Endocrinology, lecture on Receptors and 2nd messenger systems
V. Membrane Receptors ~(104+1/cell) & 2nd Messengers
A. Extracellular region
1. N-terminal portion, potentially glycosylated
2. often many cysteines, with S-S bonds forming rigid
pockets ® hormone binding
a. also link receptor chains (same or different receptors)
B. Transmembrane segments
1. helical structure (may be held together by cysteines)
a. single or multiple membrane-spanning
i. receptors with multiple-spanning segments may
bind hormone(e.g. Epi) neither inside nor outside
the cell, but within the spanning segments
2. ion channels
C. Intracytoplasmic segments
1. primarily involved in effector functions
2. coupled to a G protein
a. or may have intrinsic tyrosine protein kinase activity
3. phosphorylation sites
D. Second Messenger systems
1. cAMP
a. receptor binding catalyze GTP binding to Gs (transducer)
proteins (G refers to GTP binding)
i. GTP dephosphorylated at the a subunit of Gs
(1) a dissociates from b and g
(a) a binds to AC
(2) b and g associated with receptor
ii. free intact Gs (a,b,g) may react with receptor
(1) amplifying signal
(2) b/g may inhibit receptor
iii. inhibitory hormones, paracrines,
or neurotransmitters activate Gi
b. a from Gs turn on activity of Adenylate Cyclase
to generates production of cAMP
i. catalyzes cAMP from ATP
(1) 4 phosphorylation sites on AC
(a) AC made of 12 helices
(b) amplifies signal
ii. Gi inhibits AC
c. 4 cAMP remove inhibition (2 regulatory subunits) from
Protein Kinase A (2)
d. Protein Kinase A + ATP phosphorylates (activates - may
be inhibitory as with Gi) a protein (enzyme)
i. enzyme catalyzes cellular response
(1) e.g. make glycogen, turn on active
transport (GH release), catabolize lipids
(2) PKA may bind to transmitter receptor
(e.g. GABA) to influence ion flow
ii. PKA passes through nuclear pores,
bind to and phosphorylate CREB
(CREB = cAMP response element binding protein)
(1) phosphorylated CREB binds CBP (CREB binding protein)
(a) together CREB and CBP bind to DNA
and stimulate transcription
(i) 2x CREB bind CRE
(cAMP response element;
palindrome DNA segment)
(ii) CBP binds RNA polymerase II
which binds TATA box
(2) activates immediate-early gene products
like c-fos and c-jun
e. cause cAMP increase via Gs: GHRH, GnRH, AVP, ACTH, CRF,
FSH, LH, hCG, a-MSH, TSH,Glucagon, Gastrin,
Secretin, VIP, PTH, Calcitonin, DA (D1 receptors),
NE & Epi (b1 & b2), 5-HT, Histamine (H2), PGE
i. cAMP degraded to AMP by PDE (phosphodiesterase)
following activation of A-kinase
f. cause cAMP decrease via Gi: Somatostatin, Endorphins, Enkephalins,
Bradykinin, P (in amphibians), DA (D2), NE (a2)
2. cGMP is similar: Receptor-KHD-mGC ®+ GTP --® cGMP ®+ PKG
KHD = kinase-homology regulatory domain
a. sGC binds NO ®+ GTP --® cGMP
b. increases cGMP: ACh, Insulin, GHRIH, ANP/ANF, BNP, CNP, NO, CO
i. cGMP ®+ ion channel conductance, glycogenolysis, apoptosis, vasodilation
ii. cAMP and cGMP may have antagonizing effects
1) but cGMP requires ácAMP
c. decreases cGMP: light ® rhodopsin(7tm)conf D ®+ Gt ®+ cGMP-PDE ®- cGMP
3. Inositol Phosphate and diacylglycerol system
gene = GNAQ, GNA11...
a. receptor ® Gq/P/11/14/15 protein ® PLC or PDE ---
¯
b. PIP2 (phosphatidylinositol bisphosphate) ------® IP3 + DG
c. DG (diacylglycerol)® Kinase C
i. DG is also converted by DAGL (diacylglycerol lipase) into 2-AG
1) 2-AG (2-arachidonoyl-glycerol) is an endocannabinoid
d. IP3 (inositol trisphosphate) releases calcium from endoplasmic reticulum/
membrane Ca++ channels
e. Ca++ binds to calmodulin ® activates Ca++/calmodulin-Kinase
f. cause increase in inositol phosphate system: TRH, GnRH, AVP (V1),
OT, CCK, Ang II, VIP, substance P, Histamine (H1), Epi, NE (a1),
5-HT (5-HT2), ACh, PGF2a, NGF, EGF
4. free Ca++ activates Ca++/calmodulin-Kinase
a. calcium ion channel activated by depolarization of the membrane
i. or by a hormone or 2nd messenger (IP3)
(1) via PDE
b. PDE ® activates Guanylate cyclase ® cGMP ® G-Kinase
i. Ca++ may also stimulate cGMP
5. JaK/STAT
a. tyrosine kinase associated with the receptor
i. 2 Janus Kinase or JaK tyrosine kinase
molecules bound to receptor
(1) JaK family: JaK1, JaK2, JaK3, TyK2
b. hormone binding dimerizes receptor
i. and phosphorylates JaK
c. Jak phoshorylates the receptor
d. phosphorylated-R binds
STAT (Signal Transducers and Activation of Transcription) proteins
i. JaK phosphorylates STAT ® STAT dimerizes
e. STAT dimer translocates to the nucleus and binds to DNA
f. activate JaK/STAT: EPO, GH, IL3, PRL
6. MAP Kinase
a. hormone binding phosphorylates receptor: tyrosine PK
b. phosphorylated receptor activates
GRB2 (Growth factor Receptor Bound protein)
c. GRB2 activates Sos (conformational change)
i. Sos is the protein product of Sos gene
d. Sos binding Ras allows GTP phosphorylation of Ras
i. Ras is a G protein from the Ras gene
(1) GAP protein stimulates Ras GTPase
e. Ras activates Raf1
(conformational change and maybe association with membrane)
f. Raf1 is a kinase and phosphorylates MAP KK = MEK
i. MAP KK/MEK is a dual specificity Kinase
(1) may also be activated by PKC or Gbg
g. MEK phosphorylates MAP Kinase (Mitogen Activation Proteins)
h. MAP K/ErK phosporylates transcription factors ® translocate to nucleus
i. activate MAP K: insulin, EGF, NGF, PDGF
E. Membrane-bound Steroid Receptors
1. G-protein mediated receptors have been found in brain
and ovary for E, B, and P
a. birds, amphibians & fish
2. ligand-gated ion channels have a specific binding site
a. steroids modify channel opening and ion movement
b. GABAA, NMDA (Glu), Gly-R have been demonstrated
to have steroid site
i. family of receptors/channels/cell connectors
(1) above plus nicotinic (ACh), 5-HT3, AMPA (Glu),
KA (Glu), voltage-gated ion channels, gap-junctions
(a) all are probably steroid sensitive
