Heme carrier protein 1 (HCP1) expression and functional analysis in the retina and retinal pigment epithelium

The retina and retinal pigment epithelium (RPE) are present in the posterior segment of the eye, and the retina is dependent upon the underlying RPE for normal function. The retina is the most oxygenated tissue in the body but is isolated from the blood circulation by blood-retinal barriers. Metabolism of cellular oxygen involves heme but little is known about heme transport in the retina and RPE. Here we report the identification from bovine RPE of a heme transporter bHcp1 (bovine heme carrier protein 1) that is homologous to mouse intestinal HCP1 expressed in duodenal enterocytes. Similar to the mouse protein, bHcp1 exhibited heme uptake ability in Xenopus oocytes and localized to the cell membrane in cultured mammalian epithelium. Whereas bHcp1 expression was detected only in bovine RPE, expression of its human homologue was identified in both retina and RPE. Furthermore, the data revealed low-level wider expression of human HCP1 transcript in multiple tissues suggesting that it is responsible for heme transport in the body, not the intestine alone. Expression of HCP1 in the RPE and retina indicates the mechanism of heme transport in these ocular tissues.