Jiangjiang Zhu

PPARγ Regulates Genes Involved in Triacylglycerol Synthesis and Secretion in Mammary Gland Epithelial Cells of Dairy Goats

To explore the function of PPARγ in the goat mammary gland, we cloned the whole cDNA of the PPARγ gene. Homology alignments revealed that the goat PPARγ gene is conserved among goat, bovine, mouse, and human. Luciferase assays revealed that rosiglitazone enhanced the activity of the PPARγ response element (PPRE) in goat mammary epithelial cells (GMECs). After rosiglitazone (ROSI) treatment of GMECs, there was a significant (P < 0.05) increase in the expression of genes related to triacylglycerol synthesis and secretion: LPL, FASN, ACACA, PLIN3, FABP3, PLIN2, PNPLA2, NR1H3, SREBF1, and SCD. The decreases in expression observed after knockdown of PPARγ relative to the control group (Ad-NC) averaged 65%, 52%, 67%, 55%, 65%, 58%, 85%, 43%, 50%, and 24% for SCD, DGAT1, AGPAT6, SREBF1, ACACA, FASN, FABP3, SCAP, ATGL, and PLIN3, respectively. These results provide direct evidence that PPARγ plays a crucial role in regulating the triacylglycerol synthesis and secretion in goat mammary cells and underscore the functional importance of PPARγ in mammary gland tissue during lactation.

MiR-27a suppresses triglyceride accumulation and affects gene mRNA expression associated with fat metabolism in dairy goat mammary gland epithelial cells.

MicroRNAs (miRNAs), a well-defined group of small RNAs containing about 22 nucleotides, participate in various biological metabolic processes. miR-27a is a miRNA that is known to regulate fat synthesis and differentiation in preadipocyte cells. However, little is known regarding the role that miR-27a plays in regulating goat milk fat synthesis. In this study, we determined the miR-27a expression profile in goat mammary gland and found that miR-27a expression was correlated with the lactation cycle. Additionally, prolactin promoted miR-27a expression in goat mammary gland epithelial cells. Further functional analysis showed that over-expression of miR-27a down-regulated triglyceride accumulation and decreased the ratio of unsaturated/saturated fatty acid in mammary gland epithelial cells. miR-27a also significantly affected mRNA expression related to milk fat metabolism. Specifically, over-expression of miR-27a reduced gene mRNA expression associated with triglyceride synthesis by suppressing PPARγ protein levels. This study provides the first experimental evidence that miR-27a regulates triglyceride synthesis in goat mammary gland epithelial cells and improves our understanding about the importance of miRNAs in milk fat synthesis.

Peroxisome proliferator-activated receptor-γ stimulates the synthesis of monounsaturated fatty acids in dairy goat mammary epithelial cells via the control of stearoyl-coenzyme A desaturase.

In rodents, peroxisome proliferator-activated receptor-γ (PPARG) plays a crucial role in fatty acid (FA) metabolism through regulation of gene expression, including stearoyl-coenzyme A desaturase (SCD), which is the rate-limiting enzyme for the biosynthesis of monounsaturated FA. However, whether or how PPARG regulates the activity of mammary SCD in ruminants is unknown. This study explored the potential role of PPARG isoforms in regulating SCD mRNA expression in lactating goat mammary epithelial cells (GMEC). Using quantitative real-time PCR, we observed a positive correlation between PPARG and SCD expression in the goat mammary gland at peak lactation. Overexpression of both PPARG1 and PPARG2 in GMEC increased markedly the expression of SCD, the concentration of 16:1 and 18:1, and the desaturation indices of 16:1 and 18:1. The PPARG ligand rosiglitazone further increased SCD expression and desaturation indices in GMEC, overexpressing PPARG1 and PPARG2. Incubation with rosiglitazone alone increased the expression of SCD, but did not alter the concentration of 16- to 18-carbon FA or their desaturation indices. The results provide evidence that PPARG regulates the expression and activity of SCD in GMEC. As such, PPARG may contribute to regulation of SCD and monounsaturated FA synthesis during lactation.

MicroRNAs synergistically regulate milk fat synthesis in mammary gland epithelial cells of dairy goats.

