Novel role of RASGRP1 in ras activation in keratinocytes : implications for skin carcinogenesis

Abstract

Skin carcinogenesis is a multistage process consisting of three distinct stages: initiation, promotion and malignant conversion. The multistep nature of cancer is characterized by progressive activation or loss of signaling pathways, which dictate the behavior of tumor cells, its interaction with neighboring cells, local microenvironment and organism. One of the experimental models that have been instrumental in investigating the stages of cancer formation is the mouse multistage skin carcinogenesis protocol. In this model, a carcinogen initiates cells by introducing a mutation in a Ras gene, and the initiated population expands under treatment with phorbol esters, typically 12-Otetradecanoylphorbol-13-acetate (TPA). Eventually initiated cells outgrow the normal epidermal cells to form benign papillomas. Although phorbol esters are the prototypes of tumor promoters in skin, the nature of their interaction with Ras signaling in tumor promotion remains to be determined. RasGRP1 is a member of the guanine nucleotide exchange factor family for Ras that binds with high affinity to ultrapotent diacylglycerol analogs like the phorbol esters. The ability to bind to phorbol esters and to modulate Ras activity, make RasGRP1 an attractive candidate for an additional target of the phorbol esters in skin that could directly link phorbol ester signaling with Ras cascades and the tumorigenic process. Recent work from our lab has demonstrated expression of RasGRP1 in epidermal keratinocytes. Also, further work using a transgenic murine model that overexpresses RasGRP1 in basal keratinocytes has suggested that RasGRP1 participates in the action of tumor promoting phorbol esters like TPA in the skin. However, the relative contribution of RasGRP1 to Ras signals in keratinocytes and its role in tumor promotion and/or progression in response to the phorbol ester TPA in the absence of overexpression remains to be defined. Thus, the overall goal of this study is to investigate if RasGRP1 is a critical link to Ras activation in mouse epidermal keratinocytes in response to tumor promoter TPA and also to ascertain the relative contribution of RasGRP1 in skin carcinogenesis.