Abstract 3889: Cyclin-dependent kinase 4 levels affect the number of hair follicle stem cells in mouse epidermis

Abstract

Abstract The pRb/Cdk/cyclin/p16 pathway has been found altered in a majority of human neoplasias. In particular the Cyclin-dependent kinase 4 (CDK4) has been found mutated in familial melanoma, amplified or overexpressed in human gliomas, sporadic breast carcinomas and sarcomas. Additionally, a vast number of human cancers contain high levels of CDK4 activity due to loss of the Cdk inhibitor p16Ink4a or overexpression of its binding partner cyclin D1. We have previously demonstrated that CDK4 ablation inhibits chemically-induced mouse skin papillomas, whereas forced expression of CDK4 in mouse epidermis (K5-CDK4) accelerates malignant progression to Squamous Cell Carcinomas (SCC). However, the mechanisms by which changes in CDK4 expression levels control skin tumorigenesis have not been established. In this model topical application of DMBA and TPA results in clonal expansion of a slow cycling stem cell population localized at the bulge area of the hair follicle. Thus, we hypothesize that CDK4 deletion or overexpression affects tumorigenesis by altering the characteristic and/or the number of Keratinocytes Stem Cells (KSC). To address this, we employed the K15-EGFP transgenic mouse model, which expresses EGFP under the control of the keratin 15 promoter in the bulge area of the hair follicle, to generated K15-EGFP/K5-CDK4as well as K15-EGFP/CDK4−/− compound mice. Flow cytometry analysis of K15-EGFP/K5-CDK4 keratinocytes show 4-fold decrease in the number of KSC compared to K15-EGFP control mice. In contrast, K15-EGFP/CDK4−/− compound mice show 4-fold increase in the number of KSC compared with control siblings. Quantification of mRNA in the KSCs showed 3 to 5-fold increase in the CDK4 levels of KSC compared to non-Stem cell population, whereas mild or no changes in CDK2, Cyclin D1 and p27Kip1mRNA levels were detected. Consistent with these data, we also determined an increase number of Labeled-Retaining Cells (LRC) in the hair follicle of CDK4−/− and decreased LRC in K5-CDK4 mice. Together these results are consistent with our hypothesis that changes in CDK4 expression affects the asymmetric cell division of KSC population favoring an increase in the transit amplifying (TA) cell pool and decrease of the KSC pool in K5-CDK4 and the opposite in CDK4−/− mice. As a result, this model predicts that the number of transit amplifying or progenitor cells correlate with the susceptibility to papilloma development. Supported by NIH grant CA116328 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3889.