Synergistic regulation among microRNAs (miRNAs) is important to understand the mechanisms underlying the complex molecular regulatory networks in goats. Goat milk fat synthesis is driven by a gene network that involves many biological processes in the mammary gland. These biological processes are affected by several miRNAs rather than a single miRNA. Therefore, identifying synergistic miRNAs is necessary to further understand the functions of miRNAs and the metabolism of goat milk fat synthesis. Using qRT-PCR, we assessed the expression of 11 miRNAs that have the potential to regulate milk fat synthesis in the goat mammary gland. Six of these miRNAs exhibited expression during the lactation cycle. Additionally, we also found that prolactin, the key hormone that regulates lactation, promotes the expression of four miRNAs (miR-23a, miR-27b, miR-103, and miR-200a). Further functional analysis showed that overexpression of all four miRNAs by using recombinant adenovirus in goat mammary gland epithelial cells can affect gene mRNA expression associated with milk fat synthesis. Specifically, elevated miR-200a expression suppressed the mRNA expression of genes involved in fat droplet formation. To analyze the synergistic regulation among these four miRNAs (miR-23a, miR-27b, miR-103, and miR-200a), we used the Pearson correlation coefficient to evaluate the correlation between their expression levels in 30 lactating goats. As a result, we found a strong correlation and mutual regulation between three miRNA pairs (miR-23a and miR-27b, miR-103 and miR-200a, miR-27b and miR-200a). This study provides the first experimental evidence that miRNA expression is synergistically regulated in the goat mammary gland and has identified the potential biological role of miRNAs in goat milk fat synthesis. The identification of synergistic miRNAs is a crucial step for further understanding the molecular network of milk fat synthesis at a system-wide level.

MicroRNA-24 can control triacylglycerol synthesis in goat mammary epithelial cells by targeting the fatty acid synthase gene.

In nonruminants it has been demonstrated that microRNA-24 (miR-24) is involved in preadipocyte differentiation, hepatic lipid, and plasma triacylglycerol synthesis. However, its role in ruminant mammary gland remains unclear. In this study we measured miR-24 expression in goat mammary gland tissue at 4 different stages of lactation and observed that it had highest expression at peak lactation when compared with the dry period. Overexpression or downregulation of miR-24 in goat mammary epithelial cells (GMEC) strongly affected fatty acid profiles; in particular, miR-24 enhanced unsaturated fatty acid concentration. Additional effects of miR-24 included changes in triacylglycerol content and the expression of fatty acid synthase, sterol regulatory element binding transcription protein 1, stearoyl-CoA desaturase, glycerol-3-phosphate acyltransferase mitochondrial, and acetyl-CoA carboxylase. Luciferase reporter assay confirmed that fatty acid synthase is a target of miR-24. Taken together, these results not only highlight the physiological importance of miR-24 in fatty acid metabolism in GMEC, but also laid the foundation for further research on regulatory mechanisms among miR-24 and other microRNA expressed in GMEC.

Inhibition of FASN reduces the synthesis of medium-chain fatty acids in goat mammary gland.

Fatty acid synthase (FASN) is known as a crucial enzyme of cellular de novo fatty acid synthesis in mammary gland which has been proved as the main source of short and medium-chain fatty acids of milk. However, the regulatory role of FASN in goat-specific milk fatty acids composition remains unclear. We cloned and analyzed the full-length of FASN gene from the mammary gland of Capra hircus (Xinong Saanen dairy goat) (DQ 915966). Comparative gene expression analysis suggested that FASN is predominantly expressed in fat, small intestine and mammary gland tissues, and expresses higher level at lactation period. Inhibition of FASN activity by different concentrations (0, 5, 15, 25 and 35 μM) of orlistat, a natural inhibitor of FASN, resulted in decreased expression of acetyl-CoA carboxylase α (ACCα), lipoprotein lipase and heart-type fatty acid binding protein (H-FABP) in a concentration-dependent manner in goat mammary gland epithelial cells (GMEC). Similar results were also obtained by silencing of FASN. Additionally, reduction of FASN expression also led to apparent decline of the relative content of decanoic acid (C10:0) and lauric acid (C12:0) in GMEC. Our study provides a direct evidence for inhibition of FASN reduces cellular medium-chain fatty acids synthesis in GMEC.

Short communication: Effect of inhibition of fatty acid synthase on triglyceride accumulation and effect on lipid metabolism genes in goat mammary epithelial cells

The role of fatty acid synthase (FASN) on de novo fatty acid synthesis has been well established. In monogastrics, unlike acetyl-coenzyme A carboxylase, FASN is primarily controlled at the transcriptional level. However, no data exist on ruminant mammary cells evaluating effects of FASN knockdown on mRNA expression of lipogenic genes. Inhibition of FASN in mammary cells by C75-mediated interference, a synthetic inhibitor of FASN activity, and short hairpin RNA-mediated interference markedly reduced cellular triglyceride content at least in part by decreasing the expression of genes related to triglyceride synthesis (GPAT, AGPAT6, and DGAT2) and enhancing the expression of lipolysis-related genes (ATGL and HSL). Consistent with the markedly lower expression of genes related to lipid droplet formation and secretion (TIP47, ADFP, BTN1A1, and XDH), cellular lipid droplets also were reduced sharply after incubation with C75 or adenovirus-short-hairpin-RNA. The results underscored the essential role of FASN in the overall process of milk-fat formation in goat mammary epithelial cells.

