Hemin Control of Heme Biosynthesis and Catabolism in a Human Leukemia Cell Line

Abstract

Hemin treatment of the Philadelphia chromosome positive leukemia cell line, K562, accentuates a number of erythroid phenotypic characteristics. The nature of this hemin effect was investigated by examining heme production and heme biosynthetic and catabolic enzyme activity in untreated and 0.05 mM hemin-treated cells. Activities of δ-aminolevulinic acid synthetase (ALAS), the rate limiting heme synthetic enzyme, and δ-aminolevulinic dehydratase (ALAD) were detectable in both uninduced and induced K562 cells. However, cells treated with hemin showed a significant increase in ALAS and ALAD activity by 2.5 days of treatment compared to untreated cells, and both enzyme activities further increased over the remainder of the incubation period. Incorporation of 3H-ALA into heme was also monitored. 3H-ALA was incorporated into heme in both untreated and treated cultures, but this incorporation was significantly greater in hemin-treated cells. Incorporation of 3H-ALA into heme indicates that cellular heme biosynthesis accounts for a portion of the heme utilized for hemoglobin production and that hemin stimulates heme synthesis by increasing enzyme activities distal to ALAS in the heme biosynthetic pathway. Heme oxygenase, the rate-limiting enzyme in mammalian heme degradation, was also studied and found to be present in large amounts in treated and untreated K562 cells. However, heme oxygenase activity was significantly decreased following hemin treatment. The demonstration of heme oxygenase activity in K562 cells documents the presence of this heme catabolic enzyme in human erythropoietic stem cells. The fall in heme oxygenase activity that accompanies hemin treatment might reflect an effort by erythroid cells to conserve heme during erythroid differentiation. These studies indicate the central role that heme synthesis and catabolism play during human erythroid differentiation.