RESUMEN
Suppression of the expression or activities of enzymes that are involved in the synthesis of de novo lipogenesis (DNL) in cancer cells triggers cell death via apoptosis. The plasma membrane citrate transporter (PMCT) is the initial step that translocates citrate from blood circulation into the cytoplasm for de novo long-chain fatty acids synthesis. This study investigated the antitumor effect of the PMCT inhibitor (PMCTi) in inducing apoptosis by inhibiting the DNL pathway in HepG2 cells. The present findings showed that PMCTi reduced cell viability and enhanced apoptosis through decreased intracellular citrate levels, which consequently caused inhibition of fatty acid and triacylglycerol productions. Thus, as a result of the reduction in fatty acid synthesis, the activity of carnitine palmitoyl transferase-1 (CPT-1) was suppressed. Decreased CPT-1 activity also facilitated the disruption of mitochondrial membrane potential (ΔΨm) leading to stimulation of apoptosis. Surprisingly, primary human hepatocytes were not affected by PMCTi. Increased caspase-8 activity as a consequence of reduction in fatty acid synthesis was also found to cause disruption of ΔΨm. In addition, apoptosis induction by PMCTi was associated with an enhanced reactive oxygen species generation. Taken together, we suggest that inhibition of the DNL pathway following reduction in citrate levels is an important regulator of apoptosis in HepG2 cells via suppression of CPT-1 activity. Thus, targeting the DNL pathway mediating CPT-1 activity by PMCTi may be a selective potential anticancer therapy.
RESUMEN
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.