section 30.5
Types of Hormone Receptors
721
plasma membrane, which may maintain an elevated Ca2+
concentration at the cytoplasmic face of the membrane.
This increase, together with the diacylglycerol, may con-
tinue to activate the membrane-bound protein kinase C. In
this way, myosin light chains may remain phosphorylated
and active during smooth muscle contraction (Chapter 21)
despite a fall in myoplasmic Ca2+ to near-basal levels.
The rise in intracellular Ca2+ level also alters the ac-
tivity of metabolic pathways by calcium-binding proteins.
Of these proteins, calmodulin (M.W. 17,000), found in
the cytoplasm of every eukaryotic cell, has received the
most attention. Parvalbumin in skeletal muscle, troponin
C in skeletal and cardiac muscle, and myosin light chain
in smooth muscle serve similar functions. The affinity of
these proteins for Ca2+ is high, as reflected by dissociation
constants of 10“
6
—10
“ 8
M. Occupation of at least three of
four equivalent Ca2+ binding sites on calmodulin induces
a conformational change and, in most cases, increases its
affinity for proteins having a calmodulin binding site. A
major function of Ca2+-calmodulin is regulation of the
phosphorylation states of intracellular proteins. Systems
affected include those involved in secretion (of insulin,
for example), neurotransmitter release and neuronal func-
tion, cellular contractile processes (Chapter 21), and sev-
eral protein kinases involved in regulation of glycogen and
glucose metabolism.
A phosphatase activated by Ca2+ and calmodulin,
known as calcineurin, functions as a negative modulator
of some of the effects of Ca2+ related to protein phos-
phorylation. Calcineurin is a Ca
2
+-calmodulin-dependent
serine/threonine protein phosphotase and it participates
in many Ca2+-dependent signal transduction pathways.
The immunosuppressant drugs cyclosporin A and FK506,
after binding to their respective cytoplasmic binding pro-
teins (cyclophilin and FK506-binding protien), inhibit the
action of calcineurin. This blocks the translocation of a
factor into the nucleus of activated T lymphocytes and
prevents allograft rejection by blocking T cell cytokine
gene transcription (Chapter 35).
Receptors for Insulin and Growth Factors
Insulin exerts its effects by altering the state of phosphory-
lation of certain intracellular enzymes by a mechanism that
does not involve cAMP but that requires specific binding
to surface receptors with tyrosine kinase activity. Insulin
exerts acute (minutes), delayed-onset (hours), and long-
term (days) effects entirely by way of a single receptor.
The insulin receptor is a transmembrane heterotetramer
consisting of one pair each of two dissimilar subunits
linked by disulfide bridges (Chapter 22). Both subunits
are expressed entirely by a single gene, which is located
on chromosome 19. The two subunits,
a
and
ft,
are formed
posttranslationally from a single proreceptor following
glycosylation and proteolytic cleavage by an endopepti-
dase. The
a
subunit, which projects into the extracellular
space, is a glycosylated 135-KDa protein that specifies
the insulin binding site (one binding site per
a
subunit).
The transmembrane
ft
subunit is a glycosylated 95-KDa
subunit that contains four domains, one of which has ty-
rosine kinase activity. The tyrosine kinase activity of the
ft
subunit is suppressed by the
a
subunit in the absence of
insulin; insulin binding to the
a
subunit removes the in-
hibition and allows the /5 subunit tyrosine kinase to phos-
phorylate itself (autophosphorylation of the
ft
subunit).
Autophosphorylation is followed by phosphorylation of
one or more key intracellular proteins, one of which is in-
sulin receptor substrate-1 (IRS-1) that initiates a cascade
of intracellular phosphorylations mediated by a number of
intracellular enzymes that are incompletely characterized,
but which are critical in bringing about all of the known
cellular effects of insulin (Chapter 22).
Autophosphorylation
of the
insulin
receptor may
also be required for inactivation of the insulin signal
(“OFF” signal). Within minutes after autophosphoryla-
tion, the insulin-activated receptor complex undergoes
lateral movement toward clathrin-coated pits, the site at
which a number of activated insulin receptors congregate
or “cluster.” This clustering stimulates endocytosis of the
congregation, a process referred to as “internalization” or
“endocytosis,” which is followed by fusion of the internal-
ized insulin receptor membrane vesicle (endosome) with
a lysosome, i.e., an endolysosome formation. This allows
breakdown of both insulin and its receptor by lysosomal
proteases. It has been suggested that insulin fragments re-
sulting from this digestion process are mediators of some
of the actions of insulin within the cell, but further evi-
dence in support of this is needed.
In states of chronically elevated blood levels of insulin,
there is a substantive decrease in the density of insulin re-
ceptors in insulin-dependent cells due to a decrease in the
synthesis of insulin receptors. This phenomenon, which
is referred to as “downregulation,” represents a means by
which a cell autoregulates its receptivity to the hormone,
and is also detected by other hormones such as the cate-
cholamines, endorphins, and GnRH. The precise mecha-
nisms involved in downregulation are not known.
The insulin receptor is expressed in most cells and
tissues examined to date, including the non-insulin-
dependent tissues such as brain, kidney, and blood cells
(both red and white). Although no specific cellular re-
sponse to insulin has been described in these tissues, it is
conceivable that insulin regulates one or more intracellular
