Normal human melanocytes have been shown to respond to the
signal peptide endothelin by increased proliferation and
melanin formation. Contradictory findings, however, have been reported about which of the two
endothelin receptors (EDNRA or EDNRB) is expressed in normal melanocytes and
melanoma cells. Moreover it was not clear whether malignant cells differ from their normal precursors in this respect. Screening a melanocyte cDNA library for genes downregulated in
melanomas identified clones specific for EDNRB. Northern blots proved that the corresponding
mRNA is generally expressed in cultures of human cutaneous melanocytes and congenital
melanocytic nevus cells. In 16 of 17
melanoma cell lines, however, the expression of EDNRB
mRNA was strongly downregulated. EDNRA was only weakly expressed and detectable by northern blotting in 12 of 17 cultures of benign melanocytic cells and four of 17
melanoma cell lines. Nested
reverse transcriptase-polymerase chain reaction proved several
melanoma cell lines to be completely negative for EDNRA expression. Gene deletion as the cause of missing
endothelin receptor expression was ruled out by genomic Southern blots. Receptor binding assays confirmed
RNA data revealing 1.6 x 105
endothelin-1 binding sites per cell for a melanocyte culture and between 8.7 x 104 and 400 sites per cell for
melanoma cell lines. Expression of pigmentation genes coding for
tyrosinase,
TRP-1 and TRP-2 correlated positively with that of EDNRB but negatively with EDNRA expression. EDNRB but not EDNRA expression is therefore typical for melanocytic cells, and downregulation of EDNRB seems to be an important characteristic of
melanoma cells possibly related to
malignancy or apoptosis.