TTC39B destabilizes retinoblastoma protein promoting hepatic lipogenesis in a sex-specific fashion.

BACKGROUND & AIMS: Molecular mechanisms underlying the different susceptibility of men and women to non-alcoholic fatty liver disease (NAFLD) are poorly understood. The TTC39B locus encodes a scaffolding protein, associates with gynecological disorders and its deletion protects mice from diet-induced steatohepatitis. This study aimed to elucidate the molecular mechanisms linking TTC39B (T39) to the expression of lipogenic genes and to explore sex-specific effects. METHODS: Co-expression in HEK293A cells validated the novel T39/pRb interaction predicted by a protein-protein interaction algorithm. T39 was knocked down using an antisense oligonucleotide (ASO) in mice with dietary NAFLD and a genetic deficiency of pRb or its downstream effector E2F1, as well as in primary human hepatocytes. RESULTS: T39 interacts with pRb via its C-terminal TPR domain and promotes its proteasomal degradation. In female mice, T39 deficiency reduces the mRNA of lipogenic genes, especially Pnpla3, in a pRb- and E2F1-dependent manner. In contrast, in male mice, T39 deficiency results in a much smaller reduction in lipogenic gene expression that is independent of pRb/E2F1. T39 also interacts with VAPB via an N-terminal FFAT motif and stabilizes the interaction of VAPB with SCAP. Ovariectomy abolishes the effect of T39 knockdown on the hepatic pRb/E2F1/Pnpla3 axis. In both sexes T39 knockdown reduces SCAP independently of pRb. In primary human hepatocytes, T39 knockdown reduces expression of PNPLA3 and other lipogenic genes in women but not men. CONCLUSIONS: We have uncovered a conserved sexual dimorphism in the regulation of hepatic lipogenic genes, with effects of T39 mediated through pRb/E2F1 in females and VAPB/SCAP in both sexes. T39 inhibition could be a novel strategy to downregulate PNPLA3 and treat NAFLD in women. LAY SUMMARY: In females, the protein TTC39B degrades a tumor suppressor in the liver to promote the synthesis of new fat and the expression of a major genetic risk factor for non-alcoholic fatty liver disease. TTC39B is a potential therapeutic target for non-alcoholic fatty liver disease, especially in women.

Metformin and AMP Kinase Activation Increase Expression of the Sterol Transporters ABCG5/8 (ATP-Binding Cassette Transporter G5/G8) With Potential Antiatherogenic Consequences.

OBJECTIVE: The mechanisms underlying the cardiovascular benefit of the anti-diabetic drug metformin are poorly understood. Recent studies have suggested metformin may upregulate macrophage reverse cholesterol transport. The final steps of reverse cholesterol transport are mediated by the sterol transporters, ABCG5 (ATP-binding cassette transporter G5) and ABCG8 (ATP-binding cassette transporter G8), which facilitate hepato-biliary transport of cholesterol. This study was undertaken to assess the possibility that metformin induces Abcg5 and Abcg8 expression in liver and to elucidate the underlying mechanisms. APPROACH AND RESULTS: Metformin-treated mouse or human primary hepatocytes showed increased expression of Abcg5/8 and the bile salt export pump, Bsep. Administration of metformin to Western-type diet-fed mice showed significant upregulation of Abcg5/8 and Bsep. This resulted in increased initial clearance of 3H-cholesteryl ester HDL (high-density lipoprotein) from plasma. However, fecal 3H-cholesterol output was only marginally increased, possibly reflecting increased hepatic Ldlr (low-density lipoprotein receptor) expression, which would increase nonradiolabeled cholesterol uptake. Abcg5/8 undergo strong circadian variation. Available chromatin immunoprecipitation-Seq data suggested multiple binding sites for Period 2, a transcriptional repressor, within the Abcg5/8 locus. Addition of AMPK (5' adenosine monophosphate-activated protein kinase) agonists decreased Period 2 occupancy, suggesting derepression of Abcg5/8. Inhibition of ATP citrate lyase, which generates acetyl-CoA from citrate, also decreased Period 2 occupancy, with concomitant upregulation of Abcg5/8. This suggests a mechanistic link between feeding-induced acetyl-CoA production and decreased cholesterol excretion via Period 2, resulting in inhibition of Abcg5/8 expression. CONCLUSIONS: Our findings provide partial support for the concept that metformin may provide cardiovascular benefit via increased reverse cholesterol transport but also indicate increased Ldlr expression as a potential additional mechanism. AMPK activation or ATP citrate lyase inhibition may mediate antiatherogenic effects through increased ABCG5/8 expression.

