156 Conditional expression of oncogenic KrasG12Dand HrasG12V alleles in mouse keratinocytes reveals a dose dependent requirement for tumor formation

Abstract

The goal of the current study is to compare the tumor initiating properties of conditionally expressed endogenous KrasG12D and HrasG12V mutant alleles in primary mouse keratinocytes. Primary keratinocytes were isolated from the LSL-HrasG12Dand LSL -KrasG12D C57BL/6J mouse models that carry mutant alleles that are conditionally expressed when a Lox-STOP-Lox segment is excised by Cre recombinase adenovirus treatment. Keratinocyte cultures expressing two mutant alleles of HrasG12D (HRAS Homo) were compared with cultures expressing only one mutant allele of HrasG12D (HRAS Het) or one mutant allele of KrasG12D (KRAS Het) or one mutant allele of each (HKRAS Het). Endpoints evaluated were signal transduction pathways, gene expression, and the ability to form squamous papillomas in vivo when orthotopically transplanted on the skin of nude mice. Upon treatment with Cre, HRAS homo, HRAS Het and KRAS Het keratinocytes replicate the resistance to calcium-induced differentiation that we previously reported using v-rasHa retroviral transduction. HRAS Homo, HRAS Het, and KRAS Het keratinocytes demonstrate an upregulation of the simple epithelia keratin 8 and a downregulation of suprabasal keratins 1 and 10 through changes in both mRNA and protein expression. Furthermore, transcripts for ligands for the epidermal growth factor receptor (EGFR) are also elevated as is the phosphorylation of EGFR protein. In vivo, 100% of nude mice grafted with HRAS Homo and HKRAS Het keratinocytes developed squamous papillomas, whereas those with KRAS Het and HRAS Het keratinocytes did not form tumors. This result suggests that the HrasG12Dmutant allele may have a dose dependent influence or that wild-type Hras and Kras alleles may possess a tumor suppressing effect. We are currently exploring differences in the activation of RAS downstream signaling pathways and gene expression among the four genotypes to explain the differential ability to form tumors in vivo.