Rebalanced mitochondria restore succinate dehydrogenase activity, reduce succinate release in liver cancer

A metabolic feedback loop inside liver cancer cells may help tumors grow and spread—and researchers say disrupting it could weaken the disease. In a study of hepatocellular carcinoma, the most common primary liver cancer, scientists found that excessively fragmented mitochondria were associated with reduced activity of a key metabolic enzyme and the accumulation of succinate, a small molecule that can act as a cancer-promoting signal. Blocking the mitochondrial fission machinery restored part of the cells’ metabolic function, lowered succinate release, slowed tumor growth and reduced lung metastasis in mice. The findings, published in the Journal of Biomedical Science, reveal a previously underappreciated connection between the physical architecture of mitochondria and the tricarboxylic acid cycle, the central metabolic pathway that processes nutrients inside these organelles. They also point to circulating succinate and the mitochondrial protein SDHB as possible biomarkers of aggressive liver cancer, although neither is yet ready for routine clinical use.

Mitochondria are often described as cellular power stations, but in cancer they are also dynamic structures that constantly divide and fuse. This remodeling helps cells respond to changing energy demands, eliminate damaged organelles and distribute mitochondria during cell division. In many tumors, however, the balance shifts toward excessive fission, producing numerous short, fragmented mitochondria. A central regulator of this process is dynamin-related protein 1, or Drp1. When activated, Drp1 moves to the mitochondrial surface and constricts the organelle until it splits. The researchers observed higher levels of activated Drp1, marked by phosphorylation at serine 616, in hepatocellular carcinoma cells than in normal liver-derived cells. The cancer cells also had more of the fission-associated protein Fis1 and less of the fusion proteins mitofusin 2 and OPA1. Under the microscope, their mitochondrial networks were visibly less tubular and more fragmented.

The team focused on what this structural disruption might do to the tricarboxylic acid cycle, a sequence of reactions that converts carbon derived from carbohydrates, fats and proteins into usable metabolic intermediates and reducing equivalents. Particular attention fell on succinate dehydrogenase, or SDH, an unusual enzyme complex with a dual role. Within the tricarboxylic acid cycle, SDH converts succinate into fumarate. At the same time, it forms Complex II of the electron-transport chain, transferring electrons from FADH2 to ubiquinone and linking nutrient metabolism to oxidative phosphorylation. SDH contains four subunits, including SDHB, which helps form the enzyme’s catalytic and electron-transfer machinery. If SDH activity falls, succinate can accumulate instead of being efficiently converted into fumarate. Because succinate can influence gene regulation, inflammation and cellular signaling, its buildup may alter the tumor environment far beyond the mitochondrion.

Evidence from human samples supported this model. The investigators analyzed paired tumor and nearby non-tumorous liver tissue from 150 patients enrolled through the Taiwan Liver Cancer Network. SDHB messenger RNA was lower in tumor tissue, regardless of whether patients had hepatitis B, hepatitis C or neither infection. In a separate analysis using publicly available liver-cancer data, patients whose tumors expressed more SDHB had longer overall survival and relapse-free survival. The researchers also measured succinate in serum from the 150 patients and 47 healthy volunteers. Patients with hepatocellular carcinoma had higher circulating succinate, with a mean concentration of 23.3 micromolar and values ranging from 6.3 to 104.9 micromolar. Higher serum succinate was associated with shorter overall survival, although it was not significantly linked to relapse-free survival. The authors caution that tumor SDHB expression and serum succinate were not directly correlated in the same individual, so the clinical observations are complementary rather than proof of a single causal chain.

