Elucidating Distinct Tumorigenic Pathways in Nodular Versus Superficial Spreading Melanoma.

8544 Background: Superficial spreading melanoma (SSM) and nodular melanoma (NM), which account for 70% and 20% of all melanoma diagnoses respectively, are believed to represent sequential phases of linear progression from radial to vertical growth. Recent evidence from several groups including ours challenges this linear progression model and suggests that SSM and NM are biologically distinct. In this study, we focused on identifying tumorigenic pathways that are differentially activated in melanoma subtypes and that can be therapeutically exploited. Methods: We performed Protein Pathway Array on SSM/radial growth phase (RGP) and NM/vertical growth phase (VGP) primary melanoma cell lines to determine expression levels of 141 tumorigenic proteins/phospho-proteins. Differential protein expression was determined using the Pavlidis Template Matching algorithm in TIGR Multi Experiment Viewer. Differentially expressed proteins were validated in an expanded panel of RGP, VGP, and metastatic melanoma cell lines using western blot. Constitutively active, overexpressed proteins in VGP melanoma were down-regulated in cell lines using several small molecule inhibitors and the effects on proliferation and migration were assessed. Results: 17 (12%) proteins/phospho-proteins are significantly overexpressed in VGP versus RGP melanoma (p<0.05). Western blot confirmed the results of 5 proteins/phospho-proteins. Each of those 5 was equally overexpressed in VGP and metastatic melanoma cell lines. Phospho-p90 ribosomal s6 kinase (RSK) was prioritized for functional studies based on its role in both the MAPK and AKT signaling cascades, two tumorigenic pathways often dysregulated in melanoma. Constitutive phosphorylation of p90 RSK in VGP, but not RGP melanoma was observed in response to serum starvation. Small molecule inhibition of phospho-p90 RSK attenuated proliferation and migration (p<0.05) in VGP melanoma. Conclusions: Data demonstrate discrete tumorigenic pathways are activated in nodular versus superficial spreading melanoma and suggest that phospho-p90 RSK might be a viable target against nodular melanoma.