Lecture 3 - Regulation of protein expression (steroid signalling) Flashcards
Lipophilic/hydrophobic hormones often follow the same typical pathway as
steroids
What type of hormone can travel across the phospholipid cell membrane
Lipophilic
Steroid hormone-receptor complex
Once in the nucleus, this complex of the steroid hormone and the receptor together are able to find binding sites on the DNA and turn on or off genes (i.e. act as transcription factors) downstream of this signal
What are steroids
Molecules that are chemically based on cholesterol that are lipid soluble (cholesterol is an essential cell component that is important for membranes, as a foundation material to make up steroid hormones)
Multiple steroids with a very wide range of diverse actions but common theme dictated by their physiochemical
Where are steroids produced?
Adrenal cortex - Cortisol in response to stress
Adrenal cortex - Aldosterone in response to a fall in blood pressure
Gonads - produce a number of sex hormones with multiple roles
Female = progesterone, estradiol
Male = testosterone, dihydrotesterone
Neurosteroids - produced by neurons and glial cells
All based on the steroid pregnenolone
Multiple actions within the brain
Signalling molecules that are released from one cell to another, autocrine and paracrine actions within the brain
Lipophilic hormones
Cannot be stored therefore synthesised when needed - so when it is required, enzymes that make the steroid need to be turned on and then the steroid needs to be made and then the completed steroids can escape the cell
Require a carrier protein after being released into the blood
Cross the target cell membrane and act on intracellular receptors
Steroid hormones are generally
Slow to act
Delayed synthesis
Affect gene transcription
Long lasting
Carrier bound hormone resistant to metabolism (therefore hormone will stay around for longer and have a longer effect)
Changes in protein expression often slow to reverse - the response in the target cell is slow to come on initially due to hormone production etc and it is going to take time to reduce the hormone back to original levels - the events in the target cell are quite slow to come on but once they are on they are quite long lasting
The same is true for other lipophilic hormones such as the thyroid hormones (T3 and T4)
Cortisol requires a
carrier protein in the blood
Most of the hormone is bound
But free and bound cortisol are in an equilibrium
Only the free hormone is physiologically active
Free (1%) Bound (99%)
At any one moment in time, the molecules will dissociate themselves from the carrier molecule and then a fraction of a second in time later they become bound again - constant kinetic changing i.e. a dynamic equilibrium
The amount of carrier protein, and therefore bound hormone, can be altered by disease or medications
Important to distiguish free and bound hormones in clinical tests
Intracellular signalling pathway of a typical steroid hormone
Steroid hormones can get into any cell in your body therefore specificity is achieved by steroid receptors for that hormone only being in the target cells (you must have a receptor in order to cause a response)
The hormone-receptor complex made in the cytoplasm after binding is able to travel into the nucleus and bind to target parts of the DNA to regulate messenger RNA production and protein output
Sympathetic nervous system = releases adrenaline from the adrenal gland
Adrenal steroids - helps you recover from the fight or flight stage and it is a slow response
If this stress response goes on for too long, the system and you become exhausted
Cortisol is produced in the adrenal cortex and catecholamines are produced in the adrenal medulla
Stress response process
Adrenal cortical cells activated by ACTH
ACTH is water soluble/peptide hormone therefore it requires a cell surface receptor (this one is a member of the G protein coupled receptors)
ACTH comes from the pituitary and its function is to increase cortisol production by the cells
Acts via a G protein coupled receptor (MC2R)
Receptor that responds to ACTH
Stimulates an increase in cAMP
Increases activity of protein kinase A (PKA)
Job of protein kinases is to add phosphates onto proteins
PKA activates transcription factors
All of these are proteins that go into the nucleus that can turn things on or off, these proteins do not do anything unless activated by something like a protein kinase A (which adds a phosphate to activate them)
Increased cortisol synthesising enzymes
Transcription factors turn on these genes which make proteins that are involved in making cortisol - making more enzymes that are involved in making cortisol
These enzymes fo to different places - one to the sER, one to the mitochondria, one to the membrane - the message here is that cortisol involves many different cellular compartments e.g. cytosol, sER, mitochondria and the cell membrane
More cholesterol transported into cell
If you want to make steroids, you need cholesterol as that is what they are made from so one of the enzymes heads out to the membrane and it helps bring more cholesterol into the cell and once it is in the cell then it is fed into the system tat involves the mitochondria and sER and these end up working together to make the steroid hormone cortisol
Directs it via mitochondria and sER
Results in increased cortisol synthesis
Starts making it
And therefore cortisol release from adrenal
Cortisol’s final making step is in the mitochondria and since it is a lipid soluble molecule it can leak out of the membrane of the mitochondria and then it can leak out of the cell and now you have cortisol released for the adrenal cell
Cortisol entering the cell
Free cortisol (when it comes off the carrier protein) crosses into the target cell Cortisol will only effect if there is the right receptor in the cell and in this case it is the glucocorticoid receptor Cortisol binds to its intracellular receptor (GR)
An inhibitory regulator (Hsp) dissociated from the cortisol-GR complex
GR is kept quiet by Hsp until cortisol comes along and binds to it at which point the Hip protein dissociates and then we have an active receptor which can now enter the nucleus and regulate gene expression
Complex enters cell nucleus via nuclear pores
Binds to glucocorticoid response element (GRE) on specific genes
Regulates mRNA transcription from those genes
The cortisol-GR complex enters the nucleus via the
nuclear pore
The cortisol-GR complex enters the nucleus via the nuclear pore…
This is a multi-subunit regulated structure
Complicated to get into the cells nucleus because it contains the genes which must be kept safe as they control what the cell does as well as what every single other cell in your body does and the genes also go to the next generation therefore they must be looked after
Proteins entering the nucleus require a nuclear localisation sequence (NLS)
The NLS interacts with the protein importin to allow the cortisol-GC to enter the nucleus where it can target GREs
Importin is protein hat transports protein molecules from the cell’s cytoplasm to the nucleus. It does so by binding to specific recognition sequences, called nuclear localization sequences (NLS).
A nuclear localization sequence (NLS) is an amino acid sequence that ‘tags’ a protein for import into the cell nucleus by nuclear transport.
Additional factors now needed to remove importing from the nucleus so that it can do its job again
Cortisol, and other steroids, also signal to cells in other ways
1- The classical pathway
2- Interaction with other transcription factors
30 Steroid-receptor complex having non-transcriptional roles - such as affecting cell surface proteins, signal transduction
4- Steroids signalling via cell surface receptors
These other mechanisms are useful because one of the problems with steroid signalling is that it is quite slow for a variety of reasons and having a cell surface receptor allows for more rapid signals to be generated in the target cell as they do not have to wait for changing gene transcription, GPCR often have more to do with changing the activity of a protein than changing the amount of protein which is a much faster way of causing a response
