Progress in understanding the pathogenesis of nonalcoholic fatty liver disease

Abstract Sahai A, Malladi P, Melin‐Aldana H, Green RM, Whitington PF. Upregulation of osteopontin expression is involved in the development of nonalcoholic steatohepatitis in a dietary murine model. Am J Physiol Gastrointest Liver Physiol 2004;287:G264‐G273. (Reprinted with permission of American Physiological Society) The pathogenesis of nonalcoholic steatohepatitis (NASH) is poorly defined. Feeding mice a diet deficient in methionine and choline (MCD diet) induces experimental NASH. Osteopontin (OPN) is a Th1 cytokine that plays an important role in several fibroinflammatory diseases. We examined the role of OPN in the development of experimental NASH. A/J mice were fed MCD or control diet for up to 12 wk, and serum alanine aminotransferase (ALT), liver histology, oxidative stress, and the expressions of OPN, TNF‐α, and collagen I were assessed at various time points. MCD diet‐fed mice developed hepatic steatosis starting after 1 wk and inflammation by 2 wk; serum ALT increased from day 3. Hepatic collagen I mRNA expression increased during 1‐4 wk, and fibrosis appeared at 8 wk. OPN protein expression was markedly increased on day 1 of MCD diet and persisted up to 8 wk, whereas OPN mRNA expression was increased at week 4. TNF‐α expression was increased from day 3 to 2 wk, and evidence of oxidative stress did not appear until 8 wk. Increased expression of OPN was predominantly localized in hepatocytes. Hepatocytes in culture also produced OPN, which was stimulated by transforming growth factor‐β and TNF‐α. Moreover, MCD diet‐induced increases in serum ALT levels, hepatic inflammation, and fibrosis were markedly reduced in OPN ‐/‐ mice when compared to OPN +/+ mice. In conclusion, our results demonstrate an upregulation of OPN expression early in the development of steatohepatitis and suggest an important role for OPN in signaling the onset of liver injury and fibrosis in experimental NASH. Horie Y, Suzuki A, Kataoka E, Sasaki T, Hamada K, Sasaki J, et al. Hepatocyte‐specific Pten deficiency results in steatohepatitis and hepatocellular carcinomas. J Clin Invest 2004;113:1774‐1783. (Reproduced with permission of the Journal of Clinical Investigation.) PTEN is a tumor suppressor gene mutated in many human cancers, and its expression is reduced or absent in almost half of hepatoma patients. We used the Cre‐ loxP system to generate a hepatocyte‐specific null mutation of Pten in mice ( AlbCrePten flox/flox mice). AlbCrePten flox/flox mice showed massive hepatomegaly and steatohepatitis with triglyceride accumulation, a phenotype similar to human nonalcoholic steatohepatitis. Adipocyte‐specific genes were induced in mutant hepatocytes, implying adipogenic‐like transformation of these cells. Genes involved in lipogenesis and β‐oxidation were also induced, possibly as a result of elevated levels of the transactivating factors PPARγ and SREBP1c. Importantly, the loss of Pten function in the liver led to tumorigenesis, with 47% of AlbCrePten flox/flox livers developing liver cell adenomas by 44 weeks of age. By 74‐78 weeks of age, 100% of AlbCrePten flox/flox livers showed adenomas and 66% had hepatocellular carcinomas. AlbCrePten flox/flox mice also showed insulin hypersensitivity. In vitro, AlbCrePten flox/flox hepatocytes were hyperproliferative and showed increased hyperoxidation with abnormal activation of protein kinase B and MAPK. Pten is thus an important regulator of lipogenesis, glucose metabolism, hepatocyte homeostasis, and tumorigenesis in the liver.