Hepatocellular clear cell foci after combined intraportal pancreatic islet transplantation and knockout of chREBP in diabetic mice

Question:

Clear cell foci (CCF) of altered hepatocytes are constitutive pre-neoplastic lesions in experimental models of hepatocarcinogenesis, including the intraportal pancreatic islet transplantation (IPIT) model of diabetic rats. In this insulin mediated model, activation of the protooncogenic pathways of Akt/mTOR and Ras/raf-1 and upregulation of glycolysis and de-novo-lipogenesis, representing the lipogenic phenotype, have been described previously. Additionally, the transcription factor carbohydrate responsive element binding protein (chREBP), physiologically regulating hepatocellular glucose and lipid metabolism, was found to be activated during AKT/mTOR signaling in CCF. In this study, we transferred the IPIT model to the mouse and combined it with the knockout of chREBP.

Methods:

In short time experiments of one and four weeks, C57BL/6J Wild-type (WT) and chREBP-knockout (chREBP-KO) mice (n = 297) were matched to eight experimental and eight control groups, with or without prior Streptocotozine-induced diabetes. Experimental groups received an intraportal transplantion of 70 – 100 isologous, isolated pancreatic islets. Liver tissue was examined using histology, morphometry, enzyme- and immunohistochemistry (including BrdU labeling index) and electron microscopy.

Results:

CCF emerge at the downstream level of the islets, both in WT and even more frequently in chREBP-KO mice (WT vs. chREBP-KO one week 67% vs. 95%, four weeks 45% vs. 82%, p < 0.05). In comparison to WT lesions, CCF of chREBP-KO reveal a higher degree of glycogen accumulation due to a reduced activity of the glycogeneogentic enzyme glucose-6-phosphatase and decreased glycolysis (i.e. hexokinase II, aldolase). Lipid storage is almost missing due to reduced lipogenesis (i.e. fatty acid synthase). Proliferative activity in CCF of knockout mice is halved in comparison to WT mice (BrdU labelling index of CCF after four weeks WT vs. chREBP-KO: 11.34 ± 2.24 vs. 6.08 ± 0.70; p < 0.05). Simultaneously, signaling of the AKT/mTOR and Ras/raf-1 cascade (i.e. p-Akt, p4EBP-1, IRS1 and pERK) is depleted in CCF of chREBP-KO mice.

Conclusion:

The IPIT model is applicable to mice, as murine CCF resemble preneoplastic liver lesions from this hepatocarcinogenesis model in the rat in terms of morphological, metabolic and molecular alterations and proliferative activity, which is diminished after chREBP knockout. chREBP appears to be an essential component of AKT/mTOR mediated cell proliferation and the metabolic switch from a glycogenotic to lipogenic phenotype in precursor lesions of hepatocarcinogenesis.