Principal expression of two mRNA isoforms (ABCB 5α and ABCB 5β ) of the ATP‐binding cassette transporter gene ABCB 5 in melanoma cells and melanocytes

Summary ATP‐binding cassette (ABC) transporters play a pivotal role in physiology and pathology. We identified and cloned two novel mRNA isoforms ( ABCB 5 α and ABCB 5 β ) of the ABC transporter ABCB 5 in human melanoma cells. The deduced ABCB 5 α protein appears to be an altered splice variant containing only a putative ABC, whereas the ABCB 5 β isoform shares approximately 70% similarity with ABCB 1 ( MDR 1) and has a deduced topological arrangement similar to that of the whole carboxyl terminal half of the ABCB 1 gene product, P‐glycoprotein, including an intact ABC. Northern blot, real‐time PCR, and conventional RT‐PCR were used to verify the expression profiles of ABCB 5 α / β . We found that the melanomas included among the NCI‐60 panel of cell lines preferentially expressed both ABCB 5 α and ABCB 5 β . However, ABCB 5 α / β expression was undetectable in two amelanotic melanomas (M14 and LOX‐IMVI). The expression profile of ABCB 5 α / β in all of the other melanomas of the panel was confirmed both by RT‐PCR and by sequencing. Neither ABCB 5 α nor ABCB 5 β expression was found in normal tissues such as liver, spleen, thymus, kidney, lung, colon, small intestines or placenta. ABCB 5 α / β mRNAs were also expressed in normal melanocytes and in retinal pigment epithelial cells, suggesting that ABCB 5 α / β expression is pigment cell‐specific and might be involved in melanogenesis. Our findings indicate that expression of ABCB 5 α / β might possibly provide two novel molecular markers for differential diagnosis of melanomas and constitute potential molecular targets for therapy of melanomas.