation (= ↓ contraction)
what is the difference between long reflexes and short reflexes?
long = extrinsic nerves
- PNS interacts with ENS
- sympathetic and parasympathetic
short = ENS
- afferet, inter and efferent neurones all in ENS
- e.g. local distention -> motility (muscle contraction)
enterochromaffin cells
- what are they
- what percentage of enteroendocrine cells are these
- what does stimulation cause
- what stimulates
- what ends signal
- side effect of SSRIs
- what is linked to IBS
These are the main mechano- and chemo-sensory cells in the GI tract.
- 90% of enteroendocrine cells are enterochromaffin cells
Stimulation of ECL cells causes release of serotonin (5-HT) intracellularly.
5-HT stimulates sensory nerves via 5-HT3 receptors.
Different stimuli produce varied (stimulatory or inhibitory) responses via 5-HT signalling to the parasympathetic NS.
Stimulation -> release of serotonin (5-HT)
- Cell can be stimulated by strong forces such as distention or gentle stroking of microvilli
- Stimulation causes release of vesicles containing 5-HT into the interstitial fluid of the epithelial layer
- Released 5-HT stimulates afferent neurones via 5-HT3 receptors
- Action potential sent through enteric nervous system and then to extrinsic nerves
- SERT removes 5-HT to terminate signal
- SSRIs: side-effect = diarrhoea
- SERT mutations also linked to IBS
- 5-HT3 antagonist & 5-HT4 agonists in trial for IBS
the rectum empty most of the time, what is this due to?
A weak functional sphincter which exists between the sigmoid colon and the rectum, therefore preventing the entry of food into the rectum.
The sharp angulation at the junction between the sigmoid colon and the rectum that contributes additional resistance to filling of the rectum.
when does the desire for defecation occur? what else happens along with this?
• When a mass movement forces faeces into the rectum, the desire for defecation occurs immediately, including reflex contraction of the rectum and relaxation of the anal sphincters.
what prevents the continual dribble of faecal matter through the anus?
Tonic constriction of:
- An internal anal sphincter, a several-centimetres-long thickening of the circular smooth muscle that lies immediately inside the anus
- An external anal sphincter, composed of striated voluntary muscle that both surrounds the internal sphincter and extends distal to it.
what controls the external anal sphincter?
- Nerve fibres in the pudendal nerve
- Which is part of the somatic nervous system and therefore is under voluntary, conscious or at least subconscious control; subconsciously, the external sphincter is usually kept continuously constricted unless conscious signals inhibit the constriction.
defecation is initiated by defecation reflexes - what are these two reflexes?
- intrinsic myenteric defecation reflex
- parasympathetic defecation reflex
describe the intrinsic reflex of defecation
- what mediated by
- what happens
- how strong
mediated by the local enteric nervous system in the rectal wall.
When faeces enter the rectum, distention of the rectal wall initiates afferent signals that spread through the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing faeces toward the anus.
As the peristaltic wave approaches the anus, the internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus; if the external anal sphincter is also consciously, voluntarily relaxed at the same time, defecation occurs.
The intrinsic myenteric defecation reflex functioning by itself normally is relatively weak.
• To be effective in causing defecation, it usually must be fortified by another type of defecation reflex, a parasympathetic defecation reflex
describe the parasympathetic defecation reflex
- what does it involve
- what other effects do defecation signals entering the spinal cord initiate
involves the sacral segments of the spinal cord.
When the nerve endings in the rectum are stimulated, signals are transmitted first into the spinal cord and then reflexly back to the descending colon, sigmoid, rectum, and anus by way of parasympathetic nerve fibres in the pelvic nerves.
These parasympathetic signals greatly intensify the peristaltic waves as well as relax the internal anal sphincter, thus converting the intrinsic myenteric defecation reflex from a weak effort into a powerful process of defecation that is sometimes effective in emptying the large bowel all the way from the splenic flexure of the colon to the anus.
Defecation signals entering the spinal cord initiate other effects:
Taking a deep breath
Closure of the glottis
Contraction of the abdominal wall muscles to force the faecal contents of the colon downward and at the same time the pelvic floor is relaxed downward and pull outward on the anal ring to evaginate the faeces
how can defecation reflexes be purposely activated?
- The defecation reflexes can purposely be activated by taking a deep breath to move the diaphragm downward and then contracting the abdominal muscles to increase the pressure in the abdomen, thus forcing faecal contents into the rectum to cause new reflexes.
