670
Biological Regulation by Protein Phosphorylation
dried bodies of these beetles have been
used in China for over 2000 years as a tra-
ditional medicine for several indications
including cancer and piles. Hippocrates
also described its use for the treatment of
dropsy. It has been exploited (incorrectly)
as an aphrodisiac named Spanish Fly be-
cause it is an irritant of the urogenital tract,
and causes pelvic engorgement in women.
More recently, dermatologists have used it
as a topical agent for the treatment of warts.
Because cantharidin has a relatively sim-
ple molecular structure, a long history of
apparent medicinal value and a newly dis-
covered molecular mechanism of action,
drug hunters have begun to synthesize
derivatives of cantharidin in search of new
medicines with greater speci±city for a
subset of serine/threonine phosphatases,
thereby reducing side effects that occur
with nonselective inhibitors.
In recent years, the development of ge-
netically modi±ed organisms has provided
important tools for research. These or-
ganisms have been particularly helpful in
evaluating whether phosphorylation by a
particular kinase affects a protein’s func-
tion
in vivo
. New molecular and genetic
techniques have been developed to cre-
ate animals expressing kinases that are
driven by cell-speci±c or developmentally
regulated promoters. Knockout mice have
been generated carrying targeted muta-
tions that prevent the expression of a
functional enzyme. Animals derived from
such work have greatly enhanced our un-
derstanding of protein phosphorylation
pathways involved in pathological condi-
tions such as diabetes, Alzheimer’s dis-
ease, and cancer.
Modulators of protein phosphorylation
are useful not only to dissect and under-
stand molecular mechanisms but, as noted
above for cantharidin, may also have ther-
apeutic value. For example, FK506 and
Cyclosporin A are drugs that produce
their pharmacological effects by inhibit-
ing a serine/threonine phosphatase in
T-lymphocytes and preventing the de-
phosphorylation of a key cellular protein.
They are used therapeutically as immuno-
suppressive drugs and are prescribed to
organ-transplant patients. FK506 and Cy-
closporin A bind to their respective in-
tracellular binding proteins, FKBP12 and
cyclophilin A. Each of these complexes can
bind to PP2B and block its activation. Nor-
mally, T-cell activation causes an increase
in intracellular calcium, which stimulates
PP2B. Active PP2B dephosphorylates the
transcription factor NF-AT thereby facili-
tating its translocation to the nucleus to
initiate gene transcription. FK506 and Cy-
closporin A block the dephosphorylation
of NF-AT, preventing transcription of spe-
ci±c genes in T-lymphocytes that induce
transplant rejection.
Current drugs for cancer therapy in-
clude cytotoxic agents that target basic
cellular processes such as DNA replication
or microtubule dynamics. Drug discovery
strategies aimed at inhibiting speci±c pro-
tein kinases have led to the discovery of
compounds that, in theory, should be ef-
fective and more tolerable anticancer ther-
apies. Several dozen compounds are now
in clinical trials for a wide range of cancers.
Two protein kinase inhibitors have demon-
strated remarkable clinical value and have
been approved by the FDA: Gleevec, a
Bcr-Abl kinase inhibitor for the treatment
of chronic myelogenous leukemia, and
Iressa, an epidermal growth factor recep-
tor kinase inhibitor for the treatment of
advanced non-small-cell lung cancer.
The future holds much promise for
our increased understanding of the struc-
ture and function of protein kinases and
phosphatases. A combination of gene mu-
tation studies, X-ray crystallography and
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