Hemopexin receptor

As described later, hemopexin interacts with a variety of heme analogs, two of which, Sn-protoporphyrin IX (SnPP) and Co-protoporphyrin IX (CoPP), helped clarify the mechanism of MT-1 gene regulation by hemopexin. SnPP-hemopexin interacts with the hemopexin receptor and the SnPP (an inhibitor of HO-1, (91)) enters the cell and induces HO-1 (92). In contrast, CoPP-hemopexin interacts with the receptor, but CoPP is not internalized. Interestingly, the mere occupancy of the receptor by hemopexin is sufficient to activate signaling pathways and consequent induction of MT-1 (90, 92, 93), whereas heme uptake is required for activation of HO-1 gene transcription (92, 93). 

The ferro-complex CD spectrum shows that reduction of the heme iron alters the heme environment. Redox-induced protein conformation changes could alter the S5unmetry in the heme pocket or produce two binding modes for the reduced complex whose asymmetries nearly cancel each other. Redox-linked conformational changes are especially interesting in view of recent findings of oxido-reductase activity associated with the heme-hemopexin-receptor interaction (89). 

Tm 48°C respectively). This is consistent with a reduction step aiding heme release by weakening the complex prior to, or during, the actual release of heme effected by the hemopexin receptor. (The nondestructive mode of transport by hemopexin with recycling of intact protein demands mechanisms to reversibly lower the affinity of the complex.) With or without NaCl, lowering the pH from 7.4 to 6.5 has little effect... 

Fig. 16. Schematic depiction of the murine hemopexin receptor. The results of crosslinking studies (163) suggested that the murine receptor was composed of two subunits (a and p). The larger (65 kDa) is designated as a and is disulfide-linked to the smaller P subunit, which contains a free sulfhydryl. Hemopexin (which has no free sulfhydryl groups) can be crosslinked to the a-subunit but may also bind to the p-subunit.

Fig. 17. Hemopexin-mediated heme transport. The following is a proposed scenario for the transport of iron-porphyrins by hemopexin. Evidence is cited throughout the text that supports the major features of this working model. However, questions remain, and the molecular details of several features will require further efforts to elucidate. The role of the hemopexin receptor in activating signaling pathways is presented elsewhere (89) and thus is not considered here.

The heme—hemopexin—receptor complex is internalized via receptor-mediated endocytosis (160), presumably with the heme in the reduced state, and experiences a lower pH in the endosome. In the ferro-form, the acidic pH in the endosome may further weaken the association between heme and hemopexin, and heme is likely to be released at this... 

After transfer of heme to MHBP, either directly from hemopexin or from the hemopexin receptor, hemopexin and the receptor both recycle to the surface to undergo further rounds of transport. The heme inside the cell requires further intracellular trafficking to deliver heme to regulatory sites and to HO-1 for catabolism to biliverdin and iron, making intracellular transport an interesting focus of future research. The biliverdin is reduced and excreted as bilirubin, and the iron released, which can also have regulatory effects, is reutilized or stored on ferritin. 

It is tempting to speculate that the next comprehensive examination of hemopexin-mediated heme transport will have exciting new sections devoted to the molecular properties of MHBP and the hemopexin receptor, to the mechanism of heme release from hemopexin, to new intracellular heme transport partners, and to the links provided by the hemopexin system among heme, iron, and copper at the cellular level. 

Evidence, albeit indirect, for hemopexin receptor expression on ganglia comes from recent studies in rat adrenal phaechromocytoma (PC 12) cells in which HO-1 and MT-1 mRNA levels are induced upon incubation with heme-hemopexin (Figure 5-3). Heme-hemopexin can replace iron-transferrin as the sole source of nutrient iron and in activating protein kinase C providing evidence that hemopexin is potentially important in the regulation of cell growth under certain circumstances. When peripheral nerves are injured, e.g. axotomy of the sciatic nerve, he-... 

The isotope levels, ratios and changes with time in spleen and kidney in our study are consistent with a low level of heme uptake in these organs and some heme-hemopexin complexes reaching the bone marrow, but overall the data show that extra-hepatic uptake is low. It seems unlikely from the isotope recovery that endothelial cells lining the blood vessels express hemopexin receptors and we consider that hemopexin protects these cells by sequestering heme from them. 

It is not known whether iron-transferrin affects hemopexin receptor expression. However, the presence of iron transferrin attenuates the induction of MT-1 mRNA by heme-hemopexin in hepatoma cells, and ferric citrate is even more effective (L. Sung, M. Shibata, P. J. Morales, N. Shipulina, A. Smith, unpublished results). This suggests that the process of iron uptake into cells, which has been linked to electron transport for reduction of the iron, utilizes cofactors required for the hemopexin system. 

The cellular oxidation state is increased rapidly yet transiently by events associated with binding to the hemopexin receptor and hemopexin-mediated heme transport. High concentrations of heme-hemopexin raise the cell carbonyl content to approximately 7nmolmg protein which is equivalent to the level produced in response to 400pM H2O2 within 15 minutes. This oxidation is dose-dependent from 2-lOpM heme-hemopexin and declines after 30 minutes, presmnably due to rapid degradation since oxidized proteins are susceptible to proteolysis [118]. CoPP-hemopexin complexes which effectively bind to the hemopexin receptor but without tetrapyrrole uptake [42], do not increase carbonyl levels whereas free heme (2-10 jiM) does and to levels similar to those seen with heme-hemopexin. 

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