Figure 4.

Disruption of ER protein processing suppresses metabolic gene expression. (A) FaO rat hepatoma cells were treated in duplicate with 500 nM TG or 5 μg/ml TM for 8 or 24 hrs, followed by cell lysis and immunoblot for GRP94, BiP, C/EBPα, and C-JUN. In hepatocytes, slower-migrating phospho-C-JUN, induced by ER stress, is also observed. (B, C) Wild-type and Atf6α−/− mice were injected with vehicle or TM (1 mg/kg bodyweight). Protein lysates from liver isolated at the indicated time points after injection were probed by immunoblot for intracellular SAP (B); Plasma level of SAP was assayed by ELISA (C). (D) Total cholesterol and triglyceride levels were measured by enzymatic assay from plasma samples of mice injected with vehicle or TM for 24 hours, with a 6 hour fast preceding sacrifice. (E) Cholesterol and triglyceride represented either as total content, or in the form of LDL and VLDL particles purified by differential precipitation, was measured as in (D). In this case samples were collected 48 hours after TM injection rather than 24 hours. Note that the absolute values are somewhat different comparing panels (D) and (E), likely due to experimental variation. However, in panel (D) there is no significant difference in either cholesterol or triglyceride levels between genotypes after TM injection, while cholesterol levels recover to a greater extent in wild-type animals than Atf6α-null animals in panel (E). The fact that TM has a much greater effect on cholesterol-rich lipoprotein particles than triglyceride-rich particles suggests that TM inhibits liver lipoprotein production but not intestinal production. * = p<0.05