Docosahexaenoic acid, an omega‐3 fatty acid, induces autophagy in human breast cancer cells: role of oxidative stress‐induced growth inhibitor 1

Oxidative stress‐induced growth inhibitor 1 (OSGIN1) was identified as not only an oxidative stress response gene but also a tumor suppressor gene. Docosahexaenoic acid (DHA), a long‐chain omega‐3 polyunsaturated fatty acid that abounds in oily fish, is well documented to exhibit antitumor activity. It has been shown DHA induces both apoptosis and autophagy in cancer cells. The aim of this study was to determine whether OSGIN1 is involved in the induction of autophagy and apoptosis by DHA in human breast cancer cells and the possible mechanisms involved. MCF‐7 cells were grown to 70–80 % confluence and were then treated with or without DHA and OSGIN1 siRNA for the times indicated. Afterward, the proapoptotic protein, such as Bcl2, Bax, and cytochrome c as well as the autophagy‐related markers, including LC3B‐I, LC3B‐II, mTOR, Beclin‐1, and p62 were determined by Western Blotting. Moreover, the index of autophagy and apoptosis were assayed by using Premo™ Autophagy Tandem Sensor RFP‐GFP‐LC3B Kit and FITC Annexin V Apoptosis Detection Kit, respectively. In this study, 100 μM DHA dramatically upregulated OSGIN1 protein and mRNA expression as well as LC3B‐1/‐II and p62 level, whereas the level of mTOR and Beclin‐1 was decreased by DHA in a dose‐dependent manner. After knockdown of OSGIN1 expression by siRNA transfection, DHA‐mediated LC3B‐I and LC3B‐II expression was abrogated. OSGIN1 siRNA alleviated the suppression of mTOR and Beclin1 expression by DHA. Overexpression of OSGIN1 by transient transfection of plasmid OSGIN1 significantly increases LC3B‐II and autophagy via activation of AMPK/Raptor and inactivation mTOR/ULK ser757 signaling pathways. Furthermore, OSGIN1 overexpression mimics DHA decreases Bcl2, elevates Bax expression and cytochrome c release from mitochondria. Taken together, these results suggest that DHA induces OSGIN1 expression, and subsequently promotes autophagy through activation of AMPK/mTOR signaling pathway. Overexpression of OSGIN1 partially contributes to DHA‐mediated cytochrome c release and apoptosis in breast cancer cells.