Gene silencing of the lipogenic transcription factor sterol regulatory element binding protein-1 (SREBP1) in liver

Liver steatosis is associated with hepatic insulin resistance. SREBP1c is a transcription factor that regulates the lipogenesis pathway leading to de novo synthesis of fatty acids. In the normal liver, insulin stimulates both transcription and translation of SREBP1c. In the db/db mouse model of obesity and diabetes, levels of SREBP1c have been found significantly elevated. To gain insight into the importance of SREBP1 in hepatic insulin resistance in this animal model of type 2 diabetes, we have knocked down SREBP1 (SREBP1 KD) in the liver using RNAi. We hypothesized that silencing SREBP1 would ameliorate the accumulation of lipids by reducing lipogenesis and increasing fatty acid oxidation. This was based on the fact that SREBP1 controls Acetyl CoA Carboxylase 1 and 2 (ACC1 and ACC2) expression. In the liver, ACC1 and ACC2 regulate fatty acid oxidation by inhibiting CPT-1 activity through the production of malonyl-CoA. Using a helper-dependent (HD) adenoviral vector expressing a short hairpin RNA (shRNA) against SREBP-1, 90% reduction of SREBP1 protein was achieved for several weeks. db/db mice with hepatic SREBP1 deficiency displayed a significant reduction in total body as well as liver weight compared to animals treated with a vector expressing a scrambled shRNA sequence. Knocking down SREBP1 in the liver resulted in reduced expression of key genes in the lipogenesis pathway known to be controlled by SREBP1, such as ACC2 and SCD1. However, expression of ACC1 was not affected. As a consequence and in spite of reduced ACC2 expression, fatty acid oxidation was not improved in SREBP1 KD mice. Also, SREBP1 silencing resulted in reduced hepatic triglyceride content. Reduction of SREBP1 also altered significantly the expression of at least two genes involved in glucose metabolism: PEPCK was increased and liver pyruvate kinase was lowered, which suggests SREBP1’s involvement in the regulation of gluconeogenesis and glycolysis. Surprisingly, silencing SREBP1 promoted a decrease in glycogen synthesis. The livers of SREBP1 KD animals had less glycogen content, and reduced glycogen synthase expression and activity. Altogether, our data suggests that SREBP1 has an important role in controlling carbohydrate and fatty acid metabolism, in addition to its well-known role at regulating lipid metabolism in liver