Faculty Bibliography

BACKGROUND & AIMS/OBJECTIVE:Lipoprotein assembly in the small intestine and liver is critical for the transport of dietary and endogenous lipids. Pla2g12b has recently been shown to play a role in lipoprotein assembly in mice livers and zebrafish larvae. Pla2g12b knockout and mutant (MUT) mice with the C129Y missense mutation have low plasma cholesterol levels. However, the role of Pla2g12b in the intestine and the reason why C129Y mutation decreases plasma lipids are unknown. METHODS:) and WT control mice were used in parallel to study plasma lipids and lipoproteins, lipid absorption and hepatic lipoprotein production studies. Transmission electron microscopy was used to visualize lipid transit through enterocytes. RESULTS:We observed that Pla2g12b expression was the highest in the duodenum. Furthermore, male and female chow fed 3-month-old MUT mice and wildtype (WT) mice expressed similar amounts of Pla2g12b protein and several genes in lipid metabolism. Nonetheless, the MUT mice had significantly lower plasma triglyceride (TG), cholesterol, HDL-C, LDL-C, apoB48, and apoB100 levels than WT mice. Several mechanisms for lower plasma lipids and lipoproteins in MUT mice were investigated. C129Y mutation had no effect on the expression of Pla2g12b and several other proteins necessary for lipid transport. Therefore, the low plasma lipid levels in MUT mice were neither due to the absence of Pla2g12b protein nor due to reductions in critical proteins in lipid transport. Next, we addressed the role of Pla2g12b in hepatic lipid mobilization and intestinal lipid absorption. MUT livers exhibited normal TG synthesis, defective TG secretion, and enhanced fat accumulation. MUT mice also showed defective intestinal TG absorption, intracellular lipid accumulation, and elevated TG excretion in the feces. CONCLUSIONS:We propose that C129 in Pla2g12b is critical for the assembly and secretion of lipoproteins by the liver and intestine.

The kidneys contribute to glucose homeostasis by gluconeogenesis and glucose reabsorption. Herein, we identified previously unknown fasting-induced, glucagon-mediated inhibitory effect of the circadian clock gene basic helix-loop-helix ARNT like 1 (Bmal1) on the expression of the main proximal tubule glucose transporter solute carrier family 5 member 2 (Sglt2) in mice. During fasting, glucagon induces Bmal1, which increases expression of nuclear receptor subfamily 1, group D, member 1 (Rev-erbα). Rev-erbα represses nuclear respiratory factor 1, a transcriptional activator of Sglt2, and diminishes Sglt2 expression and thereby kidney glucose reabsorption capacity. During refeeding (lower glucagon) this process is attenuated, thereby inducing glucose reabsorption. The physiological role of this mechanism appears to ensure optimal temporal retrieval of filtered glucose during fasting/refeeding. Thus, this study demonstrates that during fasting and refeeding, glucagon regulates renal glucose reabsorption by utilizing the local cellular circadian machinery.

Atherosclerotic cardiovascular disease is a major contributor to the global disease burden. Atherosclerosis initiation depends on cholesterol accumulation in subendothelial macrophages (Mφs). To clarify the role of Bmal1 in Mφ function and atherosclerosis, we used several global and myeloid-specific Bmal1-deficient mouse models. Myeloid-specific Bmal1-deficient mice had higher Mφ cholesterol and displayed greater atherosclerosis compared with controls. Bmal1-deficient Mφs exhibited: (a) elevated expression of Cd36 and uptake of oxLDL; (b) diminished expression of Abca1 and Abcg1, and decreased cholesterol efflux and reverse cholesterol transport; and (c) reduced Npc1 and Npc2 expression and diminished cholesterol egress from lysosomes. Molecular studies revealed that Bmal1 directly regulates basal and cyclic expression of Npc1 and Npc2 by binding the E-box motif (CANNTG) sequence recognized by Bmal1 in their promoters and indirectly regulates the basal and temporal regulation of Cd36 and Abca1/Abcg1 involving Rev-erbα and Znf202 repressors, respectively. In conclusion, Mφ Bmal1 is a key regulator of the uptake of modified lipoproteins, cholesterol efflux, lysosomal cholesterol egress, and atherosclerosis and, therefore, may be a master regulator of cholesterol metabolism in Mφs. Restoration of Mφ Bmal1 expression or blocking of factors that decrease its activity may be effective in preventing atherosclerosis.

