Hemopexin Induces Nephrin-Dependent Reorganization of the Actin Cytoskeleton in Podocytes

Abstract

Hemopexin is an abundant plasma protein that effectively scavenges heme. When infused into rats, hemopexin induces reversible proteinuria, and activated hemopexin is increased in children with minimal change nephrotic syndrome. These observations suggest a role for hemopexin in glomerular disease; in this study, the effects of active hemopexin on human podocytes and glomerular endothelial cells, the two cell types that compose the glomerular filtration barrier, were investigated. Within 30 min of treatment with hemopexin, actin reorganized from stress fibers to cytoplasmic aggregates and membrane ruffles in wild-type podocytes. This did not occur in nephrin-deficient podocytes unless they were transfected with nephrin-expressing plasmids. Furthermore, hemopexin did not affect actin organization in cells that do not express nephrin, specifically human glomerular endothelial cells, fibroblasts, and HEK293 cells. The effects of hemopexin on wild-type podocytes reversed within 4 h and were inhibited by preincubation with human plasma. Treatment with hemopexin activated protein kinase B in both wild-type and nephrin-deficient podocytes but activated RhoA only in wild-type cells. In addition, hemopexin led to a selective increase in the passage of albumin across monolayers of glomerular endothelial cells and to a reduction in glycocalyx. In summary, active hemopexin causes nephrindependent remodeling of podocytes and affects permeability of the glomerular filtration barrier by degrading the glycocalyx.

Circulating Permeability Factors in Idiopathic Nephrotic Syndrome and Focal Segmental Glomerulosclerosis

“…Its secretion is enhanced by IL-2, IL-15, IL-12, and IL-18 and is inhibited by TGF␤1 (13). Hemopexin is a protease that activates protein kinase B and Rho A and induces nephrin-dependent reorganization of the actin cytoskeleton in cultured podocytes (14). It reduces endothelial glycocalyx and increases albumin diffusion across glomerular endothelial cell monolayers (14).…”

“…Hemopexin is a protease that activates protein kinase B and Rho A and induces nephrin-dependent reorganization of the actin cytoskeleton in cultured podocytes (14). It reduces endothelial glycocalyx and increases albumin diffusion across glomerular endothelial cell monolayers (14). Injection into rats causes proteinuria and glomerular alterations characteristic of MCNS (15).…”

An alternative view of the proposed alternative activities of hemopexin

“…79 Subsequently, this group reported that 100KF/ hemopexin preparations from human serum also reduced staining of ecto-ATPases in glomeruli of kidney sections 44 as well as induced proteinuria and fusion of epithelial foot processes in glomerular capillary segments following intra-renal infusion in vivo in rats. 80 This latter observation was expanded in a recent study 24 that used cultured epithelial cells (podocytes) to provide evidence that led to the conclusion that their hemopexin preparations cause reorganization of actin fibers in podocytes through a nephrin-dependent signaling cascade. Various types of hemopexin preparations have been used in publications from the Bakker group, including recombinant hemopexin, and each merits consideration.…”

“…22 The NMR spectra of the recombinant proteins included in that report provide the most extensive characterization of any recombinant hemopexin yet reported. Subsequently, some authors have reported the use of recombinant human hemopexin they expressed in Pichia pastoris 23,24 or they obtained from vendors that expressed the protein in mammalian cell culture. 25 Unfortunately, these recombinant forms of hemopexin remain essentially uncharacterized.…”

“…proposed that hemopexin in normal human plasma occurs in an inactive form 23,24 that is activated during isolation to a form of hemopexin with a serine protease activity similar to that detected in the plasma of MCNS patients in relapse 83 or with other on-going pathology. 84 Recently, this group also reported that the plasma of pregnant (but not pre-eclamptic) women produces reduced staining of glomerular ectoapyrase 82,85 and that the plasma of some renal transplant recipients has increased protease activity.…”