JAK1/2 inhibition promotes aggressive cutaneous carcinogenesis through COT kinase–mediated alternative MAPK pathway activation

Secondary malignancies are the most feared adverse effects of cancer therapies. In this study, we explored the cutaneous oncogenic effects of the JAK inhibitor ruxolitinib. First, we demonstrated an epidemiological association between ruxolitinib exposure and development of aggressive cutaneous squamous cell carcinoma (cSCC) by analyzing international pharmacovigilance data and conducting a large cohort study. We next demonstrated the causal link between ruxolitinib treatment and cSCC development using in vitro and in vivo models. Mechanistically, we revealed that ruxolitinib induces transformation of premalignant keratinocytes by activating the MAPK pathway. This effect is mostly driven by JAK2 inhibition, suggesting a pharmacological class effect. To unravel the mechanistic underpinnings of ruxolitinib-induced MAPK activation, we then applied a phosphoproteomic approach. Alternative MAPK signaling downstream of the kinase COT was pinpointed as the main activator of the pathway in this context. The central role of COT was functionally validated using chemical inhibition, genetic impairment, and inactivated COT mutant expression. Indeed, impairment of COT rescued the pro-oncogenic effect of ruxolitinib. Finally, we determined using a combination of epidemiological, in vitro, and in vivo studies that ruxolitinib-mediated oncogenic effects also result in an increased risk of melanoma, suggesting that the uncovered cellular signaling mechanism might be extrapolated to other cancers with a high dependency on the MAPK pathway.