Faculty Bibliography

Coordinated regulation of fat storage and utilization is essential for energy homeostasis, and its disruption is associated with metabolic syndrome and atherosclerosis in humans. Across species, Krüppel-like transcription factors (KLFs) have been identified as key components of adipogenesis. In humans, KLF14 acts as a master transregulator of adipose gene expression in type 2 diabetes and cis-acting expression quantitative trait locus associated with high-density lipoprotein cholesterol. Herein we report that, in Caenorhabditis elegans, mutants in klf-3 accumulate large fat droplets rich in neutral lipids in the intestine; this lipid accumulation is associated with an increase in triglyceride levels. The klf-3 mutants show normal pharyngeal pumping; however, they are sterile or semisterile. We explored important genetic interactions of klf-3 with the genes encoding enzymes involved in fatty acid (FA) β-oxidation in mitochondria or peroxisomes and FA synthesis in the cytosol, namely acyl-CoA synthetase (acs-1 and acs-2), acyl-CoA oxidase (F08A8.1 and F08A8.2), and stearoyl-CoA desaturase (fat-7). We show that mutations or RNA interference in these genes increases fat deposits in the intestine of acs-1, acs-2, F08A8.1, and F08A8 animals. We further show that acs-1 and F08A8.1 influence larval development and fertility, respectively. Thus, KLF3 may regulate FA utilization in the intestine and reproductive tissue. We demonstrate that depletion of F08A8.1 activity, but not of acs-1, acs-2, F08A8.2, or fat-7 activity, enhances the fat phenotype of the klf-3 mutant. Taken together, these results suggest that klf-3 regulates lipid metabolism, along with acs-1, acs-2, F08A8.1, and F08A8.2, by promoting FA β-oxidation and, in parallel, may contribute to normal reproductive behavior and fecundity in C. elegans.

Apolipoprotein (apo) A-IV overexpression enhances chylomicron (CM) assembly and secretion in newborn swine intestinal epithelial cells by producing larger particles (Lu S, Yao Y, Cheng X, Mitchell S, Leng S, Meng S, Gallagher JW, Shelness GS, Morris GS, Mahan J, Frase S, Mansbach CM, Weinberg RB, Black DD. J Biol Chem 281: 3473-3483, 2006). To determine the impact of apo A-IV on microsomal triglyceride transfer protein (MTTP), IPEC-1 cell lines containing a tetracycline-regulatable expression system were used to overexpress native swine apo A-IV and "piglike" human apo A-IV, a mutant human apo A-IV with deletion of the EQQQ-rich COOH-terminus, previously shown to upregulate basolateral triglyceride (TG) secretion 5-fold and 25-fold, respectively. Cells were incubated 24 h with and without doxycycline and oleic acid (OA, 0.8 mM). Overexpression of the native swine apo A-IV and piglike human apo A-IV increased MTTP lipid transfer activity by 39.7% (P = 0.006) and 53.6% (P = 0.0001), respectively, compared with controls. Changes in mRNA and protein levels generally paralleled changes in activity. Interestingly, native swine apo A-IV overexpression also increased MTTP large subunit mRNA, protein levels, and lipid transfer activity in the absence of OA, suggesting a mechanism not mediated by lipid absorption. Overexpression of piglike human apo A-IV significantly increased partitioning of radiolabeled OA from endoplasmic reticulum (ER) membrane to lumen, suggesting increased net transfer of membrane TG to luminal particles. These results suggest that the increased packaging of TG into nascent CMs in the ER lumen, induced by apo A-IV, is associated with upregulation of MTTP activity at the pretranslational level. Thus MTTP is regulated by apo A-IV in a manner to promote increased packaging of TG into the CM core, which may be important in neonatal fat absorption.

We examined the role of clock genes in the diurnal regulation of plasma triglyceride-rich apolipoprotein B-lipoproteins and their biosynthetic chaperone, microsomal triglyceride transfer protein (MTP). Clock(mt/mt) mice showed sustained hypertriglyceridemia and high MTP expression. CLOCK knockdown activated MTP promoter and reduced small heterodimer partner (SHP, NROB2). CLOCK upregulated SHP by binding to its E box. SHP suppressed MTP expression by binding to the HNF4alpha/LRH-1 at the MTP promoter. Cyclic expression of MTP after serum shock was abrogated by siCLOCK and siSHP. Plasma triglyceride and MTP showed reduced diurnal variations in Shp(-/-) mice. Whereas peaks and nadirs in SHP expression were inversely correlated with those of MTP, these changes were reduced in Clock(mt/mt) mice. Expression of Shp abrogated hypertriglyceridemia in Clock(mt/mt) mice. Together, these studies describe a role of Clock/Shp in the diurnal regulation of MTP and plasma triglyceride and indicate that disruptions in circadian regulation might cause hyperlipidemia.

