PI3K signaling mediates diverse regulation of ATF4 expression for the survival of HK-2 cells exposed to cadmium.
Cadmium exposure causes endoplasmic reticulum (ER) stress and accumulation of activating transcription factor 4 (ATF4), an ER stress marker. To elucidate the role of phosphatidylinositol-3-kinase (PI3K) signaling in this process, we examined the effects of PI3K signaling on cadmium chloride (CdCl2) exposure-induced ATF4 expression in HK-2 human renal proximal tubular cells. ATF4 knockdown by siRNA enhanced CdCl2-induced cellular damage, indicating a cytoprotective function of ATF4. Treatment with LY294002, a PI3K inhibitor, suppressed CdCl2-induced ATF4 expression and Akt phosphorylation at Thr308 with little effect on phosphorylation of eukaryotic translation initiation factor 2 subunit α at Ser51. Activation of PI3K signaling with epidermal growth factor treatment enhanced CdCl2-induced Akt phosphorylation and ATF4 expression. Suppression of CdCl2-induced ATF4 expression by LY294002 treatment was markedly blocked by cycloheximide, a translation inhibitor, but not by MG-132, a proteasome inhibitor, or actinomycin D, a transcription inhibitor. CdCl2 exposure also induced phosphorylation of mammalian target of rapamycin (mTOR) at Ser2448, glycogen synthase kinase-3α (GSK-3α) at Ser21, GSK-3β at Ser9, and 90 kDa ribosomal S6 kinase 2 (RSK2) at Ser227 in HK-2 cells. Treatment with rapamycin, an mTOR inhibitor, MK2206, an Akt inhibitor, and BI-D1870, a RSK inhibitor, partially suppressed CdCl2-induced ATF4 expression. Conversely, SB216763, a GSK-3 inhibitor, markedly inhibited the potency of LY294002 to suppress CdCl2-induced ATF4 expression. These results suggest that PI3K signaling diversely regulates the expression of ATF4 in a translation-dependent manner via downstream molecules, including mTOR, GSK-3α/β, and RSK2, and plays a role in protecting HK-2 cells from cadmium-induced damage.
Department of Hygiene and Public Health I, Tokyo Women's Medical University, Tokyo, 162-8666, Japan.
Kota
K
Fujiki
Hisako
H
Inamura
Masato
M
Matsuoka
D051701:Activating Transcription Factor 4 / Q000235:genetics / Q000378:metabolism*; D002104:Cadmium / Q000633:toxicity*; D019256:Cadmium Chloride / Q000633:toxicity; D002460:Cell Line / Q000187:drug effects; D002867:Chromones / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D055785:Gene Knockdown Techniques; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D006801:Humans; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D009025:Morpholines / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism
D051701:Activating Transcription Factor 4; D002104:Cadmium; D019256:Cadmium Chloride; D002867:Chromones; D004791:Enzyme Inhibitors; D038362:Glycogen Synthase Kinase 3; D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D051057:Proto-Oncogene Proteins c-akt; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D058570:TOR Serine-Threonine Kinases; C497109:ATF4 protein, human; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C546842:MTOR protein, human; C069330:ribosomal protein S6 kinase, 90kDa, polypeptide 3