Palmitate modulates intracellular signaling, induces endoplasmic reticulum stress, and causes apoptosis in mouse 3T3-L1 and rat primary preadipocytes.
Although fatty acids enhance preadipocyte differentiation in the presence of adequate hormone cocktails, little is known regarding their effects in the absence of these hormones. We have now shown that palmitate, a common long-chain saturated fatty acid, induced apoptosis in both mouse 3T3-L1 and rat primary preadipocytes grown in a normal serum-containing medium. Treatment of preadipocytes with palmitate induced multiple endoplasmic reticulum (ER) stress responses, evidenced by increased protein content of CHOP and GRP78 and splicing of XBP-1 mRNA, as well as altered phosphorylation of eIF2alpha and increased phosphorylation of JNK and Erk1/2. Intriguingly, palmitate induced an early activation of Akt but diminished both Akt activation and its protein mass after prolonged incubation (>6 h). In association with these changes, palmitate reduced expression of beta-catenin and its downstream target, c-Myc and cyclin D1, two key prosurvival proteins. Overexpression of constitutively active Akt did not block the apoptotic effect of palmitate. Cotreatment with unsaturated fatty acids (oleate, linoleate) or with LiCl (a glycogen synthase kinase-3beta inhibitor) attenuated the palmitate-induced apoptosis. Subsequent analysis suggested that the unsaturated fatty acids probably counteracted palmitate by reducing, not eliminating, ER stress, whereas LiCl probably improved viability by activating the Wnt signaling pathway. Cotreatment of palmitate with a standard adipogenic hormone cocktail also abolished the apoptotic effect and promoted adipocyte differentiation. Collectively, our results suggest that palmitate causes multiple cellular stresses that may lead to apoptosis in preadipocytes in the absence of adipogenic stimuli, highlighting the importance of exogenous hormones in directing cell fate in response to increased fatty acid influx.
Guo
Wen
W
Dept. of Medicine, Boston Univ. School of Medicine, 670 Albany St., #207, Boston, MA 02118, USA. wguo@bu.edu
Wong
Siu
S
Xie
Weisheng
W
Lei
Tianluo
T
Luo
Zhijun
Z
D041721:3T3-L1 Cells; D017667:Adipocytes / Q000166:cytology / Q000187:drug effects*; D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D018929:Cell Culture Techniques; D002454:Cell Differentiation / Q000187:drug effects; D004721:Endoplasmic Reticulum / Q000187:drug effects* / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D019787:Linoleic Acid / Q000494:pharmacology; D018021:Lithium Chloride / Q000494:pharmacology; D051379:Mice; D019301:Oleic Acid / Q000494:pharmacology; D019308:Palmitic Acid / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects*; D013234:Stem Cells / Q000187:drug effects / Q000378:metabolism
D000091342:Endoplasmic Reticulum Chaperone BiP; C000717248:Hspa5 protein, mouse; D019301:Oleic Acid; D019308:Palmitic Acid; D007334:Insulin-Like Growth Factor I; D019787:Linoleic Acid; D018021:Lithium Chloride