CSK-homologous kinase (CHK/MATK) is a potential colorectal cancer tumour suppressor gene epigenetically silenced by promoter methylation.

Hyperactivation of SRC-family protein kinases (SFKs) contributes to the initiation and progression of human colorectal cancer (CRC). Since oncogenic mutations of SFK genes are rare in human CRC, we investigated if SFK hyperactivation is linked to dysregulation of their upstream inhibitors, C-terminal SRC kinase (CSK) and its homolog CSK-homologous kinase (CHK/MATK). We demonstrate that expression of CHK/MATK but not CSK was significantly downregulated in CRC cell lines and primary tumours compared to normal colonic tissue. Investigation of the mechanism by which CHK/MATK expression is down-regulated in CRC cells uncovered hypermethylation of the CHK/MATK promoter in CRC cell lines and primary tumours. Promoter methylation of CHK/MATK was also observed in several other tumour types. Consistent with epigenetic silencing of CHK/MATK, genetic deletion or pharmacological inhibition of DNA methyltransferases increased CHK/MATK mRNA expression in CHK/MATK-methylated colon cancer cell lines. SFKs were hyperactivated in CHK/MATK-methylated CRC cells despite expressing enzymatically active CSK, suggesting loss of CHK/MATK contributes to SFK hyperactivation. Re-expression of CHK/MATK in CRC cell lines led to reduction in SFK activity via a non-catalytic mechanism, a reduction in anchorage-independent growth, cell proliferation and migration in vitro, and a reduction in tumour growth and metastasis in a zebrafish embryo xenotransplantation model in vivo, collectively identifying CHK/MATK as a novel putative tumour suppressor gene in CRC. Furthermore, our discovery that CHK/MATK hypermethylation occurs in the majority of tumours warrants its further investigation as a diagnostic marker of CRC.

Characterization of a novel JNK (c-Jun N-terminal kinase) inhibitory peptide.

An improved understanding of the roles of protein kinases in intracellular signalling and disease progression has driven significant advances in protein kinase inhibitor discovery. Peptide inhibitors that target the kinase protein substrate-binding site have continued to attract attention. In the present paper, we describe a novel JNK (c-Jun N-terminal kinase) inhibitory peptide PYC71N, which inhibits JNK activity in vitro towards a range of recombinant protein substrates including the transcription factors c-Jun, ATF2 (activating trancription factor 2) and Elk1, and the microtubule regulatory protein DCX (doublecortin). Analysis of cell culture studies confirmed the actions of a cell-permeable version of PYC71 to inhibit c-Jun phosphorylation during acute hyperosmotic stress. The analysis of the in vitro data for the kinetics of this inhibition indicated a substrate-inhibitor complex-mediated inhibition of JNK by PYC71N. Alanine-scanning replacement studies revealed the importance of two residues (PYC71N Phe9 or Phe11 within an FXF motif) for JNK inhibition. The importance of these residues was confirmed through interaction studies showing that each change decreased interaction of the peptide with c-Jun. Furthermore, PYC71N interacted with both non-phosphorylated (inactive) JNK1 and the substrate c-Jun, but did not recognize active JNK1. In contrast, a previously characterized JNK-inhibitory peptide TIJIP [truncated inhibitory region of JIP (JNK-interacting protein)], showed stronger interaction with active JNK1. Competition binding analysis confirmed that PYC71N inhibited the interaction of c-Jun with JNK1. Taken together, the results of the present study define novel properties of the PYC71N peptide as well as differences from the characterized TIJIP, and highlight the value of these peptides to probe the biochemistry of JNK-mediated substrate interactions and phosphorylation.