Abstract 3466: Growth inhibitory MEK-ERK signaling in the intestinal epithelium
Authors: Kaur Navneet , Lewis Robert , Black Adrian , Black Jennifer 
Published: August 2018

Abstract
Increasing evidence points to members of the PKC family of serine/threonine kinases as important tumor suppressors; however, the signaling pathways involved remain poorly defined. Our previous studies have determined that PKCα, which acts as a tumor suppressor in the epithelium of the intestine/colon, is activated coincident with growth arrest in intestinal crypts in vivo. PKCα activity induces cell cycle withdrawal in intestinal epithelial cells, a process that involves downregulation of cyclin D1 and Id1, upregulation of the cyclin dependent kinase inhibitors p21cip1 and p27kip1, and characteristic changes in pocket proteins and DNA licensing factors. Pharmacological inhibitors of the ERK pathway revealed that these changes are dependent on activation of MEK-ERK signaling, thus identifying a growth inhibitory ERK-mediated signaling axis in the intestinal epithelium. Given the interest in inhibitors of the ERK pathway for treatment of colon and other cancers, characterization of this tumor suppressive pathway will be essential for rational development of this treatment modality and to avoid and/or manage potential adverse effects of ERK inhibition in patients. To this end, the mechanistic basis for the growth suppressive effects of the PKCα→ERK module on cyclin D1, Id1 and p21cip1 are being investigated in non-transformed intestinal epithelial cells and colon cancer cells. Use of selective inhibitors and siRNA-mediated knockdown has determined that PKCα activates multiple components of the ERK pathway, including RAS, RAF, MEK and ERK. Notably, tumor suppressive effects are seen in colon cancer cells that harbor activating mutations in KRAS, indicating that growth inhibitory ERK signaling can override oncogenic signals arising from RAS mutation. PKCα signaling is associated with activation of ARAF, BRAF and CRAF and with induction of RAF heterodimers. A requirement for RAF activity was confirmed by the ability of LY3009120 or sorafenib to block growth inhibitory effects of PKCα. However, knockdown of individual RAF proteins fails to block these effects of PKCα, indicating some redundancy between the RAF proteins in mediating growth inhibitory signals. While the scaffolding protein KSR1 has been shown to be required for oncogenic RAS-ERK signaling, PKCα was still able to elicit growth inhibitory effects in KSR1 knockdown HCT-116 colon cancer cells. These data indicate that differential use of scaffolding proteins may explain both the growth inhibitory effects of PKCα-ERK signaling and their ability to override oncogenic RAS signals. A better understanding of these mechanisms may serve to guide the use of ERK inhibitors for colon cancer therapy. Supported by NIH grants DK60632, CA54807, CA157774 and CA191894.
Citation Format: Navneet Kaur, Robert Lewis, Adrian Black, Jennifer Black. Growth inhibitory MEK-ERK signaling in the intestinal epithelium [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 3466.