Altered liver parameters in a genetic mouse model of porphyria cutanea tarda and possible involvement of ABC transporters in the disease

The project sought to examine the changes in the liver associated with porphyria and whether ATP binding cassette (ABC) transporters in the hepatocyte might in some manner be connected with the disruption of porphyrin homeostasis. The animal model of porphyria selected for the study was a genetic mouse model of porphyria cutanea tarda (PCT) that spontaneously develops the disorder with maturity. This model affords the opportunity to comprehensively evaluate liver changes without the administration of any exogenous compounds, which in other animal models are used to precipitate PCT. Many changes in hepatic parameters were present in the porphyric mouse model. Total liver heme concentration was increased, select cytochrome P450 activities were decreased while others were unchanged, UDP-glucuronosyltransferase activity was unchanged while glutathione S-transferase activity was elevated. Because of their broad and overlapping substrate selectivities, changes in specific transporters are most easily investigated through changes in mRNA expression. Of twelve ABC transporters examined, mRNA expression of seven were unchanged (Abcb1a, Abcb1b, Abcb6, Abcb11, Abcc3, Abcc5 and Abcg2), three were elevated (Abca3, Abcc1 and Abcc4) and two were depressed (Abcc2 and Abcc6). The possible association of ABC transporters with porphyria arises from a consideration of how highly anionic porphyrins might or might not move across the lipophilic cellular membrane of the hepatocyte. Depressed levels of relevant transporters could contribute to the initial accumulation of porphyrins characteristic of the disease. However, the eventual excretion of porphyrins in bile and urine indicates an ability to ultimately leave the hepatocyte, and the mechanism of how this might occur required an ability to measure this efflux under controlled conditions. To this end, a procedure to isolate and maintain viable hepatocytes from a porphyric mouse was developed and used to evaluate possible perturbing influences. Isolated hepatocytes revealed that while cells contained 70% porphyrinogen, 30% porphyrin, only the porphyrin was effluxed. A variety of approaches failed, however, to provide conclusive evidence that this efflux was mediated by ABC transporters. Interesting preliminary evidence suggested that lysosomal secretion might be a mechanism responsible for the porphyrin efflux in isolated cells.