April Journal-2009.p65 - Association of Biotechnology and Pharmacy

Current Trends in Biotechnology and Pharmacy
Vol. 3 (2) 128-137, April 2009. ISSN 0973-8916
involves ubiquitination-dependent and -
independent proteasomal targeting. This occurs
through binding of its COOH terminus with the
C8 subunit of the 20S core of the proteasome.
Mitogenic signaling induced phosphorylation of
p21CIP1 is precedent in this process. CDK2,
AKT, p38MAPK or SAPK, JNK, GSK-3ß, PKA,
and PKC have been shown to phosphorylate
p21CIP1. PKC can phosphorylate Ser146 in the
COOH terminus of p21CIP1 and facilitate
degradation. Phosphorylation of p21CIP1 affects
interaction with its binding partners, regulating the
stability of the protein and its subcellular
localization. AKT phosphorylates p21CIP1 at
Thr145 and Ser146 within the binding site of
PCNA which is known to promote the
ubiquitination and degradation of p21CIP1.
Phosphorylation at Ser146 by AKT significantly
increases the p21CIP1 protein level and Thr145
phosphorylation by AKT prevents PCNA binding
and promotes the nuclear export of p21. A link
between intracellular localization and proteolysis
is better identified for the p27KIP1. Antimitogenic
signaling induced phosphorylation of p21CIP1, as
predicted, inhibits p21CIP1 degradation.
p38MAPK or SAPK and JNK1 activated by
TGFß -1 phosphorylates p21CIP1 at Ser130 and
increases p21CIP1 stability.
Activity of the target protein CDK2 can
affect p21CIP1 stability. While it is intriguing to
understand how CDK inhibitors can put the CDK
activity to use for their degradation, a novel idea
has been recently reported with an introduction
of a model of high affinity binding motif, cy1, and
low affinity binding motif, cy2, for CDK on
p21CIP1 (35, 36). In the model, CDK2 may
promote p21CIP1 degradation in a sequential
pathway. CDK2-dependent phosphorylation of
p21CIP1 at Ser130 would be recognized by a
SKP2-containing SCF complex, and ubiquitinated
and degraded by the proteasome. Direct
phosphorylation on the Thr145 of p21CIP1 by
PIM-1 stabilizes it and results in a shift in the
134
subcellular localization of p21CIP1 in H1299 cells
(29). The finding that p21CIP1 phosphorylation
at Ser114 by GSK-3ß is critical for p21CIP1
protein degradation by UV shows an example of
proteosomal degradation of p21CIP1 protein
without ubiquitination.
While clinical importance of a
cytoplasmic mislocalization of p21CIP1 in tumors
is not understood as well as that of p27KIP1,
p21CIP1 protein might be more involved in
apoptosis compared to p27KIP1 protein. For
p27KIP1, cytoplasmic localization is closely linked
to poor prognosis in human breast, colon, ovarian,
thyroid and esophageal cancers and low protein
levels have been identified and associated with
tumor progression and poor prognosis in many
different cancers including colon, breast, prostate,
ovarian, and brain cancer. At G 0
, p27KIP1
stabilization is a result of Ser10 and Thr198. Ser10
phosphorylation in quiescent cells is attributed to
MIRK/DYRK1B. Ser10 phosphorylation
interferes with the binding of p27KIP1 to cyclin/
CDKcomplexes which may reduce p27KIP1
stability. At mitogenic signal, KIS is responsible
for Ser10 phosphorylation and becomes an
important regulator of p27KIP1 (37).
Phosphorylation at Ser10 triggers the export of
p27KIP1 from the nucleus into the cytoplasm by
a CRM1-mediated export pathway. Therefore,
Ser10 phosphorylation results in nuclear export
on the mitogenic cues which might expose the
protein to the cytoplasmic proteosome and
therefore indirectly decrease the stability of
p27KIP1 (38). Thr157 located within the nuclear
localization sequence of p27KIP1 can be
phosphorylated by AKT. It is found that
phosphorylation at Thr157 and Thr198 cooperates
to enhance cytoplasmic localization of p27KIP1.
In addition to the threonine/serine phosphorylation
sites, p27KIP1 possesses three tyrosine residues,
Tyr74, Tyr78, and Tyr79, all of which are
phosphorylated by SRC family kinases. These
Regulation of CDK inhibitors