- Reflexes initiated in this way are almost never as effective as those that arise naturally, for which reason people who too often inhibit their natural reflexes are likely to become severely constipated.
describe the mucosa of the large intestine
- what does it contain
- what doesn’t it
Crypts of Lieberkühn
No villi
Epithelial cells contain almost no enzymes. They consist mostly of mucous cells that secrete only mucus.
what does the large intestine mostly secrete?
mucus
what does mucus contain?
moderate amounts of bicarbonate ions
what regulates the rate of secretion of mucus in the large intestine? what else can effect it?
• The rate of secretion of mucus is regulated principally by direct, tactile stimulation of the epithelial cells lining the large intestine and by local nervous reflexes to the mucous cells in the crypts of Lieberkühn.
• Stimulation of the pelvic nerves from the spinal cord, which carry parasympathetic innervation to the distal one half to two thirds of the large intestine, also can cause marked increase in mucus secretion.
This occurs along with increase in peristaltic motility of the colon.
• During extreme parasympathetic stimulation, often caused by emotional disturbances, so much mucus can occasionally be secreted into the large intestine that the person has a bowel movement of ropy mucus as often as every 30 minutes; this mucus often contains little or no fecal material.
what is the function of mucus in the large intestine?
Protects the intestinal wall against excoriation.
Provides an adherent medium for holding faecal matter together.
Protects the intestinal wall from the great amount of bacterial activity that takes place inside the faeces.
Plus the alkalinity of the secretion (pH of 8.0 caused by large amounts of sodium bicarbonate) provides a barrier to keep acids formed in the faeces from attacking the intestinal wall.
what causes diarrhoea? what happens in diarrhoea?
Diarrhea Caused by Excess Secretion of Water and Electrolytes in Response to Irritation
• Whenever a segment of the large intestine becomes intensely irritated, as occurs when bacterial infection becomes rampant during enteritis, the mucosa secretes extra large quantities of water and electrolytes in addition to the normal viscid alkaline mucus.
• This acts to dilute the irritating factors and to cause rapid movement of the faeces toward the anus.
• The result is diarrhoea, with loss of large quantities of water and electrolytes.
• But the diarrhoea also washes away irritant factors, which promotes earlier recovery from the disease than might otherwise occur.
where does electrogenic Cl- secretion occur? what stimulated by? what is Cl- secretion a major component of?
- Electrogenic Cl- secretion occurs in crypts of both the small and the large intestine.
- Cl- secretion is markedly stimulated by secretagogues such as ACh and other neurotransmitters.
- Cl- secretion is the major component of the ion transport events that occur during most diarrhoeal disorders.
describe the cellular model of Cl- secretion
• The cellular model of Cl- secretion includes three transport pathways on the basolateral membrane:
1. Na-K pump (sodium out, potassium in)
2. Na/K/Cl cotransporter (all three in)
3. Two types of K+ channels (IK1 and BK1) (potassium out)
• In addition, a Cl- channel (cystic fibrosis transmembrane regulator (CFTR)) is present on the apical membrane.
- This complex Cl− secretory system is energized by the Na-K pump, which generates a low [Na+]i and provides the driving force for Cl− entry across the basolateral membrane through Na/K/Cl cotransport.
- As a result, [Cl−]i is raised sufficiently that the Cl− electrochemical gradient favors the passive efflux of Cl− across the apical membrane.
what is one of the consequences of Cl- release into lumen?
- One consequence of these many transport processes is that the transepithelial voltage becomes more lumen negative, thereby promoting voltage-dependent Na+ secretion.
- This Na+ secretion that accompanies active Cl− secretion presumably occurs through the tight junctions (paracellular pathway).
- Thus, the net result is stimulation of NaCl and fluid secretion.
how much Cl- is normally released into lumen? what does secretion require?
• Normally (i.e., in the unstimulated state), the crypts secrete little Cl− because the apical membrane Cl− channels are either closed or not present.
• Cl− secretion requires activation by cyclic nucleotides or [Ca2+], which are increased by any of several secretagogues:
1. Bacterial exotoxins (i.e. enterotoxins)
2. Hormones and neurotransmitters
3. Products of cells of the immune system (e.g. histamine)
4. Laxatives
what is the primary mechanism for net colonic secretion?
passive K+ secretion
where is active K+ secretion present? what is it induced by?
present throughout the large intestine and is induced both by aldosterone and by cAMP.
is K+ secretion active or passive?
• K+ ions can be both passively and actively secreted.
where does active transport (secretion and absorption) of K+ take place?
• Active transport of K+ ions is subject to considerable segmental variation in the colon.
