Inhibition of Liver Acetyl-CoA Carboxylase (ACC) Activity Exacerbates Liver Tumorigenesis in Mice

Academic Background

Liver cancer is the sixth most common cancer globally and the third leading cause of cancer-related mortality. Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer. In recent years, the burden of liver cancer due to hepatitis B (HBV) and hepatitis C (HCV) infections has declined due to the development of vaccines and antiviral treatments. However, obesity and related fatty liver diseases, such as metabolic dysfunction-associated fatty liver disease (MAFLD), have become major risk factors for liver cancer. A hallmark of fatty liver disease is the deposition of triglycerides in the liver. Fats can be consumed through the diet or synthesized in the body via the process of de novo lipogenesis (DNL). Under normal conditions, DNL contributes 3-5% to liver fat, but in MAFLD patients, this contribution can rise to 30%.

Acetyl-CoA carboxylase (ACC) is a key enzyme in the DNL process, responsible for converting acetyl-CoA to malonyl-CoA. In mice, cytosolic ACC1 is the primary isoform driving DNL, while mitochondrial ACC2 primarily inhibits fat oxidation. However, ACC2 can also participate in DNL, as evidenced by partial compensation in liver-specific ACC1 knockout mice. Therefore, to completely block DNL, both ACC1 and ACC2 must be inhibited. ACC1 is highly upregulated in both human and mouse liver cancers, making ACC inhibition a potential strategy for treating fatty liver disease and liver cancer.

However, previous studies have shown that inhibiting ACC activity unexpectedly exacerbates liver tumorigenesis. To further investigate this phenomenon, researchers designed new experiments to better understand the impact of ACC inhibition on liver tumorigenesis using different approaches, such as selective inhibition of ACC1 or ACC2, dietary changes, and adjusting the timing of gene knockout.

Source of the Paper

This paper was co-authored by Riya Shrestha, Calum S. Vancuylenburg, Martina Beretta, and others from the School of Biotechnology and Biomolecular Sciences at the University of New South Wales (UNSW), Australia. The study was published in 2024 in the journal Cancer & Metabolism, titled Complete inhibition of liver acetyl-CoA carboxylase activity is required to exacerbate liver tumorigenesis in mice treated with diethylnitrosamine.

Research Process and Results

1. Study Design

The research team designed six different genotypes of mice, each with different combinations of ACC1 and ACC2 gene knockouts. These mice were injected with the carcinogen diethylnitrosamine (DEN) at two weeks of age, and liver-specific ACC gene knockout was achieved at nine weeks using adeno-associated virus (AAV)-mediated gene knockout technology. The mice were fed a normal diet throughout the experiment, and liver tumors were assessed at 52 weeks.

2. Experimental Process

• Mouse Genotype Design: The research team generated six different genotypes of mice, including wild-type (WT), ACC1 knockout (A1KO), ACC2 knockout (A2KO), ACC1 knockout with heterozygous ACC2 knockout (A1KO A2Het), ACC2 knockout with heterozygous ACC1 knockout (A1Het A2KO), and ACC1 and ACC2 double knockout (DKO).
• DEN-Induced Liver Cancer Model: All mice were injected with DEN at two weeks of age to induce liver cancer.
• ACC Gene Knockout: At nine weeks, liver-specific ACC gene knockout was achieved using AAV8-TBG-Cre virus-mediated gene knockout technology.
• Tumor Assessment: At 52 weeks, the researchers conducted a detailed assessment of liver tumors, including tumor number, tumor burden (total tumor volume), and tumor incidence.

3. Experimental Results

• Tumor Number and Burden: Compared to wild-type mice, ACC1 and ACC2 double knockout (DKO) mice showed a 5.5-fold increase in liver tumor number and a 40-fold increase in tumor burden. All DKO mice developed macroscopic liver tumors, while only 75% of wild-type mice developed tumors.
• Tumor Size Distribution: Most tumors in the mice were between 0-2.5 mm in size, but DKO mice had a significantly higher number of tumors, although the tumor sizes were similar to other genotypes.
• Metabolic Phenotypes: The study found that ACC1 knockout mice had approximately 40% lower liver triglyceride levels, but there were no significant differences in plasma triglycerides, liver cholesterol, or plasma cholesterol levels among the genotypes.
• Insulin Levels: At 30 weeks, ACC1 knockout mice showed significantly higher insulin levels, but other metabolic indicators (such as glucose tolerance and insulin resistance) did not differ significantly among the genotypes.

4. Conclusion

The results indicate that complete inhibition of ACC1 and ACC2 activity significantly exacerbates DEN-induced liver tumorigenesis. This phenomenon is independent of diet and persists regardless of the timing of ACC gene knockout. The study also found that retaining partial activity of either ACC1 or ACC2 can prevent tumor exacerbation, suggesting that partial ACC inhibition may be a safer strategy for liver cancer treatment.

Significance and Highlights of the Study

1. Scientific Value

This study reveals the complex role of ACC inhibition in liver tumorigenesis. Although ACC inhibition shows promise in treating fatty liver disease, complete inhibition of ACC activity exacerbates tumorigenesis in liver cancer models. This finding provides important insights for the future development of safer ACC inhibitors.

2. Practical Value

The results suggest that selective inhibition of ACC2 while retaining partial ACC1 activity may be a superior strategy for treating liver cancer. This discovery offers a new direction for future drug development, particularly in designing ACC inhibitors that consider their potential impact on tumorigenesis.

3. Research Highlights

• Multi-Genotype Design: The research team designed six different genotypes of mice to comprehensively evaluate the roles of ACC1 and ACC2 in liver tumorigenesis.
• Control of Diet and Gene Knockout Timing: By altering diet and the timing of gene knockout, the study validated the reproducibility of ACC inhibition exacerbating liver tumorigenesis under different conditions.
• In-Depth Analysis of Metabolism and Tumor Relationship: The study not only assessed tumorigenesis but also analyzed the metabolic phenotypes of the mice, revealing the impact of ACC inhibition on liver triglyceride levels.

Summary

Through systematic experimental design, this study elucidates the complex role of ACC inhibition in liver tumorigenesis. The results demonstrate that complete inhibition of ACC activity exacerbates liver tumorigenesis, while retaining partial ACC activity can prevent this phenomenon. This finding provides new insights for future liver cancer treatment, particularly in balancing the effects of ACC inhibitors on fatty liver disease and liver cancer.