Protein kinase Cα suppresses Kras-mediated lung tumor formation through activation of a p38 MAPK-TGFβ signaling axis

Protein kinase C alpha (PKC alpha) can activate both pro- and anti-tumorigenic signaling depending upon cellular context. Here, we investigated the role of PKC alpha in lung tumorigenesis in vivo. Gene expression data sets revealed that primary human non-small lung cancers (NSCLC) express significantly decreased PKC alpha levels, indicating that loss of PKC alpha expression is a recurrent event in NSCLC. We evaluated the functional relevance of PKC alpha loss during lung tumorigenesis in three murine lung adenocarcinoma models (LSL-Kras, LA2-Kras and urethane exposure). Genetic deletion of PKC alpha resulted in a significant increase in lung tumor number, size, burden and grade, bypass of oncogene-induced senescence, progression from adenoma to carcinoma and a significant decrease in survival in vivo. The tumor promoting effect of PKC alpha loss was reflected in enhanced Kras-mediated expansion of bronchio-alveolar stem cells (BASCs), putative tumor-initiating cells, both in vitro and in vivo. LSL-Kras/Prkca(-/-) mice exhibited a decrease in phospho-p38 MAPK in BASCs in vitro and in tumors in vivo, and treatment of LSL-Kras BASCs with a p38 inhibitor resulted in increased colony size indistinguishable from that observed in LSL-Kras/Prkca(-/-) BASCs. In addition, LSL-Kras/Prkca(-/-) BASCs exhibited a modest but reproducible increase in TGF beta 1 mRNA, and addition of exogenous TGF beta 1 to LSL-Kras BASCs results in enhanced growth similar to untreated BASCs from LSL-Kras/Prkca(-/-) mice. Conversely, a TGF beta R1 inhibitor reversed the effects of PKC alpha loss in LSL-Kras/Prkca(-/-) BASCs. Finally, we identified the inhibitors of DNA binding (Id) Id1-3 and the Wilm's Tumor 1 as potential downstream targets of PKC alpha-dependent tumor suppressor activity in vitro and in vivo. We conclude that PKC alpha suppresses tumor initiation and progression, at least in part, through a PKC alpha-p38MAPK-TGF beta signaling axis that regulates tumor cell proliferation and Kras-induced senescence. Our results provide the first direct evidence that PKC alpha exhibits tumor suppressor activity in the lung in vivo.