Whereas active K+ secretion occurs throughout the colon, active K+ absorption is present only in the distal segments of the large intestine.
Thus, in the recto-sigmoid colon, active K+ absorption and active K+ secretion are both operative and appear to contribute to total body homeostasis.
what is the model of active K+ secretion?
Uptake of K+ across the basolateral membrane is a result of both the Na-K pump and the Na/K/Cl cotransporter (NKCC1), which is energized by the low [Na+]i that is created by the Na-K pump.
Once K+ enters the cell across the basolateral membrane, it may exit either across the apical membrane (K+ secretion) or across the basolateral membrane (K+ recycling).
what controls how much secretion occurs of K+ ions? what normally happens to K+ ions?
The cell controls the extent to which secretion occurs, in part by K+ channels present in both the apical and the basolateral membranes.
When apical K+ channel activity is less than basolateral channel activity, K+ recycling dominates.
Indeed, in the basal state, the rate of active K+ secretion is low because the apical K+ channel activity is minimal in comparison with the K+ channel activity in the basolateral membrane.
what stimulates active K+ secretion where? how do they work?
• Aldosterone stimulates active K+ secretion in surface epithelial cells of the large intestine, whereas cAMP enhances active K+ secretion in crypt cells.
In both cases, the rate-limiting step is the apical BK K+ channel, and both secretagogues act by increasing K+ channel activity.
how does aldosterone affect overall net K+ secretion? how?
• This mineralocorticoid enhances overall net K+ secretion by two mechanisms:
1. First, it increases passive K+ secretion by increasing Na-K pump activity and thus increasing electrogenic Na+ absorption.
The net effects are to increase the lumen-negative VTE and to enhance passive K+ secretion.
2. Second, aldosterone stimulates active K+ secretion by increasing the activity of both apical K+ channels and basolateral Na-K pumps.
how do cAMP and Ca2+ affect K+ secretion? how? what affects cAMP and Ca2+?
- VIP and cholera enterotoxin both increase [cAMP]i and thus stimulate K+ secretion.
- Increases in [Ca2+]I also stimulate active K+ secretion.
- Increases [cAMP]I and [Ca2+]I increase the activity of both the apical and the basolateral K+ channels.
- Because the stimulation of K+ channels is greater at the apical than at the basolateral membrane, the result is an increase in K+ secretion from the epithelial cell across the apical membrane.
- Stimulation of K+ secretion by cAMP and Ca2+, both of which also induce active Cl− secretion, contributes to the significant faecal K+ losses that occur in many diarrheal diseases.
passive secretion of K+ ions?
- Na+/K+ pump ??
- through intercellular junctions
K+ absorption - passive and active?
passive:
- through intercellular junctions with water
- then taken up by Na+/K+ pump
active:
- K+/H+ pump in apical?
- ? channel out of basolateral
- Na+/K+ pump in basolateral
how much water and electrolytes absorbed in colon? what left over?
- Most of the water and electrolytes in the chyme are absorbed in the colon, usually leaving less than 100ml of fluid to be excreted in the faeces.
- Nearly all the ions are absorbed, leaving only 1-5mEq each of sodium and chloride ions to be lost in the faeces.
where does active absorption of sodium ions take place? what does this absorption cause?
- Active absorption (electroneutral absorption) of sodium ions takes place in the ileum and throughout the large intestine, with the exception of the most distal segment.
- This creates and electrical potential gradient which causes chloride absorption as well.
what are the tight junctions between the epithelial cells of the large intestine like compared to that of the small intestine? what does this cause?
much tighter than those of the small intestine.
• This prevents significant amounts of back-diffusion of ions through these junctions, thus allowing the large intestinal mucosa to absorb sodium ions far more completely—that is, against a much higher concentration gradient—than can occur in the small intestine.
This is especially true when large quantities of aldosterone are available because aldosterone greatly enhances sodium transport capability.
the overall electroneutral NaCl absorption process is regulated by what? what is this relevant to?
- The overall electroneutral NaCl absorptive process is regulated by both cAMP and cGMP, as well as by intracellular Ca2+.
- Increases in each of these three intracellular messengers reduce NaCl absorption.
- Decreases in [Ca2+]I increase NaCl absorption.
- Decreased NaCl absorption is important in the pathogenesis of most diarrheal disorders.
what is the primary mechanism of electrogenic Na+ absorption in the distal part of the colon?
epithelial Na+ channels
how specific are epithelial Na+ channels? what are these blocked by?
- In electrogenic Na+ absorption, Na+ entry across the apical membrane occurs through epithelial Na+ channels (ENaCs) that are highly specific for Na+.
