Cellular Protection Mechanisms against Extracellular Heme: HEME-HEMOPEXIN, BUT NOT FREE HEME, ACTIVATES THE N-TERMINAL c-Jun KINASE
Journal of Biological Chemistry(1999)
摘要
Hemopexin protects cells lacking hemopexin receptors by tightly binding heme abrogating its deleterious effects and preventing nonspecific heme uptake, whereas cells with hemopexin receptors undergo a series of cellular events upon encountering heme-hemopexin. The biochemical responses to heme-hemopexin depend on its extracellular concentration and range from stimulation of cell growth at low levels to cell survival at otherwise toxic levels of heme. High (2-10 mu M) but not low (0.01-1 mu M) concentrations of heme-hemopexin increase, albeit transiently, the protein carbonyl content of mouse hepatoma (Hepa) cells. This is due to events associated with heme transport since cobalt-protoporphyrin IX-hemopexin, which binds to the receptor and activates signaling pathways without tetrapyrrole transport, does not increase carbonyl content. The N-terminal c-Jun kinase (JNK) is rapidly activated by 2-10 mu M heme-hemopexin, yet the increased intracellular heme levels are neither toxic nor apoptotic. After 24 h exposure to 10 mu M heme-hemopexin, Hepa cells become refractory to the growth stimulation seen with 0.1-0.75 mu M heme-hemopexin but HO-1 remains responsive to induction by heme-hemopexin, Since free heme does not induce JNK, the signaling events, like phosphorylation of c-Jun via activation of JNK as well as the nuclear translocation of NF kappa B, G(2)/M arrest, and increased expression of p53 and of the cell cycle inhibitor p21(WAF1/CIP1/SDI1) generated by heme-hemopexin appear to be of paramount importance in cellular protection by heme-hemopexin.
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n terminal
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