title: Apigenin inhibits indoxyl sulfate-induced endoplasmic reticulum stress and anti-proliferative pathways, CHOP and IL-6/p21, in human renal proximal tubular cells.
abstract: OBJECTIVE
  Indoxyl sulfate (IS) has been reported to induce endoplasmic reticulum (ER) stress in tubular cells and to inhibit the cell proliferation via ER stress and ERK/IL-6/p21 pathways. This study has investigated the effect of apigenin on IS-induced ER stress in immortalized human renal proximal tubular HK-2 cells.
  
  
  METHODS
  Human Kidney 2 (HK-2) cells were treated with IS (5 mM) in the absence or presence of apigenin (10 μM) or salubrinal (20 μM) for indicated times under the serum-free condition. Cell viability was evaluated by MTT assay. The levels of protein expression and phosphorylation were evaluated by Western blot analysis.
  
  
  RESULTS
  In HK-2 cells, apigenin completely inhibited IS-induced ER stress, as indicated by decreased expression of CHOP, ATF4 and GRP78, although the phosphorylated level of eIF2α did not decrease. IS-induced expression levels of IL-6 and p21 proteins were also inhibited by apigenin, with no significant changes in ERK activation. The suppression of cell proliferation by IS was abolished by salubrinal, an ER stress inhibitor, but not by apigenin. Apigenin inhibited the phosphorylation of Akt and GSK-3β in IS-treated HK-2 cells. The phosphorylation of GSK-3β, which was inhibited by apigenin, resulted in hypo-phosphorylation of retinoblastoma (Rb) protein, which was associated with the decrease in cyclin D1 expression.
  
  
  CONCLUSIONS
  These results suggest that apigenin may inhibit IS-induced ER stress and expression of IL-6 and p21 proteins in HK-2 cells. It is most likely that apigenin, together with its inhibitory effect on ER stress, may also suppress the cell growth by inducing the loss of Rb phosphorylation, which was associated with the decrease in cyclin D1 expression by GSK-3β activation through the inhibition of PI3K/Akt pathway.
authors.affiliation: Department of Pathology, College of Korean Medicine, Wonkwang University, Iksan, Jeonbuk, Republic of Korea. omdjbh@wku.ac.kr.
authors.forename: B-J
authors.initials: BJ
authors.lastname: Jeon
authors.forename: H-M
authors.initials: HM
authors.lastname: Yang
authors.forename: Y-S
authors.initials: YS
authors.lastname: Lyu
authors.forename: H-O
authors.initials: HO
authors.lastname: Pae
authors.forename: S-M
authors.initials: SM
authors.lastname: Ju
authors.forename: B-H
authors.initials: BH
authors.lastname: Jeon
mesh_terms: D047310:Apigenin / Q000494:pharmacology*; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology; D002461:Cell Line, Transformed; D049109:Cell Proliferation / Q000187:drug effects / Q000502:physiology; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D000091342:Endoplasmic Reticulum Chaperone BiP; D059865:Endoplasmic Reticulum Stress / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007200:Indican / Q000037:antagonists & inhibitors / Q000633:toxicity*; D015850:Interleukin-6 / Q000096:biosynthesis*; D007687:Kidney Tubules, Proximal / Q000187:drug effects / Q000378:metabolism*; D051742:Transcription Factor CHOP / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D000093242:Interleukin-6 Inhibitors
chemical_list: D047310:Apigenin; D050759:Cyclin-Dependent Kinase Inhibitor p21; D000091342:Endoplasmic Reticulum Chaperone BiP; D007200:Indican; D015850:Interleukin-6; D051742:Transcription Factor CHOP; D000093242:Interleukin-6 Inhibitors; C493956:CDKN1A protein, human; C497249:DDIT3 protein, human; C000717247:HSPA5 protein, human; C508600:IL6 protein, human