- These ENaCs are blocked by the diuretic amiloride (a potassium sparing diuretic).
how efficient is Na+ absorption in the distal colon? why?
- Na+ absorption in the distal part of the colon is highly efficient.
- Because this segment of the colon is capable of absorbing Na+ against large concentration gradients, it plays an important role in Na+ conservation.
how is Na+ absorption enhanced?
- Na+ movement through electrogenic Na+ absorption is markedly enhanced by mineralocorticoids (e.g aldosterone).
- Aldosterone increases electrogenic Na+ absorption by increasing Na+ entry through the apical Na+ channel and by stimulating activity of the Na-K pump.
what are the three modes of Cl- absorption in the intestine? where does each occur?
- Voltage-dependent Cl- absorption:
o Cl− may passively diffuse from lumen to blood across the tight junctions, driven by the lumen-negative transepithelial voltage (paracellular route).
o Alternatively, Cl− may diffuse through apical and basolateral Cl− channels.
o This usually occurs in the jejunum, ileum and the distal colon. - Electroneutral Cl-HCO3 exchange:
o In the absence of a parallel Na-H exchanger, electroneutral Cl-HCO3 exchange at the apical membrane results in Cl− absorption and HCO3 secretion.
o This occurs in the ileum, proximal colon and the distal colon. - Parallel Na-H and Cl-HCO3 exchange:
o Electroneutral NaCl absorption can mediate Cl− absorption in the interdigestive period.
- Na+ in and H+ out (due to Na+/K+ pump basolateral)
- H2O and CO2 -> HCO3- -> leaves apical membrane and Cl- comes in
what does absorption of sodium and chloride ions create?
creates an osmotic gradient across the large intestinal mucosa, which in turn causes absorption of water.
what is the maximum absorption capacity of the large intestine?
5-8 litres of fluid and electrolytes each day.
where is bacteria present in the colon? which in particular? what are they capable of doing?
• Numerous bacteria, especially colon bacilli, are present even normally in the absorbing colon.
• They are capable of digesting small amounts of cellulose, in this way providing a few calories of extra nutrition for the body.
This source of energy is of little importance in human beings.
what are other substances formed as a result of bacterial activity?
- vitamin K
- vitamin B12
- thiamine
- riboflavin
- various gases that contribute to flatus in the colon, especially carbon dioxide, hydrogen gas, and methane.
which of these substances produced by bacterial activity is particuarly important and why?
The bacteria-formed vitamin K is especially important because the amount of this vitamin in the daily ingested foods is normally insufficient to maintain adequate blood coagulation.
what is the composition of faeces?
¾ water ¼ solid matter 30% dead bacteria 10-20% fat 10-20% inorganic matter 2-3% protein 30% undigested roughage from the food and dried constituents of digestive juices, such as bile pigment and sloughed epithelial cells
what causes the brown colour of faeces?
stercobilin, a derivative of bilirubin.
what causes the odour of faeces?
products of bacterial action; these products vary from one person to another, depending on each person’s colonic bacterial flora and on the type of food eaten.
The actual odoriferous products include indole, skatole, mercaptans, and hydrogen sulfide.
what is hypertrophy/atrophy?
Change in size of cells
Atrophy - cell shrinking
This may be achieved by apoptosis, reduced functional activity, loss of innervation, reduced blood supply, diminished nutrition, loss of hormonal or growth factor stimulation.
Hypertrophy - increase in size of existing cells
It is accompanied by an increase in functional capacity.
The number of cells doesn’t change, the cells just get bigger.
what is hyperplasia/aplasia?
Hyperplasia - increased number of cells caused by an increase in cell division.
Aplasia - decreased number of cells.
what is metaplasia?
Change in differentiation
Specialised cell types change their pattern of differentiation to a new mature stable cell type.
This allows them to withstand stress better.
what is metaplasia, dysplasia and anaplasia? how does this relate to cancer?
- Metaplasia - this is an adaptive response to environmental stimuli
- Dysplasia - the enlargement of tissue by the proliferation of abnormal (metaplasia) cells, as a developmental disorder or an early stage in the development of cancer.
- Anaplasia - loss of intracellular structural differentiation within a cell often with increased capacity for multiplication, as in a malignant tumour.
- Cancer»»> metaplasia followed by dysplasia followed by anaplasia.
describe interphase
• Interphase commences with G1 (G = Gap)
G1 – normal cell functions + cell growth, duplication of organelles.
