Ablation of epidermal RXRα in cooperation with activated CDK4 and oncogenic NRAS generates spontaneous and acute neonatal UVB induced malignant metastatic melanomas

Background: Understanding the underlying molecular mechanisms involved in the formation of cutaneous malignant melanoma is critical for improved diagnosis and treatment. Keratinocytic nuclear receptor Retinoid X Receptor alpha (RXR alpha) has a protective role against melanomagenesis and is involved in the regulation of keratinocyte and melanocyte homeostasis subsequent acute ultraviolet (UV) irradiation. Methods: We generated a trigenic mouse model system (RXR alpha(ep-/-)| Tyr-NRAS(Q61K) | CDK4(R24C/R24C)) harboring an epidermal knockout of Retinoid X Receptor alpha (RXR alpha(ep-/)-), combined with oncogenic NRAS(Q61K) (constitutively active RAS) and activated CDK4(R24C/R24C) (constitutively active CDK4). Those mice were subjected to a single neonatal dose of UVB treatment and the role of RXR alpha was evaluated by characterizing the molecular and cellular changes that took place in the untreated and UVB treated trigenic RXR alpha(ep-/-) mice compared to the control mice with functional RXR alpha. Results: Here we report that the trigenic mice develops spontaneous melanoma and exposure to a single neonatal UVB treatment reduces the tumor latency in those mice compared to control mice with functional RXR alpha. Melanomas from the trigenic RXR alpha(ep-/-) mice are substantial in size, show increased proliferation, exhibit increased expression of malignant melanoma markers and exhibit enhanced vascularization. Altered expression of several biomarkers including increased expression of activated AKT, p21 and cyclin D1 and reduced expression of pro-apoptotic marker BAX was observed in the tumor adjacent normal (TAN) skin of acute ultraviolet B treated trigenic RXR alpha(ep-/-) mice. Interestingly, we observed a significant increase in p21 and Cyclin D1 in the TAN skin of un-irradiated trigenic RXR alpha(ep-/-) mice, suggesting that those changes might be consequences of loss of functional RXR alpha in the melanoma microenvironment. Loss of RXR alpha in the epidermal keratinocytes in combination with oncogenic NRAS(Q61K) and CDK4(R24C/R24C) mutations in trigenic mice led to significant melanoma invasion into the draining lymph nodes as compared to controls with functional RXR alpha. Conclusions: Our study demonstrates the protective role of keratinocytic RxR alpha in (1) suppressing spontaneous and acute UVB-induced melanoma, and (2) preventing progression of the melanoma to malignancy in the presence of driver mutations like activated CDK4(R24C/R24C) and oncogenic NRAS(Q61K) .