Objective To investigate whether eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) improve lipid metabolism in 3T3-L1 adipocytes, by activating the AMPK and STAT3 signaling pathways via myogenic interleukin-6 (IL-6), respectively. Methods 1. C2C12 myotubes were treated with 50, 100, 200, or 400 μmol/L EPA or DHA for 24 h. After intervention, myotubes were co-cultured with mature adipocytes for 24 h. RT-PCR was used to detect mRNA expression of IL-6, ADAM10, ADAM17, and SOCS3 in myotubes. ELISA was performed to measure IL-6 content in the co-culture medium. 2. Myotubes were transfected with IL-6 siRNA, then treated with 200 μmol/L EPA or DHA. Meanwhile, adipocytes were pretreated for 4 h with the AMPK inhibitor dorsomorphin (compound C) or the STAT3 inhibitor FLLL32. The treated myotubes and adipocytes were then co-cultured for 24 h. RT-PCR was used to detect mRNA expression of ATGL, HSL, PGC-1α, and SREBP-1c in adipocytes. Western blot was performed to analyze protein expression of IL-6, AMPK, p-AMPK, STAT3, and p-STAT3 in adipocytes. Results EPA (100, 200, 400 μmol/L) and DHA (200, 400 μmol/L) significantly upregulated IL-6, ADAM10, and ADAM17 expression in myotubes (P<0.01). Only DHA increased SOCS3 mRNA expression (P<0.05). IL-6 secretion was significantly increased by all concentrations except 50 μmol/L DHA (P<0.05 or P<0.01). Myotubes treated with 200 μmol/L EPA or DHA specifically increased the p-STAT3/STAT3 ratio (P<0.05) and p-AMPK/AMPK ratio (P<0.01) in co-cultured adipocytes, respectively. This effect was abolished by IL-6 silencing (P>0.05). EPA-treated myotubes significantly upregulated ATGL, PGC-1α, and SREBP-1c expression in adipocytes (P<0.05 or P<0.01). IL-6 silencing inhibited ATGL upregulation (P<0.01) but did not affect the expression of PGC-1α and SREBP-1c. Inhibition with FLLL32 (alone or combined with IL-6 siRNA) significantly reduced the expression of ATGL, PGC-1α, and SREBP-1c compared to the EPA intervention group (P<0.05 or P<0.01). DHA-treated myotubes significantly upregulated ATGL, HSL, and PGC-1α (P<0.05 or P<0.01) while downregulating SREBP-1c (P<0.01) in adipocytes. Inhibition with compound C (alone or combined with IL-6 siRNA) significantly reduced ATGL, HSL, and PGC-1α expression (P<0.01) but increased SREBP-1c expression compared to the DHA intervention group (P<0.05 or P<0.01). Conclusion EPA and DHA at appropriate concentrations promote myogenic IL-6 secretion. EPA improves lipid metabolism primarily by activating the STAT3 pathway in adipocytes via IL-6, while DHA exerts its effects on lipid metabolism mainly through the IL-6-mediated AMPK pathway.
Key words
eicosapentaenoic acid (EPA) /
docosahexaenoic acid (DHA) /
lipid metabolism /
myogenic IL-6 /
AMPK/STAT3 signaling pathway
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