Targeting the oncogenic protein kinase Cι signalling pathway for the treatment of cancer

PKC (protein kinase C) isoenzymes are key signalling components involved in the regulation of normal cell proliferation, differentiation, polarity and survival. The aberrant regulation of PKC isoenzymes has been implicated in the development of many human diseases including cancer [Fields and Gustafson (2003) Methods Mol. Biol. 233, 519–537]. To date, however, only one PKC isoenzyme, the aPKC [atypical PKCι (protein kinase Cι)], has been identified as a human oncogene [Regala, Weems, Jamieson, Khoor, Edell, Lohse and Fields (2005) Cancer Res. 65, 8905–8911]. PKCι has also proven to be a useful prognostic marker and legitimate target for the development of novel pharmacological agents for the treatment of cancer. The PKCι gene resides at chromosome 3q26 and is a frequent target of tumour-specific gene amplification in multiple forms of human cancer. PKCι gene amplification in turn drives PKCι overexpression in these cancers. Genetic disruption of PKCι expression blocks multiple aspects of the transformed phenotype of human cancer cells including transformed growth in soft agar, invasion through Matrigel and growth of subcutaneous tumours in nude mice. Genetic dissection of oncogenic PKCι signalling mechanisms demonstrates that PKCι drives transformed growth by activating a PKCι → Rac1 → PAK (p21-activated kinase) → MEK [MAPK (mitogen-activated protein kinase) 1,2/ERK (extracellular-signal-regulated kinase) kinase] 1,2 signalling pathway [Regala, Weems, Jamieson, Copland, Thompson and Fields (2005) J. Biol. Chem. 280, 31109–31115]. The transforming activity of PKCι requires the N-terminal PB1 (Phox-Bem1) domain of PKCι, which serves to couple PKCι with downstream effector molecules. Hence, there exists a strong rationale for developing novel cancer therapeutics that target the PB1 domain of PKCι and thereby disrupt its interactions with effector molecules. Using a novel high-throughput drug screen, we identified compounds that can disrupt PB1–PB1 domain interactions between PKCι and the adaptor molecule Par6 [Stallings-Mann, Jamieson, Regala, Weems, Murray and Fields (2006) Cancer Res. 66, 1767–1774]. Our screen identified the gold compounds ATG (aurothioglucose) and ATM (aurothiomalate) as specific inhibitors of the PB1–PB1 domain interaction between PKCι and Par6 that exhibit anti-tumour activity against NSCLC (non-small-cell lung cancer) both in vitro and in vivo. Structural analysis, site-directed mutagenesis and modelling indicate that ATM specifically targets the PB1 domain of PKCι to mediate its anti-tumour activity [Erdogan, Lamark, Stallings-Mann, Lee, Pellechia, Thompson, Johansen and Fields (2006) J. Biol. Chem. 281, 28450–28459]. Taken together, our recent work demonstrates that PKCι signalling is required for transformed growth of human tumours and is an attractive target for development of mechanism-based cancer therapies. ATM is currently in Phase I clinical trials for the treatment of NSCLC.