Melanoma-associated brain metastasis: Molecular mechanisms and therapeutic options

Melanoma is the deadliest of skin cancers and exhibits the highest level of brain tropism of all cancer types. Melanoma brain metastases (MBMs) are diagnosed in no less than 50 % of patients with stage IV melanoma, although these lesions are identified in an even higher number of patients post-mortem. While current immune-based and targeted therapies can help against brain lesions, responses are unpredictable and resistance and cancer relapses often occur. The origin and biology of MBM are not well understood; however, increasing evidence suggests that MBMs have distinct properties from other metastatic sites. Here, using a genetically heterogeneous panel of human-derived MBM cell lines, we aimed to identify brain-specific melanoma properties and molecular processes that can be targeted in the clinic. Using functional assays, we observed that MBM cells displayed a slower intrinsic growth rate in vitro compared to extracranial cells. Reverse phase protein array analyses highlighted MBM-specific protein expression in targets associated with proliferation, survival, adhesion and migration; these targets pointed towards RAC1 which was identified as upregulated in the MBM cells, possibly involving a distinct RAC1/PAK1/JNK1 signaling axis. Knockdown of RAC1 using shRNA or its inhibition using small molecule inhibitors (alone or together with BRAF inhibition) hindered MBM functional properties in vitro, while in vivo RAC1 knockdown resulted in reduced primary tumor growth and delayed tumor appearance. Furthermore, MBM proliferation, adhesion and migration were altered in the presence of growth factors such as insulin, brain soluble factors or neuron and glia co-cultures, pointing out the importance of the microenvironment in understanding MBM cell biology. Moreover, MBM aggressive behavior upon exposure to the brain microenvironment could be modulated by RAC1 levels, as demonstrated by our knockdown studies. Finally, although resistant to PI3K/AKT/mTOR and BRAF/MEK inhibitors, MBM cells displayed an increased sensitivity to ferroptosis inducers, opening the door to future novel anti-MBM therapeutic options. Our findings emphasize the importance of microenvironment’s implication and of molecular characterization of MBM in order to achieve therapeutic efficacy.2022-04-2