Galangin inhibits proliferation of HepG2 cells by activating AMPK via increasing the AMP/TAN ratio in a LKB1-independent manner

Abstract

Galangin, a flavonol derived from Alpinia officinarum Hance and used as food additives in southern China, induces apoptosis and autophagy to suppress the proliferation of HepG2 cells. In this study, we demonstrated that galangin induced autophagy by increasing the ratio of AMP/TAN in HepG2 cells. It stimulated the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) and LKB1, but inhibited the phosphorylation of AKT and mTOR. Inhibition of AMPK activation suppressed the dephosphorylation of mTOR to block galangin-induced autophagy. AMPK activation by galangin appeared to be independent of the LKB1 signaling pathway because the down-regulation of LKB1 by its siRNA failed to affect galangin-induced autophagy. Collectively, the findings demonstrated a novel mechanism of how galangin induces autophagy via activating AMPK in a LKB1- independent manner. The induction of autophagy can thus reflect the anti-proliferation effect of galangin in HCC cells.

Introduction

Ferlay et al. (2010) and Cauchy et al. (2012) have reported that hepatocellular carcinoma (HCC) is one of the most common cancers. Although chemotherapy is a common therapeutic strategy after surgery, its use has been limited due to its toxicity to normal tissues. Therefore, it is important to screen for new nontoxic anti-cancer drugs. Natural products have been used for preventing cancer for centuries and are thus envisioned as safer alternatives for their chemical counterparts (Surh, 2003). Some dietary compounds possess anti-cancer properties such as suppressing cell proliferation and inducing cell death. Natural compounds are therefore used to enhance the efficacy of chemotherapeutic agents (Surh, 2003).

Alpinia officinarum Hance is used as a traditional Chinese medicine and about 90% Alpinia officinarum Hance in China is from Zhanjiangin regions. Galangin is a polyphenolic compound derived primarily from the rhizome of Alpinia officinarum Hance\ (Heo et al., 2001). The molecular weight of galangin is about 270.24 Da. We have shown that galangin significantly decreases cell viability and induces apoptosis of hepatocellular carcinoma cells (HCC) lines and suggested that galangin may be a potential anti-HCC agent (Zhang et al., 2012). Furthermore, a report by Wen et al. (2012) have described galangin-induced intracellular vacuolation in HepG2 cell and suggested that these vacuoles represent autophagic vesicles. These observations suggest that galangin may inhibit the proliferation of HepG2 cells by a novel mechanism. However, the precise mechanism by which galangin induces autophagy remains unclear. In this study, we demonstrated the effects of galangin on HCC cells and elucidated the mechanism by which galangin inhibits HepG2 cell proliferation.