TYRP1 mRNA level is stable and MITF-M-independent in drug-naïve, vemurafenib- and trametinib-resistant BRAF(V600E) melanoma cells

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 RAS(Q61R) and BRAF(V600E) melanoma cell lines. In BRAF(V600E) 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 RAS(Q61R) cells corresponded with high TYRP1 mRNA level, whereas undetectable TYRP1 protein in BRAF(V600E) cells was accompanied by medium mRNA level, also in cells carrying NF1(R135W) variant in addition. TYRP1 expression was MITF-M-independent, since similar transcript status was found in MITF-M(high) and MITF-M(low) 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.