Combination of Mitochondrial and Plasma Membrane Citrate Transporter Inhibitors Inhibits De Novo Lipogenesis Pathway and Triggers Apoptosis in Hepatocellular Carcinoma Cells.

Wan-Angkan Poolsri,Phornpun Phokrai,Pattamaphorn Phunsomboon,Somrudee Suwankulanan,Dumrongsak Pekthong,Narinthorn Phakdeeto,Lysiane Richert,Piyarat Srisawang,Sutatip Pongcharoen

doi:10.1155/2018/3683026

Wan-Angkan Poolsri, Phornpun Phokrai + Show 7 more

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https://doi.org/10.1155/2018/3683026

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Abstract

Increased expression levels of both mitochondrial citrate transporter (CTP) and plasma membrane citrate transporter (PMCT) proteins have been found in various cancers. The transported citrates by these two transporter proteins provide acetyl-CoA precursors for the de novo lipogenesis (DNL) pathway to support a high rate of cancer cell viability and development. Inhibition of the DNL pathway promotes cancer cell apoptosis without apparent cytotoxic to normal cells, leading to the representation of selective and powerful targets for cancer therapy. The present study demonstrates that treatments with CTP inhibitor (CTPi), PMCT inhibitor (PMCTi), and the combination of CTPi and PMCTi resulted in decreased cell viability in two hepatocellular carcinoma cell lines (HepG2 and HuH-7). Treatment with citrate transporter inhibitors caused a greater cytotoxic effect in HepG2 cells than in HuH-7 cells. A lower concentration of combined CTPi and PMCTi promotes cytotoxic effect compared with either of a single compound. An increased cell apoptosis and an induced cell cycle arrest in both cell lines were reported after administration of the combined inhibitors. A combination treatment exhibits an enhanced apoptosis through decreased intracellular citrate levels, which consequently cause inhibition of fatty acid production in HepG2 cells. Apoptosis induction through the mitochondrial-dependent pathway was found as a consequence of suppressed carnitine palmitoyl transferase-1 (CPT-1) activity and enhanced ROS generation by combined CTPi and PMCTi treatment. We showed that accumulation of malonyl-CoA did not correlate with decreasing CPT-1 activity. The present study showed that elevated ROS levels served as an inhibition on Bcl-2 activity that is at least in part responsible for apoptosis. Moreover, inhibition of the citrate transporter is selectively cytotoxic to HepG2 cells but not in primary human hepatocytes, supporting citrate-mediating fatty acid synthesis as a promising cancer therapy.

Highlights

Hepatocellular carcinoma (HCC) is a principal common global cause of cancer deaths and the fifth most frequent malignancy in patients with cirrhosis
Intermediates from oxidative phosphorylation (OXPHOS) are redirected into the de novo lipogenesis (DNL) pathway to provide precursors for long chain fatty acids (LCFAs) synthesis prevailing in cancer cells while for most normal cells their lipids come from the abundant levels in the circulation
The DNL pathway uses cytosolic citrate exported from mitochondria and transported from circulation into the cytoplasm which is converted to acetyl-CoA by ATP-citrate lyase (ACLY), followed by carboxylation to form malonyl-CoA by acetylCoA carboxylase (ACC)

Summary

Introduction

Hepatocellular carcinoma (HCC) is a principal common global cause of cancer deaths and the fifth most frequent malignancy in patients with cirrhosis. The reprogramming of energy pathways in cancers, switching the major metabolism pathway from oxidative phosphorylation (OXPHOS) to rely on aerobic glycolysis, is known as the Warburg effect [4, 5]. This hallmark feature promotes increased glucose uptake and intermediate flux for de novo synthesized biomolecules, including nucleotide, amino acids, and lipids to support high tumor proliferative and progression rate phenotypes of cancer [6, 7]. Intermediates from OXPHOS are redirected into the de novo lipogenesis (DNL) pathway to provide precursors for long chain fatty acids (LCFAs) synthesis prevailing in cancer cells while for most normal cells their lipids come from the abundant levels in the circulation. Anticancer therapy targeting the DNL enzymes has been extensively studied to become one of the most efficient therapies by promoting cancer cell apoptosis without affecting nontransformed cells [11]

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