Inside life of melanoma cell signaling, molecular insights, and therapeutic targets

Multiple signaling pathways converge on proapoptotic protein BAD to promote survival of melanocytes

Melanocyte survival is mediated by diverse signaling pathways. However, the molecular mechanisms they use and molecules that they target are incompletely understood. Here, we show that melanocyte survival is mediated by diverse, nonredundant signaling pathways, including ERK1/2, AKT, PKA, and PKC. Each of these pathways is exerting prosurvival effects by phosphorylating the BAD. While Ser112-BAD phosphorylation is regulated by pERK, pPKA and pPKC, Ser136 and Ser155 phosphorylation are exclusively controlled by pAKT and pPKA, respectively. Inhibition of these pathways individually resulted in only modest apoptosis; however, most significant apoptosis, as a result of BAD dephosphorylation, was seen when all pathways were inhibited concurrently. BAD phosphorylation was essential for survival of melanocytes as cells expressing phosphorylation-deficient BAD were not rescued by any of the identified pathway. Furthermore, melanocytes became insensitive to kinase inhibitor-induced apoptosis when BAD expression was knocked down by BAD-shRNA. Overexpression of BAD in melanocytes stimulated faster apoptosis in response to kinase inhibitors. Taken together, our results show that BAD is acting as a convergence point for diverse survival pathways in melanocytes. Understanding the molecular mechanisms of melanocyte survival provides fundamental new insights into physiological mechanisms involved in the development of various melanocyte pathologies such as melanoma and vitiligo.

Targeted molecular therapy in melanoma

Immunotherapy and chemotherapy benefit few patients with metastatic melanoma, and even fewer experience durable survival benefit. These poor results may come from treating all melanomas as though they are biologically homogeneous. Recently, it has been shown that targeting specific activated tyrosine kinases (oncogenes) can have striking clinical benefits in patients with melanoma. In 2002, a V600E mutation of the BRAF serine/threonine kinase was described as present in more than 50% of all melanomas. The mutation appeared to confer a dependency by the melanoma cancer cell on activated signaling through mitogen-activated protein kinase pathway. The frequency and focality of this mutation (>95% of all BRAF mutations being at V600 position) suggested its importance in melanoma pathophysiology and potential as a target for therapy. The recent results of a phase 1 study with PLX4032/RG7204, a small molecule RAF inhibitor, confirm this hypothesis. Mucosal and acral-lentiginous melanomas, comprising 3% of all melanomas, frequently harbor activating mutations of c-kit and drugs targeting this mutation seem to confer similar benefits for these types of tumors. Here we provide an overview of the targeted therapy development in melanoma with emphasis on BRAF inhibition because of its prevalence and possibility of transforming the care of many melanoma patients.

B-RAF inhibitors: an evolving role in the therapy of malignant melanoma

Immunotherapy and chemotherapy benefit few patients with metastatic melanoma, and even fewer experience durable survival benefit. These poor results come from treating melanoma as a single homogeneous disease. Recently, it has been shown that targeting activated tyrosine kinases (oncogenes) can mediate striking clinical benefits in several cancers. In 2002, a mutation at the V600E amino acid of the BRAF serine/threonine kinase was described as present in over 50% of melanomas. The mutation appeared to confer a dependency by the melanoma cancer cell on its activation of the MAP kinase pathway. The frequency and specificity of this mutation (95% at V600E of BRAF) suggests that it may be a potential target for therapy, and recent results with one inhibitor, PLX4032/RG7204, bare this out. This review updates the status of BRAF inhibitors in melanoma and what may be on the horizon.

MEK inhibitor PD0325901 significantly reduces the growth of papillary thyroid carcinoma cells in vitro and in vivo

Papillary thyroid carcinomas (PTC) are the most common type of thyroid malignancy. Most PTC carry one of the two mutations, RET/PTC rearrangement or BRAF mutation. Both mutations are able to activate the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) signaling transduction pathway leading to cellular proliferation, differentiation, and apoptosis. PD0325901 is a specific MEK1/2 inhibitor and therefore is a promising drug to treat thyroid cancers with either RET/PTC or BRAF mutation. In this study we tested the effects of PD0325901 on PTC cells harboring either mutation in vitro by growth curves and Western blots and in vivo using a murine orthotopic xenograft model. We found that 50% growth inhibition (GI(50)) by PD0325901 was 11 nmol/L for the PTC cells with the RET/PTC1 rearrangement and 6.3 nmol/L for PTC cells with a BRAF mutation, with both concentrations readily achievable in serum. After 1 week of oral administration of PD0325901 (20-25 mg/kg/day) in mice, no tumor growth was detected in mice inoculated with PTC cells bearing a BRAF mutation. For PTC with the RET/PTC1 rearrangement, the average tumor volume of the orthotopic tumor was reduced by 58% as compared with controls. In conclusion, our data suggested that PTC cells carrying a BRAF mutation were more sensitive to PD0325901 than were PTC cells carrying the RET/PTC1 rearrangement. Our findings support the clinical evaluation of PD0325901 for patients with PTC and potentially other carcinomas with BRAF mutations.