MiR130b-Regulation of PPARγ Coactivator- 1α Suppresses Fat Metabolism in Goat Mammary Epithelial Cells

Fat metabolism is a complicated process regulated by a series of factors. microRNAs (miRNAs) are a class of negative regulator of proteins and play crucial roles in many biological processes; including fat metabolism. Although there have been some researches indicating that miRNAs could influence the milk fat metabolism through targeting some factors, little is known about the effect of miRNAs on goat milk fat metabolism. Here we utilized an improved miRNA detection assay, S-Poly-(T), to profile the expression of miRNAs in the goat mammary gland in different periods, and found that miR-130b was abundantly and differentially expressed in goat mammary gland. Additionally, overexpressing miR-130b impaired adipogenesis while inhibiting miR-130b enhanced adipogenesis in goat mammary epithelial cells. Utilizing 3’-UTR assay and Western Blot analusis, the protein peroxisome proliferator-activated receptor coactivator-1α (PGC1α), a major regulator of fat metabolism, was demonstrated to be a potential target of miR-130b. Interestingly, miR-130b potently repressed PGC1α expression by targeting both the PGC1α mRNA coding and 3’ untranslated regions. These findings have some insight of miR-130b in mediating adipocyte differentiation by repressing PGC1α expression and this contributes to further understanding about the functional significance of miRNAs in milk fat synthesis.

Regulation of the fatty acid synthase promoter by liver X receptor α through direct and indirect mechanisms in goat mammary epithelial cells.

Fatty acid synthase (FASN) is a central enzyme of milk fat synthesis in the ruminant mammary gland. However, the mechanisms regulating goat FASN transcription remain elusive. The objective of this study was to investigate the mechanisms by which liver X receptor α (LXRα) regulates the FASN promoter in goat mammary epithelial cells (GMECs). In this study, T0901317 (T09), an agonist for LXRα, significantly enhanced the mRNA expression and promoter activity of FASN. Cloning of the dairy goat FASN promoter revealed the presence of one LXR response element (LXRE) and two sterol regulatory elements (SREs). Deletion or mutation of the FASN promoter LXRE reduced, but did not eliminate the transcriptional response of FASN to T09. While the LXRE and the SREs were both disrupted, basal transcription was severely reduced and there was no response to T09 treatment. This suggested that a complete response required one LXRE and two SREs. Knockdown of LXRα by siRNA did not alter the basal or T09-induced transcriptional activity of FASN. However, when sterol regulatory binding protein 1 (SREBP1) was knocked down, T09 significantly increased FASN transcription by wild-type GMECs, but had no effect on cells with LXRE-mutant promoters. The results suggested that LXR regulates FASN promoter activity through direct interaction with the LXRE as well as through increasing SREBP1 abundance. The present study provides insight into the transcriptional regulatory mechanisms controlling de novo fatty acid synthesis in GMECs.

Adipocyte differentiation-related protein promotes lipid accumulation in goat mammary epithelial cells

Milk fat originates from the secretion of cytosolic lipid droplets (CLD) synthesized within mammary epithelial cells. Adipocyte differentiation-related protein (ADRP; gene symbol PLIN2) is a CLD-binding protein that is crucial for synthesis of mature CLD. Our hypothesis was that ADRP regulates CLD production and metabolism in goat mammary epithelial cells (GMEC) and thus plays a role in determining milk fat content. To understand the role of ADRP in ruminant milk fat metabolism, ADRP (PLIN2) was overexpressed or knocked down in GMEC using an adenovirus system. Immunocytochemical staining revealed that ADRP localized to the surface of CLD. Supplementation with oleic acid (OA) enhanced its colocalization with CLD surface and enhanced lipid accumulation. Overexpression of ADRP increased lipid accumulation and the concentration of triacylglycerol in GMEC. In contrast, morphological examination revealed that knockdown of ADRP decreased lipid accumulation even when OA was supplemented. This response was confirmed by the reduction in mass of cellular TG when ADRP was knocked down. The fact that knockdown of ADRP did not completely eliminate lipid accumulation at a morphological level in GMEC without OA suggests that some other compensatory factors may also aid in the process of CLD formation. The ADRP reversed the decrease of CLD accumulation induced by adipose triglyceride lipase. This is highly suggestive of ADRP promoting triacylglycerol stability within CLD by preventing access to adipose triglyceride lipase. Collectively, these data provide direct in vitro evidence that ADRP plays a key role in CLD formation and stability in GMEC.