Mitochondrial carnitine palmitoyltransferase-II dysfunction: A possible novel mechanism for nonalcoholic fatty liver disease in hepatocarcinogenesis

Nonalcoholic fatty liver disease (NAFLD) or metabolic-associated fatty liver disease has been characterized by the lipid accumulation with injury of hepatocytes and has become one of the most common chronic liver diseases in the world. The complex mechanisms of NAFLD formation are still under identification. Carnitine palmitoyltransferase-II (CPT-II) on inner mitochondrial membrane (IMM) regulates long chain fatty acid β-oxidation, and its abnormality has had more and more attention paid to it by basic and clinical research in NAFLD. The sequences of its peptide chain and DNA nucleotides have been identified, and the catalytic activity of CPT-II is affected on its gene mutations, deficiency, enzymatic thermal instability, circulating carnitine level and so on. Recently, the CPT-II dysfunction has been discovered in models of liver lipid accumulation. Meanwhile, the malignant transformation of hepatocyte-related CD44⁺ stem T cell activation, high levels of tumor-related biomarkers (AFP, GPC3) and abnormal activation of Wnt3a expression as a key signal molecule of the Wnt/β-catenin pathway run parallel to the alterations of hepatocyte pathology. This review focuses on some of the progress of CPT-II inactivity on IMM with liver fatty accumulation as a possible novel pathogenesis for NAFLD in hepatocarcinogenesis.

[Dynamic expression of carnitine palmitoyltransferase II in the mitochondrial inner membrane during hepatocyte malignant transformation induced by lipid accumulation]

Objective: To investigate the dynamic expression of hepatic carnitine palmitoyltransferase-II (CPT-II) in the mitochondrial inner membrane during hepatocyte malignant transformation induced by lipid accumulation. Methods: Male Sprague-Dawley rats were divided randomly into control, fatty liver, and induced cancer groups, which were fed with normal, high-fat (HF), and HF containing 2-fluorenylacetamide (0.05%, 2-FAA) diets, respectively, for 14 weeks. One rat from each group was sacrificed every two weeks and the blood and liver samples were collected. Liver morphological changes were evaluated with hematoxylin and eosin staining, and the liver tissue samples were divided into control, fatty liver, degeneration, precancerous, and cancerous groups accordingly. Hepatic lipids were dyed by the oil red O method. The CPT-II expression was measured by immunohistochemistry and compared with the specific CPT-II concentration (ng/mg liver protein, ng/mg P) among different groups. Serum levels of circulating total cholesterol (Tch), triglyceride (TG), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) were quantitatively analyzed. Results: Massive lipid accumulation hepatocytes was seen in rats on HF and HF containing 2-FAA diets. The lipid levels in the control group were significantly lower than those in the fatty liver (t = -11.556, P < 0.001), degeneration (t = -4.847, P = 0.04), precancerous (t = -13.652, P = 0.005), and cancerous groups (t = -10.896, P = 0.008). The serum TG and Tch levels in the degeneration, precancerous, and cancerous groups were 2-3 times higher than those in the control group (P < 0.05). After 2-FAA treatment, the morphological changes of rat hepatocytes showed the progression from degeneration and precancerosis to cancerosis, with hepatocyte injury. The serum AST and ALT levels in the degeneration, precancerous, and cancerous groups were significantly higher (4-8 times) than those in the control group (P < 0.05). The specific concentration of liver CPT-II expression was significantly reduced during hepatocyte malignant transformation, as confirmed by immunohistochemistry, with the CPT-II levels significantly lower in the cancerous group than in any of other groups (P < 0.05). Conclusion: Low hepatic CPT-II expression might lead to abnormal lipid accumulation in hepatocytes, which should promote the malignant transformation of hepatocytes.

Mitochondrial carnitine palmitoyl transferase-II inactivity aggravates lipid accumulation in rat hepatocarcinogenesis

AIM

To investigate the dynamic alteration of mitochondrial carnitine palmitoyl transferase II (CPT-II) expression during malignant transformation of rat hepatocytes.

METHODS

Sprague-Dawley male rats were fed with normal, high fat (HF), and HF containing 2-fluorenylacetamide (2-FAA) diet, respectively. According to the Hematoxylin and Eosin staining of livers, rats were divided into control, fatty liver, degeneration, precancerous, and cancerous groups. Liver lipids were dyed with Oil Red O, CPT-II alterations were analyzed by immunohistochemistry, and compared with CPT-II specific concentration (μg/mg protein). Levels of total cholesterol (Tch), triglyceride (TG), and amino-transferases [alanine aminotransferase (ALT), aspartate aminotransferase (AST)] were determined by the routine methods.

RESULTS

After intake of HF and/or HF+2-FAA diets, the rat livers showed mass lipid accumulation. The lipid level in the control group was significantly lower than that in other groups. The changes of serum TG and Tch levels were abnormally increasing, 2-3 times more than those in the controls (P < 0.05). During the rat liver morphological changes from normal to cancer development process with hepatocyte injury, serum AST and ALT levels were significantly higher (4-8 times, P < 0.05) than those in the control group. The specific concentration of CPT-II in liver tissues progressively decreased during hepatocyte malignant transformation, with the lowest CPT-II levels in the cancer group than in any of the other groups (P < 0.05).

CONCLUSION

Low CPT-II expression might lead to abnormal hepatic lipid accumulation, which should promote the malignant transformation of hepatocytes.