Capacity for Compensatory Cyclin D2 Response Confers Trametinib Resistance in Canine Mucosal Melanoma

Background/objective: Mucosal melanoma (MM) is a poorly responsive, rare and aggressive subtype with few cases having targetable recurrent driver mutations, although Ras/MAPK and PI3K/AKT/mTOR signaling pathway activations are common. Eventual tumor evasion of targeted therapy continues to limit treatment success. Adequate models are necessary to address therapeutic resistance. The relatively greater incidence of naturally occurring MM in dogs, as well as its comparable clinical and pathological characteristics to human MM, represents an opportunity for study as a human MM patient surrogate. Resistance-promoting crosstalk between Ras/MAPK and PI3K/AKT/mTOR signaling under trametinib inhibition of MEK was studied in canine MM. Emphasis was placed on the suppressive effect of trametinib on cell cycle entry and its potential role in drug resistance. Methods: D-type cyclins were investigated following trametinib treatment of five MM cell lines exhibiting differential drug sensitivities. Signaling pathway activation, proliferation, survival, cell death, and cell cycle were analyzed in the context of D-type cyclin expression. Cyclin D2 expression was manipulated using siRNA knockdown or inducible recombinant overexpression. Results: Trametinib diminished cyclin D1 in all cell lines. While relatively trametinib-resistant MM cells exhibited capacity to upregulate cyclin D2, which promoted proliferation, sensitive MM cells lacked similar cyclin D2 compensation. Inhibition of the compensatory cyclin D2 in resistant cells conferred sensitivity. Induced cyclin D2 overexpression in otherwise trametinib-sensitive MM cells promoted survival. Upregulated PI3K/AKT/mTOR signaling under trametinib treatment was suppressed by mTORC1/2 inhibition, which similarly diminished cyclin D2 response. Conclusions: The compensatory switch from preferential reliance on cyclin D1 to D2 plays a role in MM resistance to MEK inhibition.