PTEN ablation in RasHa/Fos skin carcinogenesis invokes p 53-dependent p21 to delay conversion while p 53-independent p21 limits progression via cyclin D1/E2 inhibition

Abstract

To investigate tumour progression mechanism in transgenic mouse skin carcinogenesis, inducible PTEN ablation (Δ5PTENflx) was targeted to the epidermis of mice expressing activated rasHa/fos oncogenes (HK1.ras and HK1.fos). RU486-treated HK1.ras/fos-Δ5PTEN flx epidermis exhibited significant keratinocyte proliferation resulting in hyperplasia and proliferating cysts. While HK1.ras/fos- Δ5PTENflx papillomatogenesis was accelerated, malignant conversion was delayed and tumours exhibited well-differentiated squamous cell carcinoma (wdSCC) histotypes, suggesting inhibition of early-stage malignant progression. Immediate elevated p53/p21 expression was observed in HK1.ras/fos-Δ5PTENflx hyperplasia, cysts and papillomas, and while malignant conversion required p53 loss, elevated p21 expression persisted in most wdSCCs to limit further progression, unless p 21 was also lost and wdSCC progressed to more aggressive carcinomas. In contrast, TPA-promoted (that is, c-fos-activated) bi-genic HK1.ras-Δ5PTENflx cohorts lost p53/p21 expression during early papillomatogenesis and rapidly produced poorly differentiated carcinomas (pdSCCs) with high BrdU-labelling and elevated cyclin D1/E2 expression levels, indicative of a progression mechanism driven by failures in cell-cycle control. Intriguingly, HK1.ras/fos-Δ5PTEN flx wdSCCs did not exhibit similar failures, as western and immunofluorescence analysis found downregulated cyclin E2 whenever p21 persisted; further, while westerns detected elevated cyclin D1, immunofluorescence identified reduced expression in proliferative basal layer nuclei and a redistributed expression profile throughout p21-positive wdSCC keratinocytes. These data demonstrate that rapid early epidermal responses to rasHa/fos/ΔPTEN co-operation involve induction of p 53/p21 to alter differentiation and divert excessive proliferation into cyst formation. Further, despite three potent oncogenic insults p53 loss was required for malignant conversion, and following p53 loss persistent, p53-independent p21 expression possessed the potency to limit early-stage malignant progression via cyclin D1/E2 inhibition. © 2014 Macmillan Publishers Limited.