- This is the phase where the cell is sensitive to growth factors (thus entering cell cycle) and anti-proliferative factors (thus not entering cell cycle).
- Once the cell has entered the cell cycle, there is no reversal – the point at which the cell enters the cell cycle and can no longer be affected by growth/anti-proliferative factors, is called the ‘restriction point’.
(each chromosome represented by 1 chromatid)
G0 – Cells that stay in G1 for a long time, and possibly never divide again are said to be in G0
S phase (synthesis phase) – DNA replication
G2 – Chromosomes begin to condense in preparation for the next mitotic division
(each chromosome represented by 2 chromatids)
what are the stages of mitosis?
- Prophase – chromosome becomes visible, 2 pairs of centriole separate, and nucleus disintegrates.
- Metaphase – chromatids move to a midline (equator)
- Anaphase – Chromatids are pulled apart
- Telophase – Chromosomes uncoil, two nuclei formed
- Cytokinesis – Cytoplasmic division.
what controls progression through the cell cycle?
cyclic dependent kinases - enzymes - provide checkpoint control
what do cyclins do?
activate CDKs
what do checkpoint controls do?
prevent DNA replication or mitosis of damaged cells and either stop the cell cycle to allow for DNA repair or eliminate irreversibly damaged cells by apoptosis.
how do CDKs work?
by promoting DNA replication and various aspects of the mitotic process and are required for cell cycle progression.
what are cyclin kinase inhibitors? what is example?
- Cyclin Kinase Inhibitors (CKIs) inhibit CDKs.
* One particular CKI is p21, which is a potent inhibitor of CDKs.
which cyclins activate which CDKs? what order/stage in cell cycle?
- cyclinD/CDK4(/6) - G1 - before R
- cyclinE/CDK2 - G1 - after R
- cyclinA/CDK2 - S
- cyclinA/CDK1 - (S/)G2
- cyclinB/CDK1 - G2/M
cyclin B
- when in cell cycle
- what complex does it form
- what does this complex to
• Cyclin B is a protein found in high quantities in the late G2 stage, peaking during mitosis.
• It associates with cyclin-dependent kinase (CDK1) to activate it.
• Cylin B/CDK1 complex phosphorylates proteins to cause mitosis.
- phosphorylates MAP proteins, lamin and histones
what are MAPs?
microtubule associated proteins – cause spindle formation.
what is lamin?
usually makes a protein meshwork which the nuclear envelope sits on. Upon phosphorylation, this meshwork breaks down, leading to the breakdown of the nuclear envelope.
what do histones do?
cause condensation of chromosomes
what form is Ras normally in?
normally bound to GDP and is inactive
what activates Ras? what happens once active? whta actually is Ras?
• Upon binding to GTP, it becomes active.
• Once in the active form, Ras can initiate signalling cascades to cause:
Inhibition of apoptosis
Cell growth
Protein synthesis etc
- A GTPase
- It’s a switch – either on or off
- When bound to GTP it’s on, it activates all sorts of downstream signalling pathways
- Ras then hydrolyses the GTP to GDP, that switches the switch off
- When mutated, Ras can’t do this conversion and when Ras binds to GTP, it’s activated, and it can never be switched off
what is Ras? what does a mutation cause?
oncogene – its mutation causes it to remain in the active form, thus causing a significant increase in cell growth and proliferation.
what must be mutated to cause colorectal cancer?
the Wnt signalling pathway (APC and B-catenin genes)
what is Wnt? what is it associated with inside the cell?
Ligand = Wnt
- Wnt is a growth factor
Intracellular protein complex which comprises of:
GSK-3B
Adenomatous Polyposis Coli (APC) (a tumour suppressor gene)
B-catenin (an oncogene)
what is B-catenin?
an oncogene
what is the tumour supressor gene associated with Wnt? what does it do?
adenomatous polyposis coli (APC)
- degrades B-catenin
what happens in the absence of Wnt?
- In the absence of Wnt (unstimulated state), GSK-3B is active, thus phosphorylating B-catenin.
- This causes the proteolytic degradation of B-catenin.
what happens when Wnt signals are present? what are the target genes?
- When Wnt signals are present, and they bind to the receptor, the intracellular protein complex becomes disrupted.
- The GSK-3B is switched off (something moves away from it to membrane near where Wnt is), thus B-catenin is no longer phosphorylated.
- This means that B-catenin is no longer targeted for proteolytic degradation.
- B-catenin can now move to the nucleus, where it interacts with transcription factors, thus changing gene expression, and so increasing cell proliferation.
