Hypoxia-inducible factor-1α (HIF-1α) and autophagy in polycystic kidney disease (PKD)

Abstract

Cyst expansion in polycystic kidney disease (PKD) results in localized hypoxia in the kidney that may activate hypoxia-inducible factor-1α (HIF-1α). HIF-1α and autophagy, a form of programmed cell repair, are induced by hypoxia. The purposes were to determine HIF-1α expression and autophagy in rat and mouse models of PKD. HIF-1α was detected by electrochemiluminescence. Autophagy was visualized by electron microscopy (EM). LC3 and beclin-1, markers of autophagy, were detected by immunoblotting. Eight-week-old male heterozygous (Cy/+) and 4-wk-old homozygous (Cy/Cy) Han:SPRD rats, 4-wk-old cpk mice, and 112-day-old Pkd2WS25/- mice with a mutation in the Pkd2 gene were studied. HIF-1α was significantly increased in massive Cy/Cy and cpk kidneys and not smaller Cy/+ and Pkd2WS25/- kidneys. On EM, features of autophagy were seen in wild-type (+/+), Cy/+, and cpk kidneys: autophagosomes, mitophagy, and autolysosomes. Specifically, autophagosomes were found on EM in the tubular cells lining the cysts in cpk mice. The increase in LC3-II, a marker of autophagosome production and beclin, a regulator of autophagy, in Cy/Cy and cpk kidneys, followed the same pattern of increase as HIF-1α. To determine the role of HIF-1α in cyst formation and/or growth, Cy/+ rats, Cy/Cy rats, and cpk mice were treated with the HIF-1α inhibitor 2-methoxyestradiol (2ME2). 2ME2 had no significant effect on kidney volume or cyst volume density. In summary, HIF-1α is highly expressed in the late stages of PKD and is associated with an increase in LC3-II and beclin-1. The first demonstration of autophagosomes in PKD kidneys is reported. Inhibition of HIF-1α did not have a therapeutic effect.

Autophagy in renal diseases

“…Treating the cells with rapamycin led to a higher cell survival [74]. Taken together, these studies suggest an impairment of aut o p h ag y i n A D P K D , r es u l t i ng f r om a bl oc k o f autophagosome-lysosome fusion and degradation [41,73], confirming the earlier stated hypothesis that suppression of autophagic flux may be a feature of ADPKD. However, the mechanisms of crosstalk between autophagy induction, proliferation, and apoptosis in ADPKD still remain to be unraveled.…”

“…Second, localised hypoxia is present in ADPKD because of cyst expansion. This was demonstrated by higher levels of erythropoietin in ESRD ADPKD patients, and by higher levels of hypoxia-inducible factor-1α (HIF-1α) in renal cyst lining epithelial cells of different rodent disease models with varying severity of cyst formation [73]. Moreover, evidence is mounting that apoptotic cell death promotes cyst growth in polycystic kidney disease [41].…”

“…Moreover, evidence is mounting that apoptotic cell death promotes cyst growth in polycystic kidney disease [41]. Next, in tubular cyst-lining cells, the presence of autophagosomes and LC3-II was observed [73]. However, treatment with the lysosomal degradation inhibitor bafilomycin A1 in congenital polycystic kidney mice did not have an effect on LC3-II levels, while in wild-type mice it resulted in a significant increase of LC3-II.…”

Reciprocal regulation of cilia and autophagy via the MTOR and proteasome pathways

“…19 In 2011, Belibi et al reported that autophagy is suppressed in Han:SPRD rats and cpk mice, 2 animal models of PKD with ciliary dysfunction. 20 Based on this study and our pilot observations, we hypothesized that there may be a regulatory connection between autophagy and ciliogenesis. Very recently, 2 studies have demonstrated the direct interaction between autophagy and ciliary regulation.…”

Autophagy and regulation of cilia function and assembly

“…Ciliary growth leads to activation of signaling pathways that promote recruitment of specific ATG proteins to complexes already present at the ciliary base to promote maximal activation of inducible autophagy. (3) If starvation is sustained, there is a switch toward IFT20 degradation to prevent unlimited growth of the cilia and excessive activation of the autophagic process 93 showed that the presence of autophagosomes in cystic kidneys in response to hypoxia is due to a blockage of autophagic flux. In addition, mouse kidney cells that express a mutated PKD1 failed to induce autophagy in response to glucose starvation.…”