Metformin inhibits heme oxygenase-1 expression in cancer cells through inactivation of Raf-ERK-Nrf2 signaling and AMPK-independent pathways

• Published in: Toxicology and applied pharmacology 2013-09, Vol.271 (2), p.229-238
• Main Authors: Do, Minh Truong, Kim, Hyung Gyun, Khanal, Tilak, Choi, Jae Ho, Kim, Dong Hee, Jeong, Tae Cheon, Jeong, Hye Gwang
• Format: Article
• Language: English
• Subjects: 60 APPLIED LIFE SCIENCES; AMP; ANTIOXIDANTS; Biological and medical sciences; Blotting, Western; CARCINOMAS; Cell Line, Tumor; Cell Nucleus - chemistry; Cell Survival - drug effects; CHEMOTHERAPY; Cytosol - chemistry; ERK; Galactosidases - metabolism; HEME; Heme Oxygenase-1 - antagonists & inhibitors; HO-1; Humans; Hypoglycemic Agents - pharmacology; Indicators and Reagents; LIVER; LUCIFERASE; Luciferases - metabolism; LUNGS; MAP Kinase Signaling System - drug effects; Medical sciences; MESSENGER-RNA; Metformin; Metformin - pharmacology; Mitogen-Activated Protein Kinases - drug effects; Neoplasms - enzymology; NF-E2-Related Factor 2 - physiology; Nrf2; Plasmids - genetics; RADIOTHERAPY; raf Kinases - drug effects; Real-Time Polymerase Chain Reaction; RNA - biosynthesis; RNA - genetics; RNA, Small Interfering; Signal Transduction - drug effects; SIGNALS; Toxicology; Transfection
• ISSN: 0041-008X; 1096-0333
• DOI: 10.1016/j.taap.2013.05.010

Resistance to therapy is the major obstacle to more effective cancer treatment. Heme oxygenase-1 (HO-1) is often highly up-regulated in tumor tissues, and its expression is further increased in response to therapies. It has been suggested that inhibition of HO-1 expression is a potential therapeutic approach to sensitize tumors to chemotherapy and radiotherapy. In this study, we tested the hypothesis that the anti-tumor effects of metformin are mediated by suppression of HO-1 expression in cancer cells. Our results indicate that metformin strongly suppresses HO-1 mRNA and protein expression in human hepatic carcinoma HepG2, cervical cancer HeLa, and non-small-cell lung cancer A549 cells. Metformin also markedly reduced Nrf2 mRNA and protein levels in whole cell lysates and suppressed tert-butylhydroquinone (tBHQ)-induced Nrf2 protein stability and antioxidant response element (ARE)-luciferase activity in HepG2 cells. We also found that metformin regulation of Nrf2 expression is mediated by a Keap1-independent mechanism and that metformin significantly attenuated Raf-ERK signaling to suppress Nrf2 expression in cancer cells. Inhibition of Raf-ERK signaling by PD98059 decreased Nrf2 mRNA expression in HepG2 cells, confirming that the inhibition of Nrf2 expression is mediated by an attenuation of Raf-ERK signaling in cancer cells. The inactivation of AMPK by siRNA, DN-AMPK or the pharmacological AMPK inhibitor compound C, revealed that metformin reduced HO-1 expression in an AMPK-independent manner. These results highlight the Raf-ERK-Nrf2 axis as a new molecular target in anticancer therapy in response to metformin treatment. •Metformin inhibits HO-1 expression in cancer cells.•Metformin attenuates Raf-ERK-Nrf2 signaling.•Suppression of HO-1 by metformin is independent of AMPK.•HO-1 inhibition contributes to anti-proliferative effects of metformin.