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mediated remodelling of the actin cytoskeleton is involved in constitutive albumin uptake by proximal tubule cells
1 School of Biomedical Sciences, University of Queensland, Brisbane, Qld, Australia; Department of Medicine, Kolling Institute of Medical Research, University of Sydney, Sydney, NSW, Australia
2 Department of Medicine, Kolling Institute of Medical Research, University of Sydney, Sydney, NSW, Australia
3 School of Biomedical Sciences, University of Queensland, Brisbane, Qld, Australia
* To whom correspondence should be addressed. E-mail: p.poronnik{at}uq.edu.au.
One key role of the renal proximal tubule is the reabsorption of proteins from the glomerular filtrate by constitutive receptor-mediated endocytosis. In the opossum kidney (OK) renal proximal tubule cell line, inhibition of protein kinase C (PKC) reduces albumin uptake, although the isoforms involved and mechanisms by which this occurs have not been identified. We used pharmacological and molecular approaches to investigate the role of PKC
in albumin endocytosis. We found that albumin uptake in OK cells was inhibited by the pan-PKC blocker BIM-1 and the isoform specific PKC blockers Go6976 and HBDDE, indicating a role for PKC. Over-expression of a kinase deficient PKC(K368R) but not wild type PKC significantly reduced albumin endocytosis. Western blot analysis of fractionated cells showed an increased association of PKC -GFP with the membrane fraction within 10-20 min of exposure to albumin. We used phalloidin to demonstrate that albumin induces the formation of clusters of actin at the apical surface of OK cells and that these clusters correspond to the location of albumin uptake. These clusters were not present in cells grown in the absence of albumin. In cells treated either with PKC inhibitors or over-expressing kinase deficient PKC(K368R) this actin cluster formation was significantly reduced. This study identifies a role for PKC in constitutive albumin uptake in OK cells by mediating assembly of actin microfilaments at the apical membrane.
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