ERK5 is required for melanoma growth and is activated by oncogenic BRAF

Malignant melanoma is among the most aggressive cancers and its incidence is increasing worldwide. Although targeted therapies and immunotherapy have improved the survival of patients with metastatic melanoma in the last few years, available treatments are still unsatisfactory showing an urgent need to identify new therapeutic targets. While the role of BRAF-MEK1/2-ERK1/2 pathway in melanoma is well-established, the involvement of the MEK5-ERK5 signaling remains poorly explored. The Hedgehog signaling is an important pathway in melanoma, that has been shown to be required for growth, recurrence and metastasis of melanoma xenografts in mice. Several studies have shown that numerous oncogenic inputs positively modulate the activity of the HH pathway. In this study, we investigated the function of ERK5 signaling in melanoma, its regulation by oncogenic BRAF and its interplay with the HH pathway. We show that ERK5 is consistently expressed and active in human melanoma cells. Genetic silencing of ERK5 and pharmacological inhibition of the MEK5-ERK5 pathway drastically reduce the growth of melanoma cells harboring wild type (wt) or mutated BRAF (V600E), in vitro and in vivo. We also found that oncogenic BRAF positively regulates expression, phosphorylation and nuclear localization of ERK5. Importantly, BRAF enhances ERK5 kinase and transcriptional transactivator activities. Nevertheless, combined pharmacological inhibition of BRAF-V600E and MEK5 is required to decrease nuclear ERK5, that is critical for the regulation of cell proliferation. Accordingly, combination of MEK5 or ERK5 inhibitors with BRAF-V600E inhibitor Vemurafenib is more effective than single treatments in reducing the tumor growth of BRAF-V600E melanoma cells and xenografts. Moreover, we have also identified the existence of an interplay between the HH pathway and ERK5. By chemical and genetic inhibition of ERK5, we demonstrate that ERK5 positively modulates the HH pathway, increasing transcriptional activity and protein levels of the GLIs transcription factors, the final effectors of HH signaling. These data support a key role of ERK5 pathway for melanoma growth in vitro and in vivo and suggest that targeting ERK5, alone or in combination with BRAF-MEK1/2 inhibitors or HH pathway inhibitors, might represent a novel approach for melanoma treatment.