Function of SREBP1 in the milk fat synthesis of dairy cow mammary epithelial cells.

Nan Li,Qing-Zhang Li,Na Zhang,Xue-Jun Gao,Feng Zhao,Mengyao Liang,Chunmei Wang,Chenjie Wei

doi:10.3390/ijms150916998

Nan Li, Qing-Zhang Li + Show 6 more

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https://doi.org/10.3390/ijms150916998

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Abstract

Sterol regulatory element-binding proteins (SREBPs) belong to a family of nuclear transcription factors. The question of which is the most important positive regulator in milk fat synthesis in dairy cow mammary epithelial cells (DCMECs) between SREBPs or other nuclear transcription factors, such as peroxisome proliferator-activated receptor γ (PPARγ), remains a controversial one. Recent studies have found that mTORC1 (the mammalian target of rapamycin C1) regulates SREBP1 to promote fat synthesis. Thus far, however, the interaction between the SREBP1 and mTOR (the mammalian target of rapamycin) pathways in the regulation of milk fat synthesis remains poorly understood. This study aimed to identify the function of SREBP1 in milk fat synthesis and to characterize the relationship between SREBP1 and mTOR in DCMECs. The effects of SREBP1 overexpression and gene silencing on milk fat synthesis and the effects of stearic acid and serum on SREBP1 expression in the upregulation of milk fat synthesis were investigated in DCMECs using immunostaining, Western blotting, real-time quantitative PCR, lipid droplet staining, and detection kits for triglyceride content. SREBP1 was found to be a positive regulator of milk fat synthesis and was shown to be regulated by stearic acid and serum. These findings indicate that SREBP1 is the key positive regulator in milk fat synthesis.

Highlights

Sterol regulatory element-binding proteins (SREBPs) belong to the basic helix-loop-helix-leucine zipper family of transcription factors, which are synthesized as inactive precursors bound to the endoplasmic reticulum (ER)
The protein expression levels of Sterol Regulatory Element-Binding Protein 1 (SREBP1), p-SREBP1, mTOR, and p-mTOR were notably increased in cells transfected with SREBP1 compared with cells in the empty vector group (Figure 1B,C)
SREBP1 in dairy cow mammary epithelial cells (DCMECs) was found to significantly increase triglyceride secretion (Figure 1D). These findings reveal that the overexpression of SREBP1 increases milk fat synthesis in DCMECs

Summary

Introduction

Sterol regulatory element-binding proteins (SREBPs) belong to the basic helix-loop-helix-leucine zipper (bHLH-Zip) family of transcription factors, which are synthesized as inactive precursors bound to the endoplasmic reticulum (ER). The N-terminus contains the bHLH-Zip region that, upon binding to DNA, functions as a transcription factor. Each SREBP precursor exists in a hairpin-like conformation of ~1150 amino acids (~125 kDa) organized into three domains: an NH2-terminal cytoplasmic domain that functions as a DNA-binding transcription factor; a central domain containing two transmembrane segments projecting into the lumen of the ER; and a COOH-terminal cytoplasmic domain that performs essential regulatory functions. The mammalian genome encodes three SREBP isoforms designated SREBP1a, SREBP1c, and SREBP2. SREBP1a and SREBP1c are encoded by the same gene (SREBP1), which is regulated by two distinct promoters and alternative splicing [1]. The original cloning of the SREBP1 gene from human HeLa cells yielded several partial cDNA clones with alternative sequences at both the 5' and 3'

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