TYRP1 mRNA is of interest due to its potential non-coding role as a sponge sequestering tumor-suppressive miRs in melanoma.,To our knowledge, there is no report on changes in TYRP1 expression in melanomas after development of resistance to targeted therapies.,We used patient-derived drug-naïve RASQ61R and BRAFV600E melanoma cell lines.,In BRAFV600E melanoma cells, resistance to vemurafenib and trametinib was developed.,A time-lapse fluorescence microscope was used to rate proliferation, qRT-PCR and Western blotting were used to assess TYRP1 expression and MITF-M level and activity.,A high TYRP1 protein level in RASQ61R cells corresponded with high TYRP1 mRNA level, whereas undetectable TYRP1 protein in BRAFV600E cells was accompanied by medium mRNA level, also in cells carrying NF1R135W variant in addition.,TYRP1 expression was MITF-M-independent, since similar transcript status was found in MITF-Mhigh and MITF-Mlow cells.,For the first time, we showed that TYRP1 expression remained unaltered in melanoma cells that became resistant to vemurafenib or trametinib, including those cells losing MITF-M.,Also drug discontinuation in resistant cells did not substantially affect TYRP1 expression.,To verify in vitro results, publicly available microarray data were analyzed.,TYRP1 transcript levels stay unaltered in the majority of paired melanoma samples from patients before treatment and after relapse caused by resistance to targeted therapies.,As TYRP1 mRNA level remains unaltered in melanoma cells during development of resistance to vemurafenib or trametinib, therapies developed to terminate a sponge activity of TYRP1 transcript may be extended to patients that relapse with resistant disease.
Accumulating evidence supports the concept that melanoma is highly heterogeneous and sustained by a small subpopulation of melanoma stem-like cells.,Those cells are considered as responsible for tumor resistance to therapies.,Moreover, melanoma cells are characterized by their high phenotypic plasticity.,Consequently, both melanoma stem-like cells and their more differentiated progeny must be eradicated to achieve durable cure.,By reevaluating compounds in heterogeneous melanoma populations, it might be possible to select compounds with activity not only against fast-cycling cells but also against cancer stem-like cells.,Natural compounds were the focus of the present study.,We analyzed 120 compounds from The Natural Products Set II to identify compounds active against melanoma populations grown in an anchorage-independent manner and enriched with cells exerting self-renewing capacity.,Cell viability, cell cycle arrest, apoptosis, gene expression, clonogenic survival and label-retention were analyzed.,Several compounds efficiently eradicated cells with clonogenic capacity and nanaomycin A, streptonigrin and toyocamycin were effective at 0.1 µM.,Other anti-clonogenic but not highly cytotoxic compounds such as bryostatin 1, siomycin A, illudin M, michellamine B and pentoxifylline markedly reduced the frequency of ABCB5 (ATP-binding cassette, sub-family B, member 5)-positive cells.,On the contrary, treatment with maytansine and colchicine selected for cells expressing this transporter.,Maytansine, streptonigrin, toyocamycin and colchicine, even if highly cytotoxic, left a small subpopulation of slow-dividing cells unaffected.,Compounds selected in the present study differentially altered the expression of melanocyte/melanoma specific microphthalmia-associated transcription factor (MITF) and proto-oncogene c-MYC.,Selected anti-clonogenic compounds might be further investigated as potential adjuvants targeting melanoma stem-like cells in the combined anti-melanoma therapy, whereas selected cytotoxic but not anti-clonogenic compounds, which increased the frequency of ABCB5-positive cells and remained slow-cycling cells unaffected, might be considered as a tool to enrich cultures with cells exhibiting melanoma stem cell characteristics.