Experiments in cultured cells then traced the proposed mechanism. Two liver-cancer cell lines, Huh7 and HepG2, released more succinate into their surrounding medium and had lower SDH activity than THLE-2 cells, a non-cancerous human liver epithelial model. Suppressing SDHB with small interfering RNA increased succinate secretion, enhanced Drp1 activation and raised the number of fragmented mitochondria. The opposite manipulation produced the reverse effect: engineering Huh7 cells to overexpress SDHB increased SDH activity, reduced extracellular succinate, dampened Drp1 activation and restored a more tubular mitochondrial network. Adding succinate back to those SDHB-enhanced cells pushed the mitochondria toward fragmentation again. Together, these results suggest a positive feedback loop: mitochondrial fission suppresses SDH and promotes succinate accumulation, while succinate itself stimulates Drp1-dependent fission.

The researchers tested the feedback mechanism from another direction by neutralizing succinate outside the cell. An antibody directed against succinate reduced mitochondrial fragmentation and increased tubular mitochondrial forms, whereas adding succinate at a concentration of 1 millimolar increased Drp1 activation both in whole-cell extracts and in isolated mitochondrial fractions. The cancer cells also migrated less effectively after Drp1 was silenced or chemically inhibited, suggesting that the pathway affects invasive behavior as well as metabolism. Succinate is already known to function as an “oncometabolite”—a metabolic intermediate that accumulates abnormally and alters cell signaling. Inside cells, excess succinate can inhibit prolyl hydroxylase enzymes, stabilizing the transcription factor HIF-1α and mimicking aspects of low oxygen. Outside cells, it can act through surface receptors and influence neighboring cancer cells, immune cells and blood-vessel formation. The new findings add mitochondrial shape to that network of succinate-driven effects.

To see whether the observations extended beyond cell cultures, the scientists implanted Huh7 cells under the skin of immunodeficient mice. Animals received saline or Mdivi-1, a compound commonly used experimentally to inhibit Drp1-mediated mitochondrial fission, twice each week for three weeks. By the third week, tumors in treated mice were more than 50 percent smaller by volume than those in control animals, and their excised tumors weighed less. Histological analysis also showed a reduced metastatic burden in the lungs. Plasma succinate rose several-fold after tumor implantation in untreated mice but was lower in animals receiving Mdivi-1. Tumors from treated animals contained more SDHB, as well as higher levels of several fusion-associated proteins, and less activated Drp1 and Fis1. The results are consistent with the proposed model in which restoring mitochondrial balance preserves SDH and limits succinate release.

The animal findings nevertheless require careful interpretation before they can be translated into a treatment strategy. Mdivi-1 may affect biological processes beyond Drp1, and lowering plasma succinate could partly reflect the smaller tumors rather than a direct metabolic effect. The xenograft experiment used a small number of mice, outcome assessment was not blinded, and the tumors were implanted subcutaneously rather than arising in a liver with an intact immune system. The researchers also have not established how mitochondrial fragmentation lowers SDHB messenger RNA. Possible explanations include altered transcription through nuclear respiratory factors, accelerated messenger-RNA degradation by microRNAs or retrograde signals generated by mitochondrial stress, such as changes in calcium or MAP kinase activity. Nor has the study proved that extracellular succinate acts through SUCNR1, a known succinate receptor, to phosphorylate Drp1. Those unanswered questions will be important for developing more selective drugs.

Even with those limitations, the study offers a striking view of cancer metabolism as a self-reinforcing circuit rather than a collection of isolated defects. In hepatocellular carcinoma cells, fragmented mitochondria appear to compromise the SDH step of the tricarboxylic acid cycle, allowing succinate to accumulate and escape into the tumor surroundings. That succinate then feeds back onto the cells, activating Drp1 and driving further mitochondrial fragmentation. Interrupting the circuit genetically or with a pharmacological tool restored SDH activity, reduced succinate and suppressed cancer-cell migration; in mice, the intervention also reduced tumor growth and metastasis. Future work will need to test whether the same mechanism operates across genetically diverse liver tumors, whether succinate measurements can improve existing prognostic tools and whether safer, more specific inhibitors can target mitochondrial fission without disrupting healthy tissues. For now, the work identifies a potentially exploitable metabolic vulnerability—and a molecule already circulating in the blood—that may help reveal when liver cancer has entered a more aggressive state.