Activation of AMP-activated protein kinase (AMPK) mediates plumbagin-induced apoptosis and growth inhibition in cultured human colon cancer cells

Abstract

Here we report that activation of AMP-activated protein kinase (AMPK) mediates plumbagin-induced apoptosis and growth inhibition in both primary cultured human colon cancer cells and cell lines. Knocking-down of AMPKα by the target shRNA significantly inhibits plumbagin-induced cytotoxicity in cultured colon cancer cells, while forced activation of AMPK by introducing a constitutively active AMPK (CA-AMPK), or by the AMPK activator, inhibits HT-29 colon cancer cell growth. Our Western-blots and immunoprecipitation (IP) results demonstrate that plumbagin induces AMPK/Apoptosis signal regulating kinase 1 (ASK1)/TNF receptor-associated factor 2 (TRAF2) association to activate pro-apoptotic c-Jun N-terminal kinases (JNK)-p53 signal axis. Further, after plumbagin treatment, activated AMPK directly phosphorylates Raptor to inhibit mTOR complex 1 (mTORC1) activation and Bcl-2 expression in colon cancer cells. Finally, we found that exogenously-added short-chain ceramide (C6) enhances plumbagin-induced AMPK activation and facilitates cell apoptosis and growth inhibition. Our results suggest that AMPK might be the key mediator of plumbagin's anti-tumor activity.

Keywords: 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside; ACC; AICAR; AMP-activated protein kinase; AMP-activated protein kinase (AMPK); AMPK; ASK1; Colon cancer; FACS; IP; JNK; NAC; PI; Plumbagin; RNA interference; RNAi; ROS; Raptor; S6K1; TNF receptor-associated factor 2; TRAF2; TSC2; UA; acetyl-CoA carboxylase; apoptosis signal regulating kinase 1; c-Jun N-terminal kinase; fluorescence-activated cell sorting; immunoprecipitation; mTOR complex 1; mTORC1; n-acetyl-l-cysteine; propidium iodide; reactive oxygen species; regulatory associated protein of mTOR; ribosomal p70 S6 kinase; tuberous sclerosis protein 2; ursolic acid.