LXRB-SREBP1 axis regulating lipogenic homeostasis by controlling the synthesis of polyunsaturated fatty acids in goat mammary epithelial cells

Abstract Background: Liver X receptor beta (LXRB) is the predominant LXR subtype in ruminant mammary cells. Whether and how LXRB plays a role in the synthesis of polyunsaturated fatty acids (PUFA) in the mammary gland remains to be determined. In the current study, we used overexpression and knockdown of LXRB in goat primary mammary epithelial cells (GMEC) to evaluate abundance of lipogenic enzymes, fatty acid profiles, content of lipid stores and activity of the stearoyl-CoA desaturase (SCD1) promoter. Results: Overexpression of LXRB markedly upregulated the protein abundance of LXRB while incubation with siRNA targeting LXRB markedly decreased abundance of LXRB protein. Overexpression of LXRB plus ligand T0901317 (T09) dramatically upregulated genes encoding proteins related to PUFA synthesis. Compared with the control, cells overexpressing LXRB plus T09 had greater concentrations of C16:0, 16:1, 18:1n7,18:1n9 and C18:2 as well as desaturation and elongation indices of C16:0. Furthermore, LXRB overexpressing cells incubated with T09 had greater levels of triacylglycerol and cholesterol. Knockdown of LXRB in cells incubated with T09 led to downregulation of genes encoding elongases and desaturases. In contrast, knockdown of LXRB attenuated the increase in TAG and cholesterol that was induced by T09. In cells treated with DMSO, knockdown of LXRB increased the concentration of C16:0 at the expense of C18:0, while a significant decrease in C18:2 was observed in cells incubated with both siLXRB and T09. The abundance of sterol regulatory element binding transcription factor 1 (pSREBP1) and its mature fragment (nSREBP1) was upregulated by T09, but not LXRB overexpression. In the cells cultured with T09, knockdown of LXRB downregulated the abundance for pSREBP and nSREBP1. Luciferase reporter assays with site directed mutagenesis of SREBP1 response elements (SRE) revealed that that the SCD1 promoter with the wild type or SRE mutation decreased markedly when LXRB was knocked down. Activity of the SCD1 promoter that was induced by T09 was impaired when the SRE mutation was introduced. Conclusion: There is an important role of LXRB-SREBP1 axis in the synthesis of PUFA via the regulation of genes encoding elongases and desaturases. Thus, this network elicits broad effects on lipid homeostasis in ruminant mammary gland.