Sterol Regulatory Element-binding Protein-1 as a Key Transcription Factor for Nutritional Induction of Lipogenic Enzyme Genes

TLDR

To determine SREBP‑1’s role, the authors measured hepatic lipogenic enzyme mRNA levels in SREBP‑1 knockout mice following a fasting‑refeeding protocol. They quantified these mRNAs in knockout versus wild‑type mice to assess the transcriptional response to dietary manipulation. In knockout mice, fasting‑refeeding and high‑carbohydrate feeding failed to induce lipogenic enzyme mRNAs, while cholesterol‑biosynthetic genes were up‑regulated, showing that SREBP‑1 is essential for lipogenic but not cholesterol‑biosynthetic induction in the liver.

Abstract

To elucidate the physiological role of sterol regulatory element-binding protein-1 (SREBP-1), the hepatic mRNA levels of genes encoding various lipogenic enzymes were estimated in SREBP-1 gene knockout mice after a fasting-refeeding treatment, which is an established dietary manipulation for the induction of lipogenic enzymes. In the fasted state, the mRNA levels of all lipogenic enzymes were consistently low in both wild-type and<i>SREBP-1</i> ^−/− mice. However, the absence of SREBP-1 severely impaired the marked induction of hepatic mRNAs of fatty acid synthetic genes, such as acetyl-CoA carboxylase, fatty acid synthase, and stearoyl-CoA desaturase, that was observed upon refeeding in the wild-type mice. Furthermore, the refeeding responses of other lipogenic enzymes, glycerol-3-phosphate acyltransferase, ATP citrate lyase, malic enzyme, glucose-6-phosphate dehydrogenase, and S14 mRNAs, were completely abolished in<i>SREBP-1</i> ^−/− mice. In contrast, mRNA levels for cholesterol biosynthetic genes were elevated in the refed<i>SREBP-1</i> ^−/− livers accompanied by an increase in nuclear SREBP-2 protein. When fed a high carbohydrate diet for 14 days, the mRNA levels for these lipogenic enzymes were also strikingly lower in <i>SREBP-1</i> ^−/− mice than those in wild-type mice. These data demonstrate that SREBP-1 plays a crucial role in the induction of lipogenesis but not cholesterol biosynthesis in liver when excess energy by carbohydrates is consumed.