A Metabolic Pathway That Feeds Liver Cancer

The University of Maryland researchers think they’ve discovered how a specific gene plays a key role in helping liver cancer cells obtain the nutrition they need to increase. Their study (“Silencing of Solute Carrier Family 13 Member 5 Disrupts Energy Homeostasis and Inhibits Proliferation of Human Hepatocarcinoma Cells”) appears in the Journal of Biological Chemistry.

Cancer is often treated by starving it by targeting the pathways cancer cells use to meet their energy needs. The laboratory of Hongbing Wang, Ph.D., focuses on this approach as it applies to liver cancer.

“The solute carrier family 13 member 5 (SLC13A5), a sodium-coupled citrate transporter, plays a key role in importing citrate from the circulation into liver cells,” write the investigators. “Here, we sought to determine whether SLC13A5 regulates hepatic energy homeostasis and proliferation of hepatoma cells. RNAi-mediated silencing of SLC13A5 expression in two human hepatoma cell lines, HepG2 and Huh7, profoundly suppressed cell proliferation and colony formation and induced cell cycle arrest accompanied by increased expression of cyclin-dependent kinase inhibitor p21 and decreased expression of cyclin B1.”

“…our findings expand the role of SLC13A5 from facilitating hepatic energy homeostasis to influencing hepatoma cell proliferation and suggest a potential role of SLC13A5 in the progression of human hepatocellular carcinoma.”

“The liver is one of the busiest, active organs in the body,” Dr. Wang said, so the healthy liver already needs a lot of energy. In addition, liver cancer appears to be among the few cancers that seem to be on the rise, possibly associated with metabolism-related conditions such as nonalcoholic fatty liver disease.

When looking for genes that might play important roles in the metabolism of healthy and cancerous liver cells, Dr. Wang and his colleagues became interested in a gene called SLC13A5, which produces a protein that transports citrate into cells. SLC13A5 is expressed mainly in the liver, but its role is relatively understudied.

“If you search for SLC13A5 in PubMed—I searched this morning—there are 54 publications, which is not a lot,” noted Dr. Wang. Nearly half of these studies were published in the last two years. Research on SLC13A5 has focused on its role in obesity and diabetes; knocking out the SLC13A5 gene in mice prevents high-fat diet-induced obesity. If this gene plays a role in energy homeostasis and balance in the context of obesity, Dr. Wang reasoned, it could play a role in the energy requirements of liver cancer cells.

Zhihui Li, Ph.D., a postdoctoral fellow in Wang’s lab, performed experiments in which he used RNA interference (RNAi) to suppress (but not eliminate) the production of the SLC13A5 protein. He performed these experiments in cultures of two human hepatocellular carcinoma cell lines. Suppressing SLC13A5 resulted in liver cancer cells that did not die but had significantly slower growth and division. Similarly, when these cells were injected into mice, the cells in which SLC13A5 was suppressed formed barely discernable tumors compared to the unmanipulated cancer cells.

Dr. Wang hypothesizes that the extracellular citrate taken up by the SLC13A5 protein is required by the liver cancer cells for fatty acid synthesis. Because prostate cancer does not express SLC13A5, the growth of prostate cancer cells was unaffected by suppressing SLC13A5 expression. The fact that prostate cancer grew independently of the presence of SLC13A5 supports the idea that different cancers use different methods to meet their high energy requirements.

Dr. Wang emphasizes that this work is preliminary and that comparing SLC13A5 activity in healthy and cancerous human liver tissue will be necessary before studies of this pathway as a cancer drug target should be contemplated.