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 α (RXRα) 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 (RXRα ^ep-/- | Tyr-NRAS ^Q61K | CDK4 ^R24C/R24C ) harboring an epidermal knockout of Retinoid X Receptor α (RXRα ^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 α was evaluated by characterizing the molecular and cellular changes that took place in the untreated and UVB treated trigenic RXRα ^ep-/- mice compared to the control mice with functional RXRα.
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 RXRα. Melanomas from the trigenic RXRα ^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 RXRα ^ep-/- mice. Interestingly, we observed a significant increase in p21 and Cyclin D1 in the TAN skin of un-irradiated trigenic RXRα ^ep-/- mice, suggesting that those changes might be consequences of loss of functional RXRα in the melanoma microenvironment. Loss of RXRα 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 RXRα.
Conclusions: Our study demonstrates the protective role of keratinocytic RxRα 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 .