Efflux of endogenous PPIX by ABCG2. (A) K562 cells or K562/ABCG2 cells were incubated with 1 mM ALA for 21 hours to induce endogenous PPIX, with or without 1 μM Ko143, and analyzed for PPIX fluorescence. Blue line indicates K562 + ALA; red line, K562/ABCG2 + ALA; brown line, K562/ABCG2 + ALA + Ko143; green line, nontreated K562 cells; black line, nontreated K562/ABCG2 cells. (B) MEL cells were incubated with 1 mM ALA for 21 hours with or without 1 μM Ko143 and analyzed in flow cytometry for PPIX fluorescence. Red line indicates MEL + ALA; blue line, MEL + ALA + Ko143; black line, nontreated MEL cells. (C) K562 or K562/ABCG2 cells were incubated with 1 mM ALA for 21 hours and pelleted. The amount of PPIX in pelleted cells was measured in an HPLC assay. (D) An aliquot of cells also was analyzed in a flow cytometry for fluorescence. PPIX was undetectable in nontreated K562 cells as shown in the bottom panels.