Protein Kinase C Inhibition Overcomes Targeted Therapy Resistance in Cutaneous Melanoma

WNT5a expression is associated with a MAPK inhibitor resistant phenotype in melanoma driving cell polarity and invasion. No small molecules specifically targeting WNT5a are available. Promising results of targeting non-canonical WNT5a-dependent WNT signalling with a pan-PKC inhibitor in uveal melanoma prompted us to investigate the relevance of PKC inhibition in cutaneous melanoma. We revealed PKC signalling and WNT5a expression to be associated in a positive feedback loop, suggesting pan-PKC inhibitor as a potent inhibitor of WNT5a in cutaneous melanoma. Combinatorial PKC and MAPK pathway inhibition significantly reduced proliferation and invasion by induction of apoptosis in targeted therapy-resistant melanoma in vitro. In in vivo xenograft studies, we found less proliferation and apoptosis induction in the PKC inhibitor single and combination treatment group with MAPK pathway inhibitors than in the standard of care treatment group. Thus, targeting the non-canonical WNT signalling pathway via combinatorial PKC and MAPK pathway inhibition is beneficial for therapy-resistant cutaneous melanoma combating tumour heterogeneity in vivo. With our study, we are providing an alternate treatment strategy we think is worth investigating as future clinical interventions in cutaneous melanoma.