Role of osteopontin in diet-induced brown gallstone formation in rats

Background: Although osteopontin (OPN) is expressed in the liver and pigment gallstones of patients with hepatolithiasis, its role in pigment gallstone formation remains unclear. This study aimed to explore the function of OPN in pigment gallstone formation. Methods: Rats were fed a chow diet (CD) or lithogenic diet (LD) for 10 consecutive weeks; blocking tests were then performed using an OPN antibody (OPN-Ab). Incidence of gallstones and levels of several bile components, OPN, tumor necrosis factor alpha (TNF-alpha), and cholesterol 7 alpha-hydroxylase (CYP7A1) were analyzed. To determine TNF-alpha expression in hepatic macrophages and both CYP7A1 and bile acid (BA) expression in liver cells, recombinant rat OPN and recombinant rat TNF-alpha were used to treat rat hepatic macrophages and rat liver cells, respectively. Chi-square or Fisher exact tests were used to analyze qualitative data, Student t-test or one-way analysis of variance were used to analyze qualitative data. Results: Incidence of gallstones was higher in ID-fed rats than in CD-fed rats (80% vs. 10%, P < 0.05). BA content significantly decreased in bile (t = 36.08, P < 0.01) and liver tissue (t = 16.16, P < 0.01) of LD-fed rats. Both hepatic OPN protein expression (t = 9.78, P < 0.01) and TNF-alpha level (t = 8.83, P < 0.01) distinctly increased in the LD group; what's more, CYP7A1 mRNA and protein levels (t= -12.35, P < 0.01) were markedly down-regulated in the LD group. Following OPN-Ab pretreatment, gallstone formation decreased (85% vs. 25%, chi(2) = 14.55, P < 0.01), liver TNF-alpha expression (F = 20.36, P < 0.01) was down-regulated in the LD group, and CYP7A1 expression (F =17.51, P < 0.01) was up-regulated. Through CD44 and integrin receptors, OPN promoted TNF-alpha production in macrophage (F = 1041, P < 0.01), which suppressed CYP7A1 expression (F = 48.08, P < 0.01) and reduced liver BA synthesis (F = 119.4, P < 0.01). Conclusions: We provide novel evidence of OPN involvement in pigmented gallstone pathogenesis in rats.