- Target genes include Cyclin D & Myc (they drive the cell cycle)
- When the Wnt receptor is in unstimulated state, a complex with B-catenin and APC assembles, and B-catenin get’s degraded
- In the stimulated state, the Wnt binds to receptor, which inactivates the complex, B-catenin no longer gets degraded, its levels increase, it goes into nucleus where it activates gene expression, and drives the cell into proliferation
what does a mutation in B-catenin gene cause?
causes it to remain unphosphorylated and stable, thus increasing cell proliferation.
- When B-catenin gets mutation, it means there’s change in phosphorylation site, so it can never get phosphorylated and so can never get degraded, so will go into the nucleus even in the absence of Wnt signals
what stimulates Ras? what happens when stimulated? what gene expression does it affect in particular? what does this lead to? what is this describing?
Cell Cycle Entry
- Growth factors stimulate Ras, which binds to its receptors, thus activating transcription factors and affecting gene expression, especially Cyclin D1.
- Wnt signals, via B-catenin, also activate transcription factors, and affect gene expression, especially Cyclin D1.
- Cyclin D1 activates CDK4.
- CDK4 phosphorylates Rb = pRb.
- Normally Rb binds to the E2F family of transcription factors, it inhibits them.
- When CDK4 phosphorylates Rb, pRb now causes the liberation of E2F transcription factors, thus allowing them to enter the nucleus and modulating gene expression, especially of Cyclin E.
- Cyclin E can now bind to its CDK (CDK2), thus triggering the S phase of the cell cycle.
Growth factors -> Ras -> fos/jun -> cyclin D1 (upregulation and expression of proliferative genes)
Wnt signals -> B-Catenin -> Tcf/lef -> cyclin D1
- Cyclin D binds to CDK4/6
- pRb becomes phosphorylated and lets E2F go (which it was attached to)
- which switches on expression of cyclin E
- it binds to its CDK
- this drives this cell into S phase
what is Rb?
a tumour suppressor gene
what must happen to Rb for it to cause problems? what happens then?
- If one copy of this gene is mutated, it has no effect on the liberation of E2F.
- However, if both copies of Rb are mutated, then E2F is always liberated and the cell continuously enters the cell cycle and the S phase without the importance of the checkpoint control.
- its job is to bind to E2F
- if you lose both copies of Rb, E2F is now liberated, and can drive entry into S phase even when there are no growth factors around
what is the principal cell type of the epithelium of the small intestine? where are these found? what is the environment here and what influenced by?
- The principle cell type of the epithelium of the small intestine are Paneth cells.
- These are found below the stem cells in the crypts.
- These belong to the ‘stem cell niche’. This is a microenvironment where stem cells are found, where they are influenced by growth (e.g. Wnt) and differentiating factors to regulate cell fate.
S phase
- how is this part of cycle controlled
- is stability of genome maintained
- what happens if this phase doesn’t go as planned
- Here, all the control of the cell cycle happens internally, i.e. external factors don’t influence this phase of the cycle.
- Here, the stability of the genome is maintained without any damage to the DNA.
- If the S phase doesn’t go as planned, it results in the activation of p53, which will then aim to shut down the cell cycle (defense mechanism).
what is p53?
a transcription factor which is degraded normally, but in response to stimuli it can become active and remains non-degraded.
Kinases and ubiquitin ligases which would normally break down p53 are inhibited.
- it a tumour supressor gene/protein
- Transcription factor
- Normally degraded (so levels usually low)
- ‘activated’ by stabilisation (in response to damage)
- Kinases and ubiquitin ligases
p53 (guardian of the genome) - if genome encounters problems, you want to stop the cycle and fix the damage before it turns into mutation in both daughter cells
- if can’t fix damage then cell goes into apoptosis
- p53 crucial for all of this
what can activation of p53 be a result of?
Lack of nucleotides UV radiation Ionizing radiation Oncogene signalling Hypoxia Blockage of transcription factors
what does activation of p53 cause?
causes the following cascades:
Cell cycle arrest, leading to senescence or return to proliferation.
- This occurs by the upregulation of p21 (CKI). (switched on by p53 – normally p21 has low levels as well)
- This inhibits CDKs, thus arresting the cell cycle. (p21 then binds to the CDKs and inhibits them, blocking the cell cycle)
DNA repair
Block of angiogenesis
Apoptosis
- P53 drives expression of genes such as Puma and Noxa.
- These activate the BAX intrinsic
apoptotic pathway, thus leading to programmed cell death.
