Identification of novel substrates for human checkpoint kinase Chk1 and Chk2 through genome-wide screening using a consensus Chk phosphorylation motif

Checkpoint kinase 1 (Chk1) and Chk2 are effector kinases in the cellular DNA damage response and impairment of their function is closely related to tumorigenesis. Previous studies revealed several substrate proteins of Chk1 and Chk2, but identification of additional targets is still important in order to understand their tumor suppressor functions. In this study, we screened novel substrates for Chk1 and Chk2 using substrate target motifs determined previously by an oriented peptide library approach. The potential candidates were selected by genome-wide peptide database searches and were examined by in vitro kinase assays. ST5, HDAC5, PGC-1alpha, PP2A PR130, FANCG, GATA3, cyclin G, Rad51D and MAD1alpha were newly identified as in vitro substrates for Chk1 and/or Chk2. Among these, HDAC5 and PGC-1alpha were further analyzed to substantiate the screening results. Immunoprecipitation kinase assay of full-length proteins and site-directed mutagenesis analysis of the target motifs demonstrated that HDAC5 and PGC-1alpha were specific targets for Chk1 and/or Chk2 at least in vitro.

Phosphorylation of glycogen synthase kinase-3beta at serine-9 by phospholipase Cgamma1 through protein kinase C in rat 3Y1 fibroblasts

Phospholipase Cgamma1 (PLCgamma1) plays an important role in controlling cellular proliferation and differentiation. PLCgamma1 is overexpressed in some tumors, and its overexpression induces solid tumors in nude mice. However, the regulatory mechanisms underlying PLCgamma1-induced cell proliferation are not fully understood. Here we show that overexpression of PLCgamma1 highly phosphorylated glycogen synthase kinase-3beta (GSK-3beta) at serine-9 in 3Y1 fibroblasts. Inhibition of protein kinase C (PKC)s with GF109203X abrogated GSK-3beta phosphorylation by PLCgamma1. We also found that steady-state level of cyclin D1 protein, but not cyclin D1 mRNA, was highly elevated in response to serum stimulation in PLCgamma1-transfected cells as compared with vector-transfected cells. Since GSK-3beta is involved in cyclin D1 proteolysis in response to mitogenic stimulation, PLCgamma1-mediated GSK-3beta phosphorylation may function as a regulation of cyclin D1 accumulation in PLCgamma1-overexpressing cells.