MO455: Fatty Acid-Associated Albumin Induces Proliferation and Senescence of Proximal Tubular Cells

Abstract

Abstract BACKGROUND AND AIMS Proteinuria is a major risk factor for the progression of chronic kidney disease. Protein overload in the proximal tubular epithelial cells causes oxidative stress, lysosomal dysfunction, inflammation and apoptosis, resulting in proximal tubule dysfunction. Recent studies have focussed on the association between proximal tubule injury and cellular senescence and the development of drugs targeting and removing senescent proximal tubular cells. Senescent cells show resistance to apoptosis and persistently secrete inflammatory cytokines, resulting in chronic inflammation. It has been suggested that excess proliferation induced by fatty acids causes senescence of proximal tubular cells. However, the association between protein overload, proliferation and cellular senescence in proximal tubular cells remains unclear. The present study aimed to clarify the effect of protein overload on cell proliferation and senescence in a proteinuric mouse model and in an immortalized proximal tubular epithelial cell line. Moreover, it was evaluated whether the endocytic receptors for protein uptake, megalin and cubilin, affect protein overload-induced proliferation and senescence using knockout (KO) of megalin and cubilin in the proteinuric mouse model. METHOD Experiments were performed using podocin-KO (proteinuric mouse model) and megalin-cubilin-podocin triple-KO mice. Kidneys from these mice were analysed using immunohistochemical and immunofluorescent staining. Immortalized proximal tubular cells (RPTEC/TERT1, ATCC) were used for in vitro experiments, wherein RPTEC/TERT1 cells were incubated with 0.1–10 mg/mL of human serum albumin (HSA; Sigma), fatty acid-free HSA (Sigma), and transferrin (Sigma). Western blotting, quantitative rt-PCR, senescence-associated beta-galactosidase (SA-β-gal) staining and immunofluorescence were performed to analyse cellular senescence. The effect of a PKC activator (phorbol 12-myristate 13-acetate) and an inhibitor (Go6983) on proliferation and senescence was also evaluated. RESULTS The proliferation markers EdU incorporation, PCNA (Figure 1) and Ki-67 were detected in proximal tubular cells of podocin-KO mice, but not in wild-type mice. In triple-KO mice, a decrease in PCNA-positive tubules was observed. This suggests that protein reabsorption via megalin and cubilin provoked cell proliferation. Light microscopy analysis and a proliferation assay with BrdU incorporation revealed that HSA overload-induced the proliferation of RPTEC/TERT1 cells, whereas fatty acid-free HSA or transferrin had no effect on the proliferation of RPTEC/TERT1 cells. Western blot analysis showed that HSA treatment, but not fatty acid-free HSA treatment, induced alterations in proliferation (PCNA and Ki-67), cell cycle (cyclin A, D, and Rb), cellular senescence (p21 and p16), and DNA injury (γ-H2AX). Using immunofluorescence, an increase in p21 and p16 expression was also observed in HSA-treated cells. Moreover, the HSA-treated cells showed positive staining for SA-β-gal. These results suggest that fatty acids bound to HSA induce cell proliferation and senescence. Furthermore, results showed that a PKC inhibitor suppressed HSA-induced proliferation and senescence, while a PKC activator accelerated these alterations without HSA treatment. CONCLUSION The present study showed that fatty acid-associated albumin induced proliferation and senescence of the proximal tubule cells, which were dependent on megalin/cubilin endocytosis of filtered protein. PKC activation is at least in part related to cell proliferation and senescence. It is unknown which molecular switch determine the cell cycle fate.