- Bcl-X is a pro-survival protein in colon cancer (oncogenic driver in colon cancer)
what is p53 known as?
guardian of the genome
apoptosis
- what is it
- what are the pathways
- Programmed cell death
- This is a programmed sequence of intracellular events leading to the death of the cell without the release of products harmful to the surrounding cells.
- Extrinsic/ Intrinsic Pathways
what is the proteolytic cascade (apoptosis)?
- Cell shrinks and condenses.
- Digestion of the nuclear DNA into small DNA fragments.
- Cell’s cytoskeleton disassembles.
- Blebbing.
- Cell surface membrane altered to allow phagocytosis.
what is the extrinsic pathway for apoptosis?
- Ligand (TNFα or Fas) bind to a receptor on the cell surface membrane of the cell.
- The activation of the receptor causes a DISC (death inducing signaling complex) to bind to the internal aspect of the receptor.
- This causes the activation of enzymes known as caspases (known as paracaspases when inactivated).
- Proteolytic cascade occurs.
what is the intrinsic pathway?
- Cell recognises internal damage.
- BAX (in the cytoplasm) binds to a receptor on the mitochondria.
- Upon binding, BAX changes shape.
- Proteins (cytochrome C) in the intramembranous space of the mitochondria leak out into the cytoplasm.
- Cytochrome C activates the paracaspases into caspases, leading to apoptosis.
what is necrosis? what due to?
• Cell death due to:
1. Trauma
2. Disease
3. Ischaemia as a result of lack of blood supply
• Release of intracellular content into the surrounding tissue.
familial cancers
- what percentage
- what is cause
- what are most
- what most due to
- what need in somatic cells
1% of all cancers.
Single gene mutations (Mendelian disorders)
Most are inherited as autosomal dominant traits.
Most due to inherited mutations of tumour suppressor genes.
Further genetic events are necessary if the mutation is in somatic cells. Even though the mutated gene is inherited, it isn’t sufficient for malignancy.
The inherited mutated gene increases cancer susceptibility.
sporadic cancers
- what percentage
- what result of
- what results in
99% of all cancers.
Result of exposure to carcinogenic agents and unrepaired DNA replication errors.
Results in somatic activation/ inactivation of cancer genes.
compare familial and sporadic cancers
- onset
- how many tumours
- how many types of tumours
- tumour cells - how many copies of inactivated TS gene
- other cells - how many copies of inactivated TS gene?
Familial cancer:
- Early onset
- Multiple tumours of same type
- Other types of tumours
- Tumour cells: both copies of tumour suppressor gene inactivated
- All other cells: one copy of tumour suppressor gene inactivated
Sporadic cancer:
- Late onset
- Single tumour usually
- No other tumours usually
- Tumour cells: both copies of tumour suppressor genes inactivated
- All other cells: normal
what are the different types of cancer? what most common?
- Adenoma: cancer of the glands (glandular cells)
- Carcinoma: epithelial cells (more than 90% of all cancers)
- Lymphoma: lymphocytes or lymphatic system
- Sarcoma: connective tissue
- Blastoma: immature/ pre-cursor cells (dendrites – white blood cells)
- Papilloma: surface epithelia (skin)
what is neoplasia?
means new growth (tumour) and it is described as malignant.
describe characteristics of benign tumour
- Capsule surrounds tumour
- Well differentiated cells
- Structure is similar to tissue organ
- LOW MITOTIC ACTIVITY – slow rate of growth
- NO INVASION of surrounding tissue
- NO METASTASIS
describe characteristics of malignant tumour
- No capsule
- Lack of differentiation (anaplasia)
- Structure is different to tissue organ
- HIGH MITOTIC ACTIVITY – rapid rate of growth
- INVASION of surrounding tissue
- METASTASIS – cause multiple organ failure
genomic instability
- what is this
- what are types
• Cancer genomes are unstable:
Either chromosomal instability (CIN) – result of many numerical and structural abnormalities.
Or microsatellite instability (MIN) – result of impaired DNA Mismatch Repair (MMR).
- normal genomes are stable
- cancer genomes are unstable
- cancer cells are constantly shuffling around their chromosomes
- chromosomal instability -> either whole chromosomes or parts of chromosomes are duplicated or deleted
- genome stability – increased mutation rate
what is telomerase?
an enzyme that prevents the shortening of the telomere, thus preventing senescence (specific number of cell divisions).
telomere = a region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes.
Overtime, telomeres get a bit shorter, until they get so short that it triggers a DNA damage response and it triggers the cells into senescence – they no longer proliferate
Cancer cells find a way of bypassing this, which leads to telomere erosion, which leads to chromosome fusion
- telomeres – T-loops, DSB (double strand breaks) & senescence
what is senescence?
