3.4. Linagliptin exerts direct and indirect effects on hepatocytes during the improvement of hepatic steatosis
To elucidate whether the effect of linagliptin on hepatic steatosis was mediated through the direct action of linagliptin, GLP-1 receptor signaling in hepatocytes, or an indirect pathway through components such as humoral factors, we treated hepatocyte AML-12 cells with the DPP-4 inhibitor linagliptin or the GLP-1 receptor agonist liraglutide in the presence of OSI-906 (Figure 7). In AML-12 hepatocyte cells, linagliptin did not reverse the effects of OSI-906 on gluconeogenic gene expressions, consistent with the in vivo results. In contrast, the decrease in Tnf expression in OSI-906-treated cells was not reversed by incubation with linagliptin or liraglutide, unlike thein vivo results, implying an indirect effect of linagliptin and DPP-4 inhibition on hepatocytes. In contrast, liraglutide canceled the OSI-906-induced increases in gluconeogenic gene expressions. The expression of Plin2 was induced by OSI-906 and was diminished by the treatment with linagliptin, but not with liraglutide. These results indicate that linagliptin influences hepatocytes through multiple mechanisms that are independent of GLP-1 receptor signaling.