Sfragano, Y. | Search bioRxiv

bioRxiv2026

ERK5 inhibition triggers CDK6 proteasomal degradation and enhances palbociclib efficacy in cancer cells

Advanced endometrial cancer (EC) and melanoma are two malignancies with poor treatment options at advanced stages. CDK4 and CDK6 kinases play a central role in the regulation of cell proliferation by controlling progression through G1/S transition. CDK4/CDK6 inhibitors are currently in clinical trials for advanced ECs, whereas melanoma tumors frequently carry mutations affecting the CDK4/CDK6 pathways that support the therapeutic potential of these kinases. The MAP kinase ERK5 promotes tumor progression by driving cell-cycle progression, yet its functional interplay with CDK4/6-dependent cell cycle control remains poorly defined. Here, we investigated the relationship between ERK5 and CDK4/6 in melanoma and serous EC cells, uncovering the benefits of their co-targeting. Both genetic and pharmacological inhibition of ERK5 induced CDK6 proteasomal degradation, without affecting CDK4 protein levels. Co-treatment with the ERK5 inhibitor JWG-071 and palbociclib synergistically reduced cell viability and increased apoptosis in both melanoma and EC cells, compared with single-agent treatments. Mechanistically, combined inhibition of ERK5 and CDK4/6 reinforced cell-cycle inhibitory signaling through p21 induction and reduced phospho-retinoblastoma levels. These findings suggest that targeting ERK5 may improve the anticancer efficacy of palbociclib, at least in melanoma and serous EC tumors.

cancer biology↗

bioRxiv2026

R-Ras coordinates reciprocal activation of ERK5 and ERK1/2 under single pathway inhibition in melanoma

Malignant melanoma is frequently driven by constitutive activation of the RAS-RAF-MEK1/2-ERK1/2 pathway, yet adaptive signaling limits the long-term efficacy of MAPK-targeted therapies. Although activation of the MEK5-ERK5 pathway has emerged as a mechanism of resistance to RAF-MEK1/2-ERK1/2 inhibition, whether ERK5 inhibition reciprocally activates the canonical MAPK cascade and the molecular basis of this crosstalk remain unknown. Here, we show that genetic and pharmacological inhibition of ERK5 induces further activation of the MEK1/2-ERK1/2 pathway in BRAFV600E melanoma cells. Based on our previous transcriptomic analyses, we investigated the role of the small GTPase R-Ras, identified among the genes upregulated following ERK5 silencing. Accordingly, R-Ras mRNA and protein levels increased upon both genetic and pharmacological ERK5 inhibition, whereas R-Ras silencing abolished ERK1/2 hyperactivation and potentiated the anti-proliferative and pro-apoptotic effects of ERK5 targeting. Conversely, inhibition of the RAF-MEK1/2-ERK1/2 pathway increased R-Ras expression and ERK5 activation, both of which were prevented by R-Ras depletion. Besides ERK1/2, overexpression of a constitutively active mutant of R-Ras promoted ERK5 activation, placing R-Ras upstream of both signaling cascades. Finally, the pan-Ras inhibitor RMC-6236 potentiated the antitumor activity of either ERK5- or RAF-MEK1/2-ERK1/2-targeted therapies in either two-dimensional cultures or melanoma spheroids. Collectively, these findings identify R-Ras as a central regulator of reciprocal rewiring between ERK1/2 and ERK5 pathways under targeted MAPK inhibition. Functional disruption of this signaling circuit enhances melanoma cell death, providing a mechanistic rationale for co-targeting R-Ras together with MAPK signaling to limit adaptive responses to targeted therapy in BRAFV600E melanoma.

cancer biology↗