LECTURE 4 - hormone action and synthesis Flashcards
What are the hormone superfamilies?
Peptide
Steroid
Amino acid derivatives
How can hormones affect proteins?
FAST response = modify proteins (i.e. phosphorylation) - takes seconds-mins
SLOW response = activate new gene to make new protein (altered gene transcription) - takes mins-hours
What are the properties of a hormone receptor?
- must be visible to hormone
- binds hormone specifically and be able to detect it among other related molecules
- bind the hormone with enough affinity to detect hormone in the blood
- must only be on specific tissues
- must be saturable and must have limited number of binding sites (prevents constant activation, allows reaction to speed up and slow down and allows reversibility)
- must mediate some biological response
What are the 2 types of hormone receptors?
- Cell surface receptors
(either linked to TK or G proteins) - Intracellular receptors (steroid hormone receptors)
What are the subgroups of cell surface receptors linked to TK?
- Growth factor receptors (intrinsic TK) e.g. insulin, IGF1. EGF - all have TK domain built in
- Cytokine receptors (recruit TK) e.g. growth hormone, prolactin, leptin
What is TK?
Tyrosine kinase
- an enzyme that transfers a phosphate group from ATP to a tyrosine residue in a protein (phosphorylation)
- phosphorylation induces conformational changes
- TK activity can be either intrinsic or recruited
What are the properties of the extracellular EGF receptor?
EGF = epidermal growth factor receptor
- 4 family members (EGF 1-4)
- ligand-induced dimerisation: peptide ligands cleaved to yield active hormone
- autocrine, paracrine cell signalling
- signal transduction processes
- -> Ras
- -> Phosphatidylinositide 3-kinase (PI 3-kinase)
- -> JAK-STAT
Define autocrine and autocrine signalling
Autocrine = a substance that has an effect on the cell by which it was produced
Autocrine signalling = a form of cell signalling in which a cell secretes a hormone or chemical messenger that binds to autocrine receptors on that same cell, leading to changes in the cell
Define paracrine and paracrine signalling
Paracrine = referring to a hormone that only has effect in the vicinity of the gland secreting it
Paracrine signalling = a form of cell signalling or cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells
What is the EGF receptor composed of?
- hormone binding site
- 2 cysteine-rich regions
- a single trans-membrane region
- kinase domain
Describe the basic mechanism of receptor modification
- hormone binds to receptor –> dimerisation = allows conformational shape change
- -> exposes kinase domain
- exposed kinase domain allows for phosphorylation to occur
- activation now allows for receptor to recruit other molecules (e.g. GRB + SOS)
- factors can enable second messenger pathway (e.g. by activating Ras) leading to activated transcription factors
How do receptors normally work?
- work as dimers
- dimerisation allows for conformational shape change
- exposes kinase domain –> allows for phosphorylation –> then activated to recruit other factors
What is Ras?
- family of related proteins all belonging to a class of proteins called small GTPase
- exists in 2 forms - 1 bound to GTP and 1 bound to GDP, becomes active when bound to GTP
Explain how recruited tyrosine kinase activity works
- receptors that do not have TK domain built in but still want to be able to phosphorylate
- use a secondary system (JAK-STAT)
JAK = kinase, STAT = transcription factor - cytokine hormone binds to receptor = induces shape change
- shape change allows for recruitment for JAK
- JAK phosphorylates receptor, can then recruit STAT molecule which affects transcription
What are G protein coupled receptors composed of?
- much more complex than those linked to TK
- 7 transmembrane domains
What are G proteins?
- inactive when bound to GDP
- 3 subunits, a,b,y (gamma)
- -> a subunit as single one
- -> b,y form single functional unit
How does G coupled receptor signalling work?
- resting G protein a subunit associated with GDP
- activation of receptor by hormone = conformational shape change to receptor
- -> conformational change to a subunit allowing for exchange of GPD for GTP
- a subunit released and activates secondary messenger
What effect can Ca2+ have when working coupled with G proteins?
- Diaglycerol (DAG) (secondary messenger) or Ca2+ can activate protein kinase C –> phosphorylated proteins
- Ca2+ sensitive enzymes –> modified substrates
- calmodulin activated protein kinase –> phosphorylated proteins
What are steroid hormone receptors?
- found within the cell, typically in cytoplasm or nucleus
- ligands are small lipophilic molecules
- receptor encoded by a single gene
- able to bind to DNA
- function as transcription factors
How do steroid hormones work?
- most hydrophobic steroid are bound to plasma protein carriers as they cannot travel in water, only unbound hormones can diffuse into target cell
- bind to receptor that use secondary messengers
- receptor-hormone complex binds to DNA and activates or represses one or more genes
- activated genes create new mRNA –> protein synthesis
What are type 1 nuclear receptors and how do they work?
- work as homo-dimers
- normally found in cytoplasm
- e.g. glucocorticoids, mineralocorticoids, androgens
- when hormone binds, move from cytoplasm to uncles and bind to sequence of DNA called hormone response elements (HRE’s)
- nuclear receptor/DNA complex then recruits other proteins which transcribe DNA downstream from HRE to mRNA and eventually protein causing a change in cell function
What are type 2 nuclear receptors and how do they work?
- work at hetero-dimers
- all bind as heterosexual-dimers with RXR (retinoid X receptor)
- e.g. VDR (vitamin D), RAR and TR
- found in nucleus normally bound to response element already
- these receptors have proteins on them (co-repressors) which stop transcription
- activated by removing co-repressor when ligand binds
Describe the structure of the steroid hormone receptors
- 3 domains: transactivation, DNA binding and hormone binding
- within those there are 5 further domains: A,B,C,D,E and sometimes F
A/B: AF-1 = always on, when hormone binds to receptor, AF-2 works with AF-1 to drastically up regulate gene transcription (synergisation)
C/D: DNA binding areas, NLS = nuclear locational signal (allows entry into nucleus to bind to DNA). Hinge that allows dimerisation is here
E: within domain there is AF-2- which is a transcriptional activation domain, becomes active when hormone bound
What is a zinc finger?
- within the C domain
- enables proteins to bind to DNA
- second zinc finger domain involved in dimerisation
What are HRE’s?
Hormone response elements
- within receptor there is a set sequence recognition of DNA
- allow for specificity
- dimeric and 2 halves for 2 receptors as its dimeric
- vary in structure
How does a hormone alter gene transcription?
- binding via receptor to target sequences of DNA
- HRE
- located in regulatory regions of target gene
- usually 5’, close to core promoter
- 6bp hexamer - core recognition motif
- usually 2 half-sites, intervening base pairs
How does a receptor recognise a specific HRE?
P-box contains:
- zinc fingers which enable DNA docking
- HRE which allows for specificity
What are resistance syndromes?
- altered binding of hormones to its receptor results in disease
3 examples:
- Vitamin D resistant rickets
- Glucocorticoid resistance
- Insulin resistance
