RhoB promotes cancer initiation by protecting keratinocytes from UVB-induced apoptosis but limits tumor aggressiveness

The role of UVB-induced apoptosis in the formation of squamous cell carcinoma (SCC) is recognized. We previously identified the small RhoB (Ras homolog gene family, member B) GTPase, an early response gene to cellular stress, as a critical protein controlling apoptosis of human keratinocytes after UVB exposure. Here we generated SKH1 (hairless immunocompetent mouse) mice invalidated for RhoB to evaluate its role in UVB-induced skin carcinogenesis in vivo. We show that rhob-/- mice have a lower risk of developing UVB-induced keratotic tumors and actinic keratosis that is associated with a higher sensitivity of UVB-exposed keratinocytes to apoptosis. We extend this observation to primary cultures of normal human keratinocytes in which RhoB was downregulated with small interfering RNA (siRNA) and further show that the hypersensitivity to apoptosis depends on B-cell lymphoma 2 (Bcl-2) downregulation. In rhob-/- mice, the UVB-induced tumors were preferentially undifferentiated and highly proliferative. Finally, we show in humans an almost constant loss of RhoB expression in undifferentiated SCCs. These undifferentiated and RhoB-deficient tumors have elevated phosphorylated histone H2AX (γH2AX) and 53BP1, two markers of DNA double-strand breaks. Together, our results indicate that UVB-induced RhoB expression participates in in vivo SCC initiation by increasing keratinocyte survival. Conversely, RhoB may limit tumor aggressiveness as loss of RhoB expression in tumor cells is associated with tumor progression.