RhoB Promotes Cancer Initiation by Protecting Keratinocytes from UVB-Induced Apoptosis but Limits Tumor Aggressiveness

Abstract

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.

RhoB Promotes γH2AX Dephosphorylation and DNA Double-Strand Break Repair

“…6). We recently reported that RhoB-deficient human skin tumors also have elevated ␥H2AX levels compared to RhoB-proficient tumors (63). Hence, our findings suggest that loss of RhoB could promote oncogenesis by increasing DSB-mediated genomic instability.…”

Actinic keratosis modelling in mice: A translational study

“…Therefore, correlation of experimental observations made in murine models and in human tissue is of prime importance for demonstrating relevance to human disease. Mouse models, most notably those employing the SKH-1 hairless immunocompetent mouse, proved to be useful and allowed the identification of critical molecular and biologic changes that trigger skin tumour development [[14], [15]]. They are also commonly used to assess the preclinical efficacy of novel treatments on SCC however, rather than on AK lesions [[10]; [9], [8], [16]].…”

The Rac GTPase in Cancer: From Old Concepts to New Paradigms

“…For example, early studies revealed that ablation of the RhoB gene in mice (a gene deleted in many human cancers) enhances carcinogen-induced skin cancer (15). Rhob −/− mice are less prone to form squamous cell carcinomas following chronic exposure to UVB, arguing that RhoB favors early stages of oncogenesis, however RhoB could also limit the progression to highly aggressive tumors (16). Deletion of the RhoC gene in the MMTV-PyVT mouse model of breast cancer significantly reduces lung metastasis of mammary tumors (17).…”