Biol. Pharm. Bull. 30(6) 1144—1146 (2007)

family transport various substrates coupled with hydrolysis of ATP. The human ABC transporter superfamily consists of 49 subtypes, some of which function as drug efflux transporters (ABCB1, ABCC subfamily and ABCG2) or sterol transporters (ABCA1, ABCA7, ABCG1, ABCG5 and ABCG8). However, the role of the ABCA5-like transporters remains largely unknown. The ABCA family forms the second largest gene family, consisting of 12 subtypes, after the ABCC family. Evolutional analysis has revealed a gene cluster encoding ABCA5, ABCA6, ABCA8, ABCA9 and ABCA10, which are known as the ABCA5-like transporters. Interestingly, although these five ABCA transporters form a unique cluster on human Chr17q24, most other ABC transporter genes are dispersed in the mammalian genome. Among the ABCA5-like transporters, ABCA8 expressed in oocytes was shown to exhibit ATP-dependent transport of estradiol-b-glucuronide. In the testis, ABCA5 is localized in Leydig cells, which form the blood-testis barrier, and ABCA5 knockout mice develop an enlarged heart, injured liver and decreased plasma levels of thyroid hormones. These observations imply the physiological importance of ABCA5-like transporters. Since ABCA8 has been reported to act as a transporter, it is possible that ABCA5-like transporters have functions related to multi-drug resistance in tumor cells. The purpose of this study is, therefore, to investigate mRNA expression of ABCA5-like transporters in human tissues and tumors, in order to establish whether there is a relationship between ABCA5-like transporters and tumor development. MATERIALS AND METHODS