Inhibitor of DNA binding (Id) proteins (Id1-Id4) are dominant negative antagonists of basic helix-loop-helix (bHLH) transcription factors with recognized functions in development, stem cell self-renewal, and cell fate determination. Recent studies have identified Id proteins as master regulators of cancer-initiating cells and tumor aggressiveness, with critical roles in regulation of central hallmarks of cancer, including cell proliferation, survival, senescence, angiogenesis, migration, metastasis, and chemoresistance. In colorectal cancer (CRC), Id1 overexpression is associated with aggressive clinical behavior and poor prognosis, while decreasing sensitivity to chemotherapeutic agents. Protein kinase C α (PKCα) is a serine/threonine kinase that functions as a tumor suppressor in CRC. PKCα signaling induces cell cycle arrest in nontransformed IEC-18 intestinal epithelial cells via an ERK-dependent mechanism. A major target of PKCα negative control in these cells is Id1. To further characterize mechanisms underlying the tumor-suppressive effects of PKCα, we modulated PKCα expression and activation in CRC cells and nontransformed IEC-18 cells and performed RNA-seq analysis to identify changes in gene expression. This analysis identified transforming growth factor beta receptor 1 (TGFβR1) as an mRNA upregulated by PKCα signaling. Subsequent RT-PCR and Western blot analysis confirmed that PKCα upregulates TGFβR1 at both the mRNA and protein levels. The MEK inhibitors U0126 and PD98059 blocked PKCα upregulation of TGFβR1, pointing to an ERK-dependent mechanism. Notably, inhibition of TGFβR1 with GW788388 prevented PKCα-mediated Id1 downregulation, linking TGF-β signaling to the tumor-suppressive effects of PKCα. Further studies indicated that TGFβR1 activates a p38-mediated noncanonical pathway to downregulate Id1 in response to PKCα activity. Collectively, these findings identify a PKCα→Erk→TGFβR1→p38→Id1 signaling axis in intestinal epithelial cells. Both PKCα and TGF-β function as tumor suppressors in the intestinal epithelium and both pathways are inactivated during intestinal tumor progression. By providing a direct link between two tumor-suppressive signaling pathways with established effects on intestinal tumorigenesis, our findings broaden our understanding of CRC and open avenues for potential novel therapeutic approaches to this disease. (Supported by NIH grants DK60632, CA54807, CA036727 and CA191894.)

Citation Format: Xinyue Li, Michelle Lum, Adrian Black, Jennifer Black. Crosstalk between PKCalpha and TGF-beta signaling suppresses inhibitor of DNA binding 1 (Id1) expression in intestinal epithelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3463.