Clock genes respond to external stimuli and exhibit circadian rhythms. This study investigated the expression of clock genes in the small intestine and their contribution in the regulation of nutrient absorption by enterocytes. We examined expression of clock genes and macronutrient transport proteins in the small intestines of wild-type and Clock mutant (Clk(mt/mt)) mice with free or limited access to food. In addition, we studied absorption of macronutrients in these mice. Intestinal clock genes show circadian expression and respond to food entrainment in wild-type mice. Dominant negative Clock in Clk(mt/mt) mice disrupts circadian expression and food entrainment of clock genes. The absorption of lipids and monosaccharides was high in Clk(mt/mt) mice whereas peptide absorption was reduced. Molecular studies revealed that Clock regulates several transport proteins involved in nutrient absorption. Clock plays an important role in light and food entrainment of intestinal functions by regulating nutrient transport proteins. Disruptions in intestinal circadian activity may contribute to hyperlipidemia and hyperglycemia.

Light and food are two major environmental factors that impact daily life. Light entrainment is centrally controlled by suprachiasmatic nuclei of the hypothalamus. Food entrainment might require cooperation between the intestine and dorsomedial hypothalamus. Clock genes that are essential for light entrainment also play a part in food entrainment. Understanding the role of clock genes in the entrainment of intestinal functions, as well as in gut-brain communication during food entrainment, will enhance our understanding of gastrointestinal and metabolic disorders. This review highlights recent studies examining light- and food-entrained regulation of plasma lipids and of various intestinal activities and offers insight into the role of the intestine in food entrainment.

Serine palmitoyltransferase (SPT) is the key enzyme for the biosynthesis of sphingolipids. It has been reported that oral administration of myriocin (an SPT inhibitor) decreases plasma sphingomyelin (SM) and cholesterol levels, and reduces atherosclerosis in apoE knockout (KO) mice. We studied cholesterol absorption in myriocin-treated WT or apoE KO animals and found that, after myriocin treatment, the mice absorbed significantly less cholesterol than controls, with no observable pathological changes in the small intestine. More importantly, we found that heterozygous Sptlc1 (a subunit of SPT) KO mice also absorbed significantly less cholesterol than controls. To understand the mechanism, we measured protein levels of Niemann-Pick C1-like 1 (NPC1L1), ABCG5, and ABCA1, three key factors involved in intestinal cholesterol absorption. We found that NPC1L1 and ABCA1 were decreased, whereas ABCG5 was increased in the SPT deficient small intestine. SM levels on the apical membrane were also measured and they were significantly decreased in SPT deficient mice, compared with controls. In conclusion, SPT deficiency might reduce intestinal cholesterol absorption by altering NPC1L1 and ABCG5 protein levels in the apical membranes of enterocytes through lowering apical membrane SM levels. This may be also true for ABCA1 which locates on basal membrane of enterocytes. Manipulation of SPT activity could thus provide a novel alternative treatment for dyslipidemia.

PURPOSE OF REVIEW/OBJECTIVE:This review summarizes recent advances about the role of microsomal triglyceride transfer protein in plasma and tissue lipid homeostasis. RECENT FINDINGS/RESULTS:Microsomal triglyceride transfer protein emerged as a phospholipid transfer protein and acquired triacylglycerol transfer activity during evolution from invertebrates to vertebrates. These activities are proposed to participate in 'nucleation' and 'desorption' steps during the biosynthesis of primordial apoB-containing lipoproteins. Microsomal triglyceride transfer protein also transfers phospholipids to the glycolipid antigen presentation molecule CD1d. Under physiologic conditions, plasma apoB-containing lipoproteins and microsomal triglyceride transfer protein expression exhibit diurnal variations synchronized by food and light. Microsomal triglyceride transfer protein is regulated at the transcriptional level. HNF4alpha is critical for its transcription. Other transcription factors along with coactivators and corepressors modulate microsomal triglyceride transfer protein expression. Reductions in microsomal triglyceride transfer protein mRNA and activity are related to steatosis in HCV-3 infected patients. CCl4 induces steatosis by enhancing proteasomal degradation of microsomal triglyceride transfer protein and can be partially avoided by inhibiting this degradation. Chemical antagonists cause hepatosteatosis, but this was not seen in the absence of fatty acid binding protein. SUMMARY/CONCLUSIONS:Microsomal triglyceride transfer protein is a target to lower plasma lipids and to reduce inflammation in certain immune disorders. More knowledge is required, however, regarding its regulation and its role in the biosynthesis of apoB-containing lipoproteins and CD1d.