MLANA/MART1 and SILV/PMEL17/GP100 are transcriptionally regulated by MITF in melanocytes and melanoma

Abstract

The clinically important melanoma diagnostic antibodies HMB-45, melan-A, and MITF (D5) recognize gene products of the melanocyte-lineage genes SILV/PMEL17/GP100, MLANA/MART1, and MITF, respectively. MITF encodes a transcription factor that is essential for normal melanocyte development and appears to regulate expression of several pigmentation genes. In this report, the possibility was examined that MITF might additionally regulate expression of the SILV and MLANA genes. Both genes contain conserved MITF consensus DNA sequences that were bound by MITF in vitro and in vivo, based on electrophoretic mobility shift assay and chromatin-immunoprecipitation. In addition, MITF regulated their promoter/enhancer regions in reporter assays, and up- or down-regulation of MITF produced corresponding modulation of endogenous SILV and MLANA in melanoma cells. Expression patterns were compared with these factors in a series of melanoma cell lines whose mutational status of the proto-oncogene BRAF was also known. SILV and MLANA expression correlated with MITF, while no clear correlation was seen relative to BRAF mutation. Finally, mRNA expression array analysis of primary human melanomas demonstrated a tight correlation in their expression levels in clinical tumor specimens. Collectively, this study links three important melanoma antigens into a common transcriptional pathway regulated by MITF.

Figure 1.

The SILV and MLANA promoters/enhancers. Initial transcribed exons are denoted by large arrows and E-boxes are represented by vertical bars.

Figure 2.

Binding of MITF to SILV enhancer and MLANA promoter in vitro and in vivo. A: Chromatin immunoprecipitations were performed on materials isolated from melanoma cells. DNA from lysates before immunoprecipitation was used as positive input control. The SILV enhancer primer set amplifies the intronic regulatory region which is important for gene expression, while the control primer set amplifies the region containing the upstream E-box which is dispensable for gene expression (negative control), based on reporter assays (see below and data not shown). The MLANA promoter primer set spanning the upstream E-boxes, while the control primer set amplifies a portion of the 3′ UTR (negative control). B: EMSA assays were performed on melanoma lysates using probes containing SILV intronic E-box (E1) and MLANA proximate E-box (E2), respectively. Similar binding of MITF was seen at the E1 site of MLANA as well (data not shown).

Figure 3.

Reporter assays of SILV and MLANA point mutants. SILV and MLANA reporters were transfected into 293 cells. Firefly luciferase activities in samples were normalized to Renilla luciferase activities in the same specimens. Relative luciferase activities are shown. To recapitulate the putative SILV intronic enhancer, the genomic locus was incorporated into the pGL3 reporter with luciferase placed downstream of a SILV splice acceptor sequence. As described in Results, correct splicing of the reporter transcript was separately confirmed.

Figure 4.

Regulation of endogenous SILV and MLANA mRNA levels by MITF. A: SILV mRNA levels were assayed after adenovirus infection (top panel). Total RNA was isolated from melanoma cells infected with adenoviruses 72 hours after infection. Quantitative RT-PCR was performed and data were normalized to endogenous GAPDH levels. Expression levels for MITF (wt) and MITF (dn)-virus infected samples were normalized to the control samples. B: MLANA mRNA levels were assayed after adenovirus infection (bottom panel). Total RNA was harvested 72 hours after adenovirus infection and Northern analysis was performed.

Figure 5.

Regulation of endogenous SILV and MLANA protein levels by MITF. Western analysis was performed on melanoma lysates harvested 72 hours after adenovirus infection.

Figure 6.

Correlation between MITF, MLANA, and SILV expression and BRAF mutation status. Western analysis were performed on cell lysates harvested from various melanoma cell lines and human primary melanomas with antibodies specified in Materials and Methods. BRAF mutation status was assessed by PCR and subsequent sequencing on corresponding genomic DNA. Whereas MITF and SILV expression correlated quite closely, no simple correlation was seen between MITF and BRAF mutational state or the various transcriptional regulators shown (which have been suggested to regulate MITF expression in specific contexts).

Figure 7.

Correlations between MITF, SILV, and MLANA mRNA levels in cultured melanoma cell lines and human primary melanomas. NCI human melanoma cell lines (A) and human primary melanomas (B). Quantitative mRNA levels for all three genes were acquired from Affymetrix microarrays as described in Materials and Methods, and deviations from mean expression are plotted across each series.