3-Phosphoinositide-dependent protein kinase-1 (
PDK1) is a key regulator of
cell proliferation and survival
signal transduction.
PDK1 is known to be constitutively active and is further activated by Src-mediated
phosphorylation at the tyrosine-9, -373, and -376 residues. To identify novel regulators of
PDK1, we performed E. coli-based
two-hybrid screening and revealed that
tumor suppressor candidate 4 (
TUSC4), also known as
nitrogen permease regulator-like 2 (NPRL2), formed a complex with
PDK1 and suppressed Src-dependent
tyrosine phosphorylation and
activation of
PDK1 in vitro and in cells. The NH(2)-terminal 133
amino acid residues of
TUSC4 were involved in binding to
PDK1. The deletion
mutant of
TUSC4 that lacked the NH(2)-terminal domain showed no
inhibitory effects on
PDK1 tyrosine phosphorylation or
activation. Thus, complex formation is indispensable for TUSC4-mediated
PDK1 inactivation. The siRNA-mediated down-regulation of
TUSC4 induced
cell proliferation, while
ectopic TUSC4 expression inactivated the
PDK1 downstream
signaling pathway, including
Akt and p70
ribosomal protein S6 kinase, and increased cancer cell sensitivity to several
anticancer drugs. Our results suggest that
TUSC4/NPRL2, a novel PDK1-interacting protein, plays a role in regulating the Src/
PDK1 signaling pathway and cell sensitivity to multiple cancer
chemotherapeutic drugs.
