Mitochondrial and anabolic pathways in hepatocellular carcinoma.

Despite significant improvements in healthcare in the United States and in the world, liver cancer remains a formidable challenge with limited therapeutic approaches.¹ The development of such approaches is limited by an insufficient understanding of the molecular mechanisms of liver cancer. Hepatocellular carcinoma (HCC) is one of the leading liver cancers that affects many patients of different ages.^1,2 Numerous mutational studies and studies of global transcriptome and proteome have emphasized extraordinary complexity of alterations that are observed in cancer cells.³ The liver is a key tissue in metabolic homeostasis regulation and recent discoveries have shown that certain metabolic alterations are critical in the development of HCC.^{2, 3} However, very little is known about the specific mechanisms and key factors that drive the metabolic-dependent pathways of HCC. In this issue of Hepatology, Piccinin and colleagues have investigated the role of mitochondrial and fatty acid metabolisms in HCC by focusing on a key regulator of these processes: the peroxisome proliferator-activated receptor γ (PPARγ) co-activator −1β (PGC-1β). This elegant work demonstrates that hepatic PGC-1β drives alterations of the metabolic programs that lead to the development of HCC.⁴

Liver specific mitochondrial metabolism is a critical process which provides energetic support for the entire body. Members of a family of nuclear receptors peroxisome proliferator activated receptors, PPARα, β, and γ are critical transcription factors that regulate energy homeostasis, and mitochondrial functions.⁵ Although PPARα is the major member of this family, expressed in the liver, many studies have shown that PPARγ is also a critical regulator of hepatic mitochondrial functions. The activity of PPAR proteins is under tight control, including by co-activators PGC-1α and β which are the main regulators of the PPAR family. These regulators are involved not only in liver metabolism and cancer, but also exert in biological functions and cancer susceptibility in other tissues.⁶ Piccinin and colleagues have focused on PGC-1β since this gene mainly regulates de novo lipogenesis and is likely involved in both mitochondrial and fatty acid metabolism. The authors generated two animal models with hepatic-specific overexpression of PGC-1β (LivPGC-1β TR mice), and with hepatic-specific deletion of PGC-1β (LivPGC-1β KO mice) and used these mice to investigate the role of PGC-1β in the diethylnitrosamine (DEN) model of liver carcinogenesis. Their findings at different times after injection of DEN into LivPGC-1β TR mice show that overexpression of PGC-1β promotes the development of liver cancer. The appearance of tumor nodules was detectable in LivPGC-1β TR mice 6 months after DEN injection, at which time WT mice showed no tumors. Studies conducted at 10 months after DEN injections revealed the early appearance of liver tumors in WT mice, but LivPGC-1β TR mice developed much larger tumor load. The acceleration of liver cancer was further confirmed by the examination of blood parameters that showed higher elevations of ALT and AST.

Since PGC-1β regulates levels of reactive oxygen species (ROS) and lipid metabolism⁷, the authors further examined if these two processes are affected in tumors of LivPGC-1β TR mice. It has been shown that ROS simulate the development of liver cancer by inducing mutations in carcinogens.⁸ However, recent studies showed that de-toxification of ROS might also promote cancer under certain conditions.⁹ Therefore, the authors examined if overexpression of PGC-1β might utilize this mechanism for the promotion of liver cancer in DEN treated LivPGC-1β TR mice. The examination of ROS and ROS-dependent DNA damage showed that indeed PGC-1β significantly reduces ROS injury in tumors of DEN-treated LivPGC-1β TR mice. It is well documented that fatty liver and increased lipid metabolism are risk factors for the development of HCC.^1,2 Consistent with this, the examination of enzymes involved in the de novo lipogenesis and triglyceride synthesis found significant activation of mRNAs encoding ACC, DGAT1, and Scd1 in liver tumors of LivPGC-1β TR mice compared to WT mice which correlated with elevation of triglycerides and cholesterol in the serum of DEN-treated LivPGC-1β TR mice.

To further examine the role of PGC-1β in liver cancer the authors generated Abcb4^−/−LivPGC-1β mice by breeding Abcb4^−/− and LivPGC-1β TR mice. Abcb4 promotes the secretion of phosphatidylcholine into bile and its deletion results in progressive liver injury and HCC.¹⁰ Therefore, the authors asked if the Abcb4^−/−LivPGC-1β TR mice might have different abilities to develop liver cancer under conditions of altered biliary lipid metabolism. These studies revealed that the number and size of spontaneous tumors in Abcb4^−/−LivPGC-1β TR mice were dramatically increased compared to Abcb4^−/− mice. The acceleration of liver cancer development in Abcb4^−/−LivPGC-1β TR mice was associated with increased ROS scavenger activity and with increased expression of lipogenic enzyme mRNAs. These studies again confirmed the critical role of PGC-1β in the promotion of liver cancer via an increase of lipid metabolism. Since overexpression studies might be associated with non-physiological levels of PGC-1β, the authors generated mice lacking hepatic expression of PGC-1β (LivPGC-1β KO mice). These mice were treated with DEN and showed that liver cancer was inhibited in mice lacking PGC-1β, findings that correlated with reduced liver damage. Together, these results show that PGC-1β promotes liver cancer through enhancement of ROS detoxification and via the increase of fatty acid metabolism (Figure 1).

Figure 1.

PGC-1β promotes hepatocellular carcinoma through alterations of mitochondrial and lipid metabolisms (see text).

The results of the studies conducted by Piccinin and colleagues are highly significant and provide an important step in understanding some of metabolic underpinnings of liver cancer. However, DEN-mediated carcinogenesis does not exactly recapitulate the pathways of spontaneous liver cancer. In this regard, it would be important to determine if LivPGC-1β TR mice develop liver cancer with age even without chemical interventions. In addition, it might be of interest to ascertain if the development of human HCC is associated with alterations of PGC-1β expression. For example, to determine if HCC patients exhibit alterations in PGC-1β expression and/or if they harbor somatic mutations that produce gain or loss of function.

In conclusion, the paper by Piccinin and colleagues is a significant step in understanding the mechanisms of liver cancer. Future work might considerably include development of PGC-1β-based therapeutic approaches to treat HCC once we understand the molecular mechanisms by which PGC-1β promotes liver cancer. It would be critical to find out the first molecule downstream of PGC-1β and investigate if there is a possibility to block its oncogenic activity using small drug inhibitors. Along these lines, it will be important to understand the role of PPARγ in mediating effects of PGC-1β in promoting liver cancer, since this might simplify the search for appropriate drugs. Alternatively, if the tumor promotion activity of PGC-1β is mediated through several direct targets, inhibition of oncogenic activity of PGC-1β might require approaches targeting several pathways.