Hormonal And Cytokine Actions At A Cellular Level

Question 0

PIP2 cleaved to form second messengers

Answer 0

Cleaved by phospholipase (PLC)/phosphodiesterase (PDE)in the membrane-> IP3 and DAG

• PLC is first activated by either tyrosine kinase growth factor PLCy or G protien signal transduction PLCb
• > IP3 is water soluble so diffuses into the cytosol-> binds to ion channels on ER-> increase cytosol Ca-> Ca binds calmodulin on CAM kinase-> cell actions
• > DAG is lipophilic-> remains in bilayer-> diffuses along-> meets protein kinases-> increases activity of protein kinases-> eg increases protein kinase C-> increased proteins/genes

Question 1

Inositol signalling pathway

Answer 1

Phosphatidylinositol phosphate phosphorylated by kinases-> phosphatidylinositol 4,5 bis phosphate -> all present in inner half of plasma -> PIP2
Two things can happen to PIP2:
1) cleaved to form second messengers
2) phosphorylated to PIP3

Question 2

PIP2 phosphorylated to PIP3

Answer 2

By PIP3 kinase which is activated by GBy subunit of G proteins or tyrosine growth factor receptors
Has many effects
-> mainly pro growth and pro survival
Eg AKT recruitment with PDK1 to block apoptosis
PIP3 kinase is antagonised by PTEN (phosphatase)
Both often mutated in human cancers

Question 3

Receptor tyrosine kinase

Answer 3

Enzyme couple surface receptor
Often activate same pathways as GPCR as well as distinct pathways
60 genes encoding human RTK’s -> 16 structural families
Binding of signal-> intracellular tyrosine kinase domain activated-> phosphorylates selected tyrosine side chains-> become a binding surface for other proteins.
Generally monomers with single membrane spanning helix
One molecule of growth factor generally binds two monomers-> dimerisation

Question 4

Insulin receptor

Answer 4

Pre formed dimer
Coded by a single gene
2 alpha and 2 beta chains-> beta chains span membrane, linked by sulphate bonds
Located in the membrane of all mammalian cells, number dependent on glucose requirements
-> regulation of carb and lipid metabolism
Stimulates growth and protein synthesis

Question 5

Insulin receptor mechanism

Answer 5

Insulin binds-> conformational change-> tyrosine kinase domain on intracellular end of beta chains activated-> autophosphorylate each other
-> phospho-tyrosines at specific sites in TK domain-> act as specific binding surface for signal proteins containing SH2 domain
Eg IRS-1(insulin receptor substrate)-> binds and is phosphorylated by TK-> phosphates then provide binding sites for other protiens with SH2 domains
-> PIP3 k-> protein B kinase activation via phosphorylation by PDK1-> proliferation/apoptosis, glycogen metabolism, protein synthesis
-> Syp
-> Grb 2-> Ras GDP/GTP-> activation of Ras and MAP kinase pathway

Question 6

mTOR

Answer 6

mammalian Target of Rapamycin
Integrates inputs from various sources-> GF’s, nutrients, amino acids-> turn on mTOR -> cell growth
Established link to tuberous sclerosis-> disjunction of mTOR
Dominantly inherited disease
-> formation of hamortomas/growths/tubers
-> epilepsy, learning disabilities, autism, kidney problems, but variable Reduced control of cellular pathways

Question 7

Downstream targets of enzyme and G protein coupled receptors

Answer 7

GPCR
-> G protein-> adenylate cyclase-> cAMP-> PKA
-> G protein-> phospholipase C->
-> IP3-> Ca2+ -> calmodulin-> CAM kinase
-> DAG-> PKC
TKR
->Grb2-> Ras-GEF-> Ras-> MAP kinase kinase kinase-> MAP kinase kinase-> MAP kinase
-> PI3 kinase-> PI3,4,5P3-> PDK-> AKT kinase
All end in gene regulatory proteins and many target proteins

Question 8

Wnt

Answer 8

Developmental signalling
Conical Wnt pathway
-> frizzled GPCR > doesn’t use G protiens
-> frizzled+LRP-> bind Wnt
No Wnt-> B-catenin continually phosphorylated in the cytosol -> inactive-> destroyed as recognise-> no transcription -> no cellular division
Wnt-> phosphorylates LRP-> recruits axin, GSK3 and DVL therefore removing them from B-catenin-> B-catenin no longer phosphorylated-> into nucleus-> increase Wnt genes-> increased DNA transcription and cell division
Mutations found in gut cancer-> polyps
- in APC-> no B catenin complex-> different so B catenin stabilised-> stimulation in absence of Wnt
- or B-catenin mutation-> can’t be phosphorylated

Question 9

Human Epidermal Growth Factor Receptor

Answer 9

Found on surface of normal mammary cells
Involved in cascading growth signals from outside to inside the cell
-> growth division
Breast cancer -> cells have elevated numbers of HER2 receptors-> results in elevated growth and division-> very aggressive cancer
Herceptin-> breast cancer drug-> binds HER2 receptor and stops signalling-> prevents further growth, also targets them for destruction by immune system