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We have cloned from a chicken intestinal cDNA library Cmdr1, the first avian P-glycoprotein. Cmdr1 is 67% and 69% identical to proteins encoded by the human  MDR1  and  MDR2  genes, respectively. Functional expression of Cmdr1 in both mouse NIH 3T3 and yeast cells demonstrated that Cmdr1 represents the avian ortholog of human  Mdr1 , since it confers resistance to several anticancer drugs and the fluorescent dye rhodamine 6G. Northern and immunoblot analysis showed that  CMDR1  is highly expressed throughout the intestine and in the liver, and to a considerable extent in kidney, brain, lung, heart, eye and follicles.  In situ  hybridization revealed a cell type-specific expression of  CMDR1  in the intestinal epithelium, with high levels in the villi of the small and large intestine as well as crypt cells. These data suggest that Cmdr1 could play a role in intestinal detoxification. Most interestingly, immunoblotting showed that Cmdr1 is also expressed in ovarian tissues, particularly in theca cells, the major site for ovarian estrogen production in birds. Indeed, competition experiments indicated that Cmdr1 interacts with estradiol, since rhodamine 6G efflux was efficiently blocked by estradiol in NIH 3T3 cells expressing Cmdr1. Rhodamine efflux was also blocked by PSC-833, a specific inhibitor of steroid-transporting P-glycoproteins from mammalian cells. We propose that Cmdr1 in ovarian cells could be involved in the cell type-specific transport or release of estrogen that is essential for avian follicular development.

Cmdr1, a Chicken P-Glycoprotein, Confers Multidrug Resistance and Interacts with Estradiol