CAMKIIγ suppresses an efferocytosis pathway in macrophages and promotes atherosclerotic plaque necrosis.

Atherosclerosis is the underlying etiology of cardiovascular disease, the leading cause of death worldwide. Atherosclerosis is a heterogeneous disease in which only a small fraction of lesions lead to heart attack, stroke, or sudden cardiac death. A distinct type of plaque containing large necrotic cores with thin fibrous caps often precipitates these acute events. Here, we show that Ca2+/calmodulin-dependent protein kinase γ (CaMKIIγ) in macrophages plays a major role in the development of necrotic, thin-capped plaques. Macrophages in necrotic and symptomatic atherosclerotic plaques in humans as well as advanced atherosclerotic lesions in mice demonstrated activation of CaMKII. Western diet-fed LDL receptor-deficient (Ldlr-/-) mice with myeloid-specific deletion of CaMKII had smaller necrotic cores with concomitantly thicker collagen caps. These lesions demonstrated evidence of enhanced efferocytosis, which was associated with increased expression of the macrophage efferocytosis receptor MerTK. Mechanistic studies revealed that CaMKIIγ-deficient macrophages and atherosclerotic lesions lacking myeloid CaMKIIγ had increased expression of the transcription factor ATF6. We determined that ATF6 induces liver X receptor-α (LXRα), an Mertk-inducing transcription factor, and that increased MerTK expression and efferocytosis in CaMKIIγ-deficient macrophages is dependent on LXRα. These findings identify a macrophage CaMKIIγ/ATF6/LXRα/MerTK pathway as a key factor in the development of necrotic atherosclerotic plaques.

GLP-1 and GLP-2 as yin and yang of intestinal lipoprotein production: evidence for predominance of GLP-2-stimulated postprandial lipemia in normal and insulin-resistant states.

The glucagon-like peptides (GLP-1 and GLP-2) are processed from the proglucagon polypeptide and secreted in equimolar amounts but have opposite effects on chylomicron (CM) production, with GLP-1 significantly reducing and GLP-2 increasing postprandial chylomicronemia. In the current study, we evaluated the apparent paradoxical roles of GLP-1 and GLP-2 under physiological conditions in the Syrian golden hamster, a model with close similarity to humans in terms of lipoprotein metabolism. A short (30-min) intravenous infusion of GLP-2 resulted in a marked increase in postprandial apolipoprotein B48 (apoB48) and triglyceride (TG) levels in the TG-rich lipoprotein (TRL) fraction, whereas GLP-1 infusion decreased lipid absorption and levels of TRL-TG and apoB48. GLP-1 and GLP-2 coinfusion resulted in net increased lipid absorption and an increase in TRL-TG and apoB48. However, prolonged (120-min) coinfusion of GLP-1 and GLP-2 decreased postprandial lipemia. Blocking dipeptidyl peptidase-4 activity resulted in decreased postprandial lipemia. Interestingly, fructose-fed, insulin-resistant hamsters showed a more pronounced response, including possible hypersensitivity to GLP-2 or reduced sensitivity to GLP-1. In conclusion, under normal physiological conditions, the actions of GLP-2 predominate; however, when GLP-1 activity is sustained, the hypolipidemic action of GLP-1 predominates. Pharmacological inhibition of GLP-1 degradation tips the balance toward an inhibitory effect on intestinal production of atherogenic CM particles.

Regulation of intestinal chylomicron production by glucagon-like peptides.

Apolipoprotein B48 (apoB48)-containing triglyceride-rich lipoproteins are atherogenic and therefore it is important to understand factors that regulate their metabolism in the intestine. Insulin resistant states are associated with increased intestinal output of apo B48, but the mechanistic studies explaining this overproduction have relied heavily on models of diet-induced insulin resistance. There is evidence that glucagon-like peptide (GLP) secretion is diminished in insulin resistant and diabetic states, which may have implications for postprandial lipid secretion. This review presents a survey of studies on GLPs and intestinal lipoprotein metabolism, along with some insights into the net physiological significance of GLP action in postprandial lipoprotein metabolism. Studies with GLP-1 receptor agonists and dipeptidyl peptidase-IV inhibitors have indicated that GLP-1 in pharmacological and physiological doses can inhibit intestinal TRL production. On the other hand, GLP-2 has an acute stimulatory effect on intestinal apoB48 secretion. Modulating GLP action may serve as a strategy to improve postprandial dyslipidemia in insulin resistant states.