ROLE OF HEME-HEMOPEXIN IN HUMAN T-LYMPHOCYTE PROLIFERATION

Heme-hemopexin supports and stimulates proliferation of human acute T- lymphoblastic (MOLT-3) cells, suggesting the participation of heme in cell growth and division. MOLT-3 cells express approximately 58,000 he mopexin receptors per cell (apparent K-d 20 nM), of which about 20% ar e on the cell surface. Binding is dose- and temperature-dependent, and growth in serum-free IMDM medium is stimulated by 100-1000 nM heme-he mopexin, consistent with the high affinity of the receptor for hemopex in, and maximal growth is seen in response to 500 nM complex. Growth w as similar in defined minimal medium supplemented with either low conc entrations of heme-hemopexin or iron-transferrin, and either of these complexes were about 80% as effective as a serum supplement. Heme-hemo pexin, but not apo-hemopexin, reversed the growth inhibition caused by desferrioxamine showing that heme-iron derived from heme catabolism i s used for cell growth. Cobalt-protoporphyrin (CoPP)-hemopexin, which binds to the receptor but is not transported intracellularly [Smith ct al., (1993) J. Biol. Chem. 268, 7365], also stimulated cell prolifera tion in serum-free IMDM but did not ''rescue'' the cells from desferri oxamine. Furthermore, CoPP-hemopexin effectively competed for the hemo pexin receptor with heme-hemopexin and diminished its growth stimulato ry effects. In addition, protein kinase C (PKC) is translocated to the plasma membrane within 5 min after heme-hemopexin is added to the med ium, reaches maximum activity within 5-10 min, and declines to unstimu lated levels by 30 min. Heme-hemopexin and CoPP-hemopexin both augment ed MOLT-3 cell growth stimulated by serum. Thus, heme-hemopexin not on ly functions as an iron source for T-cells but occupancy of the hemope xin receptor itself triggers signaling pathway(s) involved in the regu lation of cell growth. The stimulation of growth of human T-lymphocyte s by heme-hemopexin is likely to be a physiologically relevant mechani sm at sites of injury, infection, and inflammation. (C) 1997 Academic Press.