Suppressed de novo lipogenesis by plasma membrane citrate transporter inhibitor promotes apoptosis in HepG2 cells

Abstract

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 ( PMCT i) in inducing apoptosis by inhibiting the DNL pathway in HepG2 cells. The present findings showed that PMCT i 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 PMCT i. 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 PMCT i 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 PMCT i may be a selective potential anticancer therapy.

NaCT/SLC13A5 facilitates citrate import and metabolism under nutrient-limited conditions

“…No major changes in metabolism were observed beyond acetyl-CoA and TCA metabolism, and phenotypic responses were dependent upon expression of functional NaCT as well as citrate supplementation. Importantly, some prior studies observed signaling and growth phenotypes in cells upon knockdown or inhibition of NaCT in the absence of extracellular citrate (Li et al, 2017;Phokrai et al, 2018;Poolsri et al, 2018), but our results indicate that citrate transport is a key function of the SLC13A5 gene product. Indeed, we also found that citrate uptake by NaCT was protective against Zn 2+ cytotoxicity in Huh7 cells.…”

“…Additionally, hepatic SLC13A5 expression correlated positively with both body and liver fat in a cohort of non-alcoholic fatty liver disease (NAFLD) patients (von Loeffelholz et al, 2017). Functional impacts of NaCT inhibition or knockdown have been proposed in cells, although findings are not necessarily dependent on the presence of extracellular citrate (Li et al, 2017;Phokrai et al, 2018;Poolsri et al, 2018). Citrate transport may also influence acetyl-CoA metabolism and ionic homeostasis in the nervous system, as loss-of-function mutations in SLC13A5 have been linked to pediatric epilepsy, Kohlsch€ utter-Tö nz syndrome, and other brain disorders (Hardies et al, 2015;Klotz et al, 2016;Matricardi et al, 2020;Schossig et al, 2017;Thevenon et al, 2014).…”

Metabolic reprogramming and immune regulation in viral diseases

“…It is well known that activated immune cells alter their metabolic functions to hasten energy production, such as switching to glycolysis. 45 On the other hand, viruses exploit host cellular metabolisms, including changes in the TCA cycle that transports citrate to the cytosol to facilitate its availability for free fatty acid (FFA) synthesis, 46 assembling viral machinery in infected cells. Although activation of the NLRP3 inflammasome has been reported to confer host protection against viruses, 47 it must also be emphasized that certain viruses can also activate the NLRP3 inflammasome to exacerbate disease severity.…”

NaCT (SLC13A5) facilitates citrate import and metabolism under nutrient-limited conditions

“…Additionally, hepatic SLC13A5 expression correlated positively with both body and liver fat in a cohort of non-alcoholic fatty liver disease (NAFLD) patients (von Loeffelholz et al, 2017). Functional impacts of NaCT inhibition or knockdown have been proposed in cells, though findings are not necessarily dependent on the presence of extracellular citrate (Li et al, 2017;Phokrai et al, 2018;Poolsri et al, 2018). Citrate transport may also influence AcCoA metabolism and ionic homeostasis in the nervous system, as loss-of-function mutations in SLC13A5 have been linked to pediatric epilepsy, Kohlschütter-Tönz syndrome, and other brain disorders (Hardies et al, 2015;Klotz et al, 2016;Matricardi et al, 2020;Schossig et al, 2017;Thevenon et al, 2014).…”