Lecture
#5: Hormone Action: Signal Transduction, Part II
I. EFFECTS OF SIGNAL
TRANSDUCTION ON ION GATING
A. INTRODUCTORY REMARKS ABOUT ION GATING
1. One of the
principal effects of G-protein-mediated signal
transduction is the gating of ion channels in the
plasma membrane. (or,
for greater depth on this topic, click
HERE.)
2. One established
example of this ion gating is when cAMP-dependent
protein kinase (i.e., PKA) phorphorylates
Ca++ channels and promotes a rapid influx of this ion into the cell.
(Fig. D)
B. G-PROTEIN
MEDIATED REGULATION OF THE Na+-K+ PUMP
1. The plasma
membranes of all cells contain Na+-K+
pumps. Like most ion channels, this pump is a complex transmembrane
protein that is a specific enzyme with a - and b
-subunits (Fig. G).
2. By extruding Na+
out of the cell while taking K+ into a cell, the pump creates a
resting membrane potential that is necessary for every "excitable"
tissue. (Fig. G)
3. The enzyme is
called Na+-K+-ATPase because it
utilizes the energy derived from ATP hydrolysis to bring about the active countertransport of Na+-K+ across the
plasma membrane.
4. The importance
of Na+-K+ pumps is confirmed by the fact that more than 25% of all
ATP-energy is utilized by these constantly operating pumps in all living cells.
5. Hormone
stimulated receptors mediate signal transduction processes (via G-proteins)
which cause acute alterations in the Na+-K+-ATPase activity. The signal transduction processes either
activate or inhibit protein kinases and phosphorylases.
6. PKA
regulation of Na+-K+-ATPase
activity: (Fig. G)
a.
hormone/receptor coupling that
leads to activation of adenylate cyclase
by Gs-proteins results in cAMP, cAMP-dependent protein kinase
(PKA), and PLA2, all of which inhibit Na+-K+-ATPase activity.
b.
the mechanism by which this PKA
pathway inhibits Na+-K+-ATPase
is not clear, but it may involve phosphorylation of
the a -subunit of the enzyme.
c. in any event, the
inhibition of Na+-K+-ATPase
activity results in a depolarization of the plasma membrane (i.e., it makes the
membrane more easily excitable).
7. PKC
regulation of Na+-K+-ATPase
activity:
a. hormone/receptor
coupling associated with G-proteins that activate PLC leads to PKC activity
that can either inhibit or stimulate Na+-K+-ATPase depending on the experimental conditions or the
tissue examined (i.e., the effect is not consistent).
8. Eicosanoid regulation of Na+-K+-ATPase activity:
a.
Na+-K+-ATPase activity is
inhibited by some of the metabolites of arachidonic
acid, especially PGE2 and 12-HETE, but the mechanism of action of
these "fourth messengers" is not clear.
II. TERMINATION OF SIGNAL TRANSDUCTION
PROCESSES
Like
most biological processes, hormonal activation of G-protein coupled receptors
cannot proceed indefinitely.
Occasionally, the termination of signaling is brought about by
dissociation of the hormone (ligand) from its
receptor. However, there are other
built-in mechanisms to terminate the signaling process that is initiated by the
interaction of hormones with G-protein-coupled receptors.
I have
summarized two of these mechanisms below:
A. GTPase-ACTIVATING PROTEINS AND RELATED PHENOMENA
(G-proteins are complex, globular molecules that can have
functional units within their sequences.)
1. Termination of
G-protein signaling usually occurs when the bg-subunit re-associates with the a-subunit. (Fig.
4.A)
2. This
recombination of G-protein entities can occur naturally, albeit slowly, by GTPase
activity that is intrinsic to the a-subunit. The GTPase converts GTP into GDP on the a-subunit, and thereby allows reunion with the bg-subunit.
3. However, it is
now recognized that there are a number of proteins known as GTPase-activating proteins (i.e., GAPs) that promote hydrolysis of GTP to GDP on the a-subunit.
4. Recently, a superfamily of evolutionarily-conserved GAPs
known as regulators of G-protein signaling (RGS) have been
characterized.
5. These RGS
proteins are now recognized as an important superfamily
that regulates the duration and intensity of signal transduction
processes. They cause a 100-1000-fold
increase in the GTPase activity in the a-subunits.
B. UNCOUPLING AND
SEQUESTRATION OF MEMBRANE RECEPTORS
1. Agonist
Removal: reuptake or enzymatic
degradation in extracellular fluid. (Fig.
3.19.a)
2. Receptor
Uncoupling: phosphorylation
of activated receptors by GRKs (arrestins) (Fig.
3.19.b)
3. Receptor Endocytosis: endocytosis by
receptor cell causes desensitization (Fig.
3.19.c)
4. Receptor Down-Regulation: increased degradation and diminished
synthesis of GPCR (Fig.
3.19.d)
III. A
SUMMARY OF SIGNAL TRANSDUCTION