Heat shock protein 90 (Hsp90) has emerged as a promising therapeutic target for the treatment of cancer. Several Hsp90 inhibitors have entered clinical trials. However, some toxicological detriments have arisen, such as cardiotoxicity resulting from hERG inhibition following the administration of Hsp90 inhibitors. We sought to investigate this toxicity as hERG has been previously reported as a client protein that depends upon Hsp90 for its maturation and functional trafficking. In this study we show that hERG depends upon a single Hsp90 isoform. hERG preferentially co-immunoprecipitated with Hsp90α and genetic knockdown of Hsp90α, but not Hsp90β, resulted in a trafficking-defective hERG channel. This study demonstrates the importance of delineating the isoform dependence of Hsp90 client proteins and provides rationale for the design of isoform-selective Hsp90 inhibitors that avoid detrimental effects.

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 M) but not low (0.01-1 M) concentrations of hemehemopexin 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. WAF1/CIP1/SDI1 generated by heme-hemopexin appear to be of paramount importance in cellular protection by heme-hemopexin.

Heme-hemopexin (2-10 microM) is used as a model for intravenous heme released in trauma, stroke, and ischemia-reperfusion. A transient increase in cellular protein oxidation occurs during receptor-mediated heme transport from hemopexin which is inhibited by the nonpermeable Cu(I) chelator, bathocuproinedisulfonate. Thus, participation of surface redox process involving Cu(I) generation are proposed to be linked to the induction of the protective proteins heme oxygenase-1 (HO-1) and metallothionein-1 (MT-1) by heme-hemopexin. The region (-153 to -42) in the proximal promoter of the mouse MT-1 gene responds to heme- and CoPP-hemopexin in transient transfection assays and contains metal-responsive elements for MTF-1 and an antioxidant-responsive element (ARE) overlapping a GC-rich E-box to which USF-1 and -2 bind. No decreases in DNA binding of the diamide-oxidation sensitive USF-1 and -2 occur upon exposure of cells to heme-hemopexin. MTF-1 and the ARE-binding proteins are relatively resistant to diamide oxidation and are induced approximately eight- and two-fold, respectively, by heme-hemopexin. BCDS prevents the nuclear translocation of MTF-1 by both heme- and CoPP-hemopexin complexes as well as MT-1 mRNA induction by CoPP-hemopexin. Thus, copper is needed for the surface oxidation events and yet the nuclear translocation of MTF-1 in response to hemopexin occurs via copper, probably Cu(I),-dependent signaling cascades from the hemopexin receptor rather than the oxidation per se.

No abstract

This review provides a synopsis of the published literature over the past two years on the heme-binding protein hemopexin (HPX), with some background information on the biochemistry of the HPX system. One focus is on the mechanisms of heme-driven pathology in the context of heme and iron homeostasis in human health and disease. The heme-binding protein hemopexin is a multi-functional protectant against hemoglobin (Hb)-derived heme toxicity as well as mitigating heme-mediated effects on immune cells, endothelial cells, and stem cells that collectively contribute to driving inflammation, perturbing vascular hemostasis and blood–brain barrier function. Heme toxicity, which may lead to iron toxicity, is recognized increasingly in a wide range of conditions involving hemolysis and immune system activation and, in this review, we highlight some newly identified actions of heme and hemopexin especially in situations where normal processes fail to maintain heme and iron homeostasis. Finally, we present preliminary data showing that the cytokine IL-6 cross talks with activation of the c-Jun N-terminal kinase pathway in response to heme-hemopexin in models of hepatocytes. This indicates another level of complexity in the cell responses to elevated heme via the HPX system when the immune system is activated and/or in the presence of inflammation.