Senescence is the process by which cells irreversibly stop dividing and enter a state of permanent growth arrest without undergoing cell death. Senescence can be induced by unrepaired DNA damage or other cellular stresses.
is telomerase normally switched on or off? what about in malignant cells? what does this lead to?
- In normal cells, telomerase is switched off.
- In malignant cells, telomerase is switched on, thus inhibiting senescence.
Telomere repeats maintained by telomerase:
- OFF in normal cells
- ON in tumour cells
what does successful carcinogenesis require?
Either mutations that increase the rate of cell proliferation, so as to provide an expanded target for further mutations (clonal evolution).
Or mutations that destabilise the genome, so as to increase the rate of further mutations.
for a cancer cell to become malignant, it must what? (hallmarks of cancer)
- Become independent of external growth signals (this means not responding to signals which tell it to grow. The cell should be able to grow independently).
- Become insensitive to external anti-growth signals (this means not responding to signals which tell the cell to stop dividing. The cell should be able to divide independently).
- Become able to avoid apoptosis.
- Become capable of indefinite replication (cells have a finite number of times they divide before dying – senescence).
- Become capable of sustained angiogenesis (must be able to cause blood vessels to enter the tumour to provide nutrition).
- Become capable of tissue invasion and metastasis.
- Self-sufficiency in growth signals
- Insensitivity to anti-growth signals
- Tissue invasion & metastasis
- Limitless replicative potential
- Sustained angiogenesis
- Evading apoptosis
- Genomic instability
- Deregulated metabolism
- Avoiding immune destruction
- Tumour promoting inflammation
what is cancer mediated by?
Genes – oncogenes and TSGs Telomeres – telomerase and senescene Signalling Pathways – Wnt and Ras Genome Instability – CIN/ MMR P53 and Apoptosis Cell Cycle Control – cyclin/ CDK/CKI(p21)/
what are the cancer genes?
- (Proto-)Oncogenes
• Gain of function
• Dominant (only need one mutated allele to be activated) - Tumour Suppressor Genes
• Loss of function
• Recessive (need two mutated alleles to be inactivated)
proto-oncogenes
- what is it
- what is the resultant protein called
- what do these genes do
- A proto-oncogene is a normal gene that may be activated into an oncogene due to mutations or increased expression.
- The resultant protein may be termed an oncoprotein.
- Proto-oncogenes promote cell division, survival and growth.
how can proto-oncogenes be activated?
- Point mutations that increase protein function.
- Gene amplification that causes overexpression.
- Chromosomal translocation (e.g. MYC).
- Viral stimulation that may lead to addition/deletion of genes when viral DNA is integrated with human DNA.
what does the translocation of MYC cause? what is MYC?
MYC - This is a regulator gene that codes for a transcription factor. If translocated, MYC will be continually expressed, causing unregulated expression of many genes, some of which are involved in cell proliferation (e.g. cyclin D1) and results in the formation of cancer.
oncogenes
- what are they
- what happens
- what do they usually code for? and examples for each of these?
• An oncogene is a gene that has the potential to cause cancer (e.g. β-catenin and KRAS)
• When one allele is mutated, there is a gain in protein function.
• In tumour cells, they are often mutated or expressed in high levels.
• Oncogenes usually code for:
1. Secreted growth factors (e.g. EGF/ Wnt/ Ras)
2. Cell surface receptors (e.g. HER)
3. Signal transduction system components (ABL)
4. Nuclear proteins, transcription factors (e.g. MYC)
5. Cyclins/ cyclin-dependent kinases (cyclin D1, CDK4)
tumour suppressor genes
- what are they
- what happens
- what does familial predisposition mean
- examples
- Tumour suppressor genes are the body’s natural defence mechanism against malignancy.
- When both alleles of this gene are mutated, there is a loss in protein function.
- If there is a familial predisposition, then one allele has already been mutated, therefore, only a second hit is required for fatal effects (“two-hit hypothesis”).
- If one copy already mutated, only 1 random hit required
- Two random hits required
- APC & p53
- Familial form of retinoblastoma – already have one form of the gene
- Sporadic form of retinoblastoma – no mutations, have to get both throughout life to develop cancer
what is inactivation of TSGs caused by?
- Mutations
- Chromosomal abnormalities
- Methylation of promoters
- Interaction with viral proteins
- methylation -> reduced binding of transcription factors -> inactive gene -> decrease gene expression
