Rapid Communication: Period2 gene silencing increases the synthesis of αs-casein protein in bovine mammary epithelial cells

Circadian Gene PER2 Silencing Downregulates PPARG and SREBF1 and Suppresses Lipid Synthesis in Bovine Mammary Epithelial Cells

Simple Summary

The present study was constructed to determine the effects of the core circadian clock gene, Period 2 (PER2), on lipid synthesis in bovine mammary epithelial cells (BMECs). Data revealed that PER2-regulated genes were involved in fatty acid de novo synthesis, desaturation, TAG accumulation, and lipid droplet secretion in primary BMECs, partly by inhibiting PPARG and SREBF1. Our overall data suggests that PER2 in bovine mammary cells plays a role in regulating milk fat synthesis directly, or via the activation of the transcription regulators PPARG and SREBF1. This study provides molecular evidence underscoring a link between the circadian clock and lipid metabolism in bovines.

Abstract

PER2, a circadian clock gene, is associated with mammary gland development and lipid synthesis in rodents, partly via regulating peroxisome proliferator-activated receptor gamma (PPARG). Whether such a type of molecular link existed in bovines was unclear. We hypothesized that PER2 was associated with lipid metabolism and regulated cell cycles and apoptosis in bovine mammary epithelial cells (BMECs). To test this hypothesis, BMECs isolated from three mid-lactation (average 110 d postpartum) cows were used. The transient transfection of small interfering RNA (siRNA) was used to inhibit PER2 transcription in primary BMECs. The silencing of PER2 led to lower concentrations of cellular lipid droplets and triacylglycerol along with the downregulation of lipogenic-related genes such as ACACA, FASN, LPIN1, and SCD, suggesting an overall inhibition of lipogenesis and desaturation. The downregulation of PPARG and SREBF1 in response to PER2 silencing underscored the importance of circadian clock signaling and the transcriptional regulation of lipogenesis. Although the proliferation of BMECs was not influenced by PER2 silencing, the number of cells in the G2/GM phase was upregulated. PER2 silencing did not affect cell apoptosis. Overall, the data provided evidence that PER2 participated in the coordination of mammary lipid metabolism and was potentially a component of the control of lipid droplets and TAG synthesis in ruminant mammary cells. The present data suggested that such an effect could occur through direct effects on transcriptional regulators.

The Circadian Physiology: Implications in Livestock Health

Circadian rhythms exist in almost all types of cells in mammals. Thousands of genes exhibit approximately 24 h oscillations in their expression levels, making the circadian clock a crucial regulator of their normal functioning. In this regard, environmental factors to which internal physiological processes are synchronized (e.g., nutrition, feeding/eating patterns, timing and light exposure), become critical to optimize animal physiology, both by managing energy use and by realigning the incompatible processes. Once the circadian clock is disrupted, animals will face the increased risks of diseases, especially metabolic phenotypes. However, little is known about the molecular components of these clocks in domestic species and by which they respond to external stimuli. Here we review evidence for rhythmic control of livestock production and summarize the associated physiological functions, and the molecular mechanisms of the circadian regulation in pig, sheep and cattle. Identification of environmental and physiological inputs that affect circadian gene expressions will help development of novel targets and the corresponding approaches to optimize production efficiency in farm animals.

Ruminal epithelial cell proliferation and short-chain fatty acid transporters in vitro are associated with abundance of period circadian regulator 2 (PER2)

The major circadian clock gene PER2 is closely related to cell proliferation and lipid metabolism in various nonruminant cell types. Objectives of the study were to evaluate circadian clock-related mRNA abundance in cultured goat ruminal epithelial cells (REC), and to determine effects of PER2 on cell proliferation and mRNA abundance of short-chain fatty acid (SCFA) transporters, genes associated with lipid metabolism, cell proliferation, and apoptosis. Ruminal epithelial cells were isolated from weaned Boer goats (n = 3; 2 mo old; ∼10 kg of body weight) by serial trypsin digestion and cultured at 37°C for 24 h. Abundance of CLOCK and PER2 proteins in cells was determined by immunofluorescence. The role of PER2 was assessed through the use of a knockout model with short interfering RNA, and sodium butyrate (15 mM) was used to assess the effect of upregulating PER2. Both CLOCK and PER2 were expressed in REC in vitro. Sodium butyrate stimulation increased mRNA and protein abundance of PER2 and PER3. Furthermore, PER2 gene silencing enhanced cell proliferation and reduced cellular apoptosis in isolated REC. In contrast, PER2 overexpression in response to sodium butyrate led to lower cellular proliferation and ratio of cells in the S phase along with greater ratio of cells in the G₂/M phase. Those responses were accompanied by downregulated mRNA abundance of CCND1, CCNB1, CDK1, and CDK2. Among the SCFA transporters, PER2 silencing upregulated mRNA abundance of MCT1 and MCT4. However, it downregulated mRNA abundance of PPARA and PPARG. Overexpression of PER2 resulted in lower mRNA abundance of MCT1 and MCT4, and greater PPARA abundance. Overall, data suggest that CLOCK and PER2 might play a role in the control of cell proliferation, SCFA, and lipid metabolism. Further studies should be conducted to evaluate potential mechanistic relationships between circadian clock and SCFA absorption in vivo.