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Glucosamine‐Induced Sp1 O‐GlcNAcylation Ameliorates Hypoxia‐Induced SGLT Dysfunction in Primary Cultured Renal Proximal Tubule Cells

The aim of this study is to determine whether GlcN could recover the endoplasmic reticulum (ER) stress‐induced dysfunction of Na+/glucose cotransporter (SGLT) in renal proximal tubule cells (PTCs) under hypoxia. With the rabbit model, the renal ischemia induced tubulointerstitial abnormalities and decreased SGLTs expression in tubular brush‐border, which were recovered by GlcN. Thus, the protective mechanism of GlcN against renal ischemia was being examined by using PTCs. Hypoxia decreased the level of protein O‐GlcNAc and the expression of O‐GlcNAc transferase (OGT) while increased O‐GlcNAcase (OGA) and these were reversed by GlcN. Hypoxia also decreased the expression of SGLTs (SGLT1 and 2) and [14C]‐α‐methyl‐D‐glucopyranoside (α‐MG) uptake which were recovered by GlcN and PUGNAc (OGA inhibitor). Hypoxia enhanced reactive oxygen species (ROS) and then ER stress proteins, glucose‐regulated protein 78 (GRP78), and C/EBP‐homologous protein (CHOP). However, the expression of GRP78 increased till 6 h and then decreased whereas CHOP increased gradually. Moreover, decreased GRP78 and increased CHOP were reversed by NAC (antioxidant) and GlcN. GlcN ameliorated hypoxia‐induced decrease of O‐GlcNAc modification of Sp1 but OGT or Sp1 siRNAs blocked the recovery effect of GlcN on SGLT expression and α‐MG uptake. In addition, hypoxia‐decreased GRP78 and HIF‐1α expression was reversed by GlcN but OGT siRNA or Sp1 siRNA ameliorated the effect of GlcN. When PTCs were transfected with GRP78 siRNA or HIF‐1α siRNA, SGLT expression and α‐MG uptake was decreased. Taken together, these data suggest that GlcN‐induced O‐GlcNAc modified Sp1 with stimulating GRP78 and HIF‐1α activity ameliorate hypoxia‐induced SGLT dysfunction in renal PTCs. J. Cell. Physiol. 229: 1557–1568, 2014. © 2014 Wiley Periodicals, Inc.