Abstract
Melanoma plasticity creates a plethora of opportunities for cancer cells to escape treatment. Thus, therapies must target all cancer cell subpopulations bearing the potential to contribute to disease. The role of the differentiation/pigmentation program in intrinsic and acquired drug resistance is largely uncharacterized. MITF level and expression of MITF-dependent pigmentation-related genes, MLANA, PMEL, TYR, and DCT, in drug-naïve and vemurafenib- or trametinib-treated patient-derived melanoma cell lines and their drug-resistant counterparts were analysed and referred to genomic alterations. Variability in execution of pigmentation/differentiation program was detected in patient-derived melanoma cell lines. Acute treatment with vemurafenib or trametinib enhanced expression of pigmentation-related genes in MITF-Mhigh melanoma cells, partially as the consequence of transcriptional reprograming. During development of resistance, changes in pigmentation program were not unidirectional, but also not universal as expression of different pigmentation-related genes was diversely affected. In selected resistant cell lines, differentiation/pigmentation was promoted and might be considered as one of drug-tolerant phenotypes. In other resistant lines, dedifferentiation was induced. Upon drug withdrawal (“drug holiday”), the dedifferentiation process in resistant cells either was enhanced but reversed by drug reexposure suggesting involvement of epigenetic mechanisms or was irreversible. The irreversible dedifferentiation might be connected with homozygous loss-of-function mutation in MC1R, as MC1RR151C +/+ variant was found exclusively in drug-naïve MITF-Mlow dedifferentiated cells and drug-resistant cells derived from MITFhigh/MC1RWT cells undergoing irreversible dedifferentiation. MC1RR151C +/+ variant might be further investigated as a parameter potentially impacting melanoma patient stratification and aiding in treatment decision.
Highlights
Targeted therapies brought hope for melanoma patients; the initial clinical response is not achieved in every patient and the development of drug resistance is observed in the majority of responders within one year
Using our preclinical model of patient-derived melanoma cells cultured in stem cell medium (SCM), we investigated effects of targeted drugs, vemurafenib and trametinib, on MITF level and expression of MITF-dependent pigmentation/differentiation genes
When we aligned patient-derived melanoma cell characteristics shown in this and our previous [5, 37] studies with differentiation subtypes described in this model, we found that the drug-naıve melanoma cell lines belong to one of three subtypes: invasive/neural crestlike: MITFlow/SOX10high/NGFRhigh/AXLhigh (DMBC11 and DMBC12 cell lines), melanocyte-like: MITFhigh/SOX10high/ NGFRlow/AXLlow (DMBC17 cell line), and neural crest/ pigmentation: MITFhigh/SOX10high/NGFRmedium/AXLlow
Summary
Targeted therapies brought hope for melanoma patients; the initial clinical response is not achieved in every patient and the development of drug resistance is observed in the majority of responders within one year. The possible switch between diverse phenotypic states creates a plethora of opportunities for melanoma cells to escape the treatment [4] In this respect, the role of the differentiation program in intrinsic and acquired resistance to targeted drugs is not sufficiently elaborated. Our previous study indicates that vemurafenib (PLX4032, Zelboraf), an inhibitor of V600EBRAF, and trametinib, an inhibitor of MEK1/2 (GSK1120212, Mekinist), increase percentages of CD271 (NGFR)-positive melanoma cells (stemlike/neural crest-like phenotype) while reducing the percentages of Ki-67-positive cells (proliferative phenotype) [5] This would suggest that, as targeted therapies reduce percentages of proliferating cells and increase those of more primitive cells [5,6,7], they should diminish a subpopulation executing differentiation/pigmentation program if drug-induced changes follow the rheostat model of MITFM (M isoform of microphthalmia-associated transcription factor) activity [8].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
