[31] In these mice, hepatic expression of OPN protein was markedly increased at 1 day after the beginning of MCD diet and persisted

check details up to 8 weeks, whereas OPN mRNA expression was increased at 4 weeks.[31] OPN protein expression was predominantly localized to hepatocytes, not inflammatory cells, assessed by immunohistochemistry at 3 days and at 8 weeks. Moreover, hepatic inflammation induced by MCD diet was markedly reduced in OPN–/– mice compared with OPN+/+ mice, while histological steatosis and liver triglyceride levels were similar among these mice.[31] This may be because mice fed MCD diet lose weight and do not show insulin resistance, unlike human or other diet-induced rodent models of NAFLD. Osteopontin mRNA was increased during culture-related activation of rat quiescent hepatic stellate cells to myofibroblastic stellate cells.[32] Furthermore, incubation of hepatic stellate cells with OPN induced their collagen production, transforming growth factor-β receptor upregulation, proliferation and migration.[32] These results suggested a potential role for OPN in the progression of hepatic fibrosis. Hepatic fibrosis induced by MCD diet was HKI-272 mw attenuated in OPN–/–

mice compared with OPN+/+ mice.[31, 33] Moreover, hepatic OPN mRNA and protein levels were not affected by feeding an MCD diet in genetically leptin-deficient, obese and diabetic ob/ob mice, which developed steatohepatitis but not liver fibrosis after the feeding.[34] In contrast, the diet had a stimulatory effect on OPN mRNA and protein levels in hyperleptinemic, obese and diabetic db/db mice, which exhibited a lesser degree of steatosis, but greater histological inflammation and marked pericellular fibrosis by the diet.[34]

Recently, it was reported that OPN was induced by Hedgehog signaling and directly promoted profibrogenic responses in steatohepatitis (Fig. 2). Hedgehog pathway can promote activation of quiescent hepatic stellate cells to myofibroblastic stellate cells.[35] In patients with NAFLD, accumulation of Hedgehog ligands and expression of Hedgehog-target genes were significantly correlated with Amylase hepatic fibrosis stage.[36] Furthermore, Hedgehog-mediated accumulation of natural killer T (NKT) cells contributed to fibrosis progression of NASH in mice and humans.[37] As shown in Figure 3, after the binding of the Hedgehog ligand to the Patched (Ptc) receptor, glioma-associated oncogene (Gli) is activated by release from a large protein complex and translocated to the nucleus to function as a transcriptional activator. The consensus DNA-binding sequences of Gli-1 were indentified in the 5′ regions of OPN, and gel shift analysis confirmed Gli-1 protein could bind to the oligonucleotides of OPN promoter region.[38] Syn et al. analyzed hepatic OPN expression and fibrosis grade, using Ptc-deficient (Ptc+/−) mice with haploinsufficiency of Ptc, which exhibit overly active Hedgehog signaling.