Melanoma remains the deadliest form of skin cancer, with a five year survival of only 15%. The advent of drugs specifically targeting mutated BRAF has represented a major improvement in melanoma patient care. Yet, these therapies are hampered by the development of resistance within six to twelve months of treatment, and are applicable exclusively to patients with mutated BRAF that include roughly 50% of melanoma cases. Here we will investigate the therapeutic benefits and molecular mechanisms of a novel approach targeting the developmental pathways Notch and ERBB as alternative treatments that can benefit patients across the spectrum of mutations that drive melanoma. Notch and ERBB are evolutionarily conserved signaling cascades that play essential roles in embryogenesis and stem cell renewal, but are inappropriately activated in various cancers. We have recently identified a functional cross-talk between Notch1 and ERBB3 signaling in melanoma. We find that active Notch1 (Notch1NIC) and active (phosphorylated) ERBB3 and ERBB2 correlate significantly in over 70% of melanoma tumors, suggesting these receptors are co-reactivated in melanoma. We have shown that Notch1 directly promotes ERBB3 activation by regulating NRG1 (neuregulin1) expression, the ligand for ERBB3 and 4. Once activated, ERBB3 specifically co-opts ERBB2 to promote melanoma cell survival. Together, our data suggest Notch1 and ERBB3/ERBB2 signaling are concurrently reactivated in melanoma where their coordinate activity contributes to cell survival and tumor expansion. Indeed, while blocking either pathway triggers modest effects, combining a gamma-secretase inhibitor (GSI) to block Notch activation, and a tyrosine kinase inhibitor (TKI) to inhibit ERBB3/2 elicits synergistic effects, leading to 90% loss of melanoma cell viability regardless of whether cells carry wild type or mutated BRAF. Preliminary data also show that by using a well-tolerated delivery schedule, a combination therapy promotes melanoma tumor regression with no overt weight loss, a side effect often associated with GSI treatments. Mechanistically, we show for the first time that Notch1 and ERBB3 inhibition results in the deregulation of KEAP1, a (BTB)-Kelch protein that by promoting ubiquitination and proteasome-dependent degradation of IKK-beta leads to complete ablation of NFkB activity. Thus, we propose that the developmental pathways Notch and ERBB are pivotal in melanoma survival and expansion, and that blocking these pathways may have previously unappreciated therapeutic implications. We will: 1) Determine the efficacy of a combined anti Notch-ERBB targeted therapy in BRAF and RAS dependent transgenic mouse melanoma models that best recapitulate the mutations and biological features of the human disease;2) Dissect the mechanisms of regulation of KEAP1 by Notch1 and ERBB3 and the role of NFkB downstream of Notch and ERBB signaling;and 3) examine the association between active Notch1 (Notch1NIC) and phosphorylated ERBB3 and NFkB expression and their correlation with melanoma patient survival. We expect the knowledge gained from this work to provide the rationale for novel treatments that can benefit a larger melanoma patient population.
Although anti BRAF therapies have been a significant advance in the management of melanoma patients, they are limited by the appearance of resistance within a few months of treatment and exclude roughly half of the melanoma patient population who does not express mutated BRAF. Our novel approach instead focuses on the developmental pathways Notch and ERBB signaling as alternative therapeutic targets. Because blockade of Notch and ERBB effectively impairs melanoma cells regardless of whether they express wild type or mutated BRAF, our approach can not only serve as adjuvant therapy to BRAF inhibitors to overcome resistance;but can extend beyond BRAF driven melanomas, potentially serving a broader patient population. Finally, inhibitors of the Notch and ERBB pathways are already in use in the clinic or in clinical trials. The availability of such treatment to melanoma patients is therefore a concrete possibility in the short-term.