Exploring the main source of coproporphyrins: Observations on transport in red blood cells

Coproporphyrins (CPs) I and III are byproducts of heme biosynthesis and have been proposed as tier 1 biomarkers for organic anion transporting polypeptide 1B-mediated drug-drug interactions. Although approximately 85% of total CPs are produced in erythroid cells, the mechanisms underlying their extrusion from human red blood cells remain unknown. The aim of this study was to investigate efflux transporters potentially involved in CP export from hematopoietic cells. Proteomic data from human reticulocytes indicate the presence of ATP-binding cassette (ABC) transporters multidrug resistance-associated protein (MRP)4, ABCB6, and breast cancer resistance protein (BCRP). We confirmed the expression of these transporters in CD71-positive reticulocytes from peripheral blood using immunofluorescence microscopy. To explore the impact of erythroid differentiation on CP levels, we treated K562 cells with imatinib to induce Hb synthesis. This led to a concurrent increase in extracellular CP levels and upregulation of ABCB6 and ABCG2 mRNA. However, only BCRP protein levels increased, whereas MRP4 exhibited a shift in molecular weight suggestive of posttranslational modification. Using CP transport assays in double-transfected HeLa cells and in erythrocyte membrane vesicles, we demonstrated that all 3 transporters-MRP4, ABCB6, and BCRP-can mediate the export of CPI and CPIII. In conclusion, to our knowledge, our findings provide the first evidence that active efflux by ABC transporters contributes to the release of CPs from erythroid cells. SIGNIFICANCE STATEMENT: Coproporphyrin (CP) I is a novel biomarker for predicting OATP1B-associated DDIs. So far, there is no clear consensus on the role of the blood compartment on CP plasma levels. This study addressed the question of how CPs are released from the red blood cells and found that active transport mechanisms, especially mediated by MRP4, ABCB6, and breast cancer resistance protein are likely involved in the efflux of CPs from erythroid cells. This suggests the reduced function of these efflux transporters may affect plasma CP levels.