Macrophages are essential immune cells that play crucial roles in inflammation and tissue homeostasis, and are important regulators of metabolic processes, such as the metabolism of glucose, lipids, and amino acids. The regulation of macrophage metabolism by circadian clock genes has been emphasized in many studies. Changes in metabolic profiles occurring after the perturbation of macrophage circadian cycles may underlie the etiology of several diseases. Specifically, chronic inflammatory disorders, such as atherosclerosis, diabetes, cardiovascular diseases, and liver dysfunction, are associated with poor macrophage metabolism. Developing treatment approaches that target metabolic and immunological ailments requires an understanding of the complex relationships among clock genes, disease etiology, and macrophage metabolism. This review explores the molecular mechanisms through which clock genes regulate lipid, amino acid, and glucose metabolism in macrophages, and discusses their potential roles in the development and progression of metabolic disorders. The findings underscore the importance of maintaining circadian homeostasis in macrophage function as a promising avenue for therapeutic intervention in diseases involving metabolic dysregulation, given its key roles in inflammation and tissue homeostasis. Moreover, reviewing the therapeutic implications of circadian rhythm in macrophages can help minimize the side effects of treatment. Novel strategies may be beneficial in treating immune-related diseases cause by shifted and blunted circadian rhythms via light exposure, jet lag, seasonal changes, and shift work or disruption to the internal clock (such as stress or disease).

Circadian rhythms are intrinsic, 24 h cycles that regulate key physiological, mental, and behavioral processes, including sleep-wake cycles, hormone secretion, and metabolism. These rhythms are controlled by the brain's suprachiasmatic nucleus, which synchronizes with environmental signals, such as light and temperature, and consequently maintains alignment with the day-night cycle. Molecular feedback loops, driven by core circadian "clock genes", such as Clock, Bmal1, Per, and Cry, are essential for rhythmic gene expression; disruptions in these feedback loops are associated with various health issues. Dysregulated lipid metabolism in the brain has been implicated in the pathogenesis of neurological disorders by contributing to oxidative stress, neuroinflammation, and synaptic dysfunction, as observed in conditions such as Alzheimer's and Parkinson's diseases. Disruptions in circadian gene expression have been shown to perturb lipid regulatory mechanisms in the brain, thereby triggering neuroinflammatory responses and oxidative damage. This review synthesizes current insights into the interconnections between circadian rhythms and lipid metabolism, with a focus on their roles in neurological health and disease. It further examines how the desynchronization of circadian genes affects lipid metabolism and explores the potential mechanisms through which disrupted circadian signaling might contribute to the pathophysiology of neurodegenerative disorders.

Circadian rhythms controlled by clock genes affect plasma lipids, known risk factors for atherosclerosis. Clock and Bmal1 are critical clock genes that positively regulate circadian rhythms. We have shown that Clock plays an important role in lipid absorption. Additionally, we showed that global ablation of Bmal1 in Apoe–/– and Ldlr–/– mice, and liver-specific ablation of Bmal1 in Apoe–/– (L-Bmal1–/–Apoe–/–) mice increases atherosclerosis. However, it is unknown whether Bmal1 regulates intestinal lipid absorption.We studied cholesterol and triglyceride absorption in normal and lipase-inhibited Bmal1–/– and Bmal1–/–Apoe–/– mice with global Bmal1 deficiency and compared it with Bmal1+/+ and Bmal1+/+Apoe–/–, respectively. To study the effect of liver-specific Bmal1 deficiency, studies were performed in L-Bmal1–/– and L-Bmal1–/–Apoe–/– mice and data were compared with Bmal1fl/fl and Bmal1fl/flApoe–/– mice. In addition, enterocytes isolated from these mice were used to study uptake and secretion of cholesterol and oleic acid. Further, protein and mRNA levels of candidate genes involved in lipid uptake, lipoprotein assembly and secretion were quantified. Both normal and lipase-inhibited Bmal1–/– mice absorbed significantly higher amounts of cholesterol and triglyceride. Further, uptake and secretion of cholesterol and fatty acids by enterocytes was increased in Bmal1–/– mice. As reported previously, liver-specific Bmal1 ablation increased VLDL production. Surprisingly, postprandial plasma lipids were also significantly increased in L-Bmal1–/– mice than in control Bmal1fl/fl mice. Further, enterocytes isolated from L-Bmal1–/– took up more lipids and secreted more lipoproteins. Moreover, in vivo lipid absorption in L-Bmal1–/– was increased. Gene expression quantifications revealed that intestinal MTP, DGAT1, CD36 and NPC1L1 mRNA levels were increased in global and hepatic Bmal1 deficient animals. Global and liver-specific ablation of Bmal1 increases intestinal lipoprotein production by increasing the expression of genes involved in lipid uptake and lipoprotein assembly. These data for the first time suggest that intestinal lipoprotein assembly is regulated by hepatic Bmal1.