CREBH normalizes dyslipidemia and halts atherosclerosis in diabetes by decreasing circulating remnant lipoproteins

Elevated apolipoprotein C3 augments diabetic kidney disease and associated atherosclerosis in type 2 diabetes

Diabetes increases the risk of both cardiovascular disease and kidney disease. Notably, most of the excess cardiovascular risk in people with diabetes is in those with kidney disease. Apolipoprotein C3 (APOC3) is a key regulator of plasma triglycerides, and it has recently been suggested to play a role in both type 1 diabetes-accelerated atherosclerosis and kidney disease progression. To investigate if APOC3 plays a role in kidney disease in people with type 2 diabetes, we analyzed plasma levels of APOC3 from the Veterans Affairs Diabetes Trial. Elevated baseline APOC3 levels predicted a greater loss of renal function. To mechanistically test if APOC3 plays a role in diabetic kidney disease and associated atherosclerosis, we treated black and tan, brachyury, WT and leptin-deficient (OB; diabetic) mice, a model of type 2 diabetes, with an antisense oligonucleotide (ASO) to APOC3 or a control ASO, all in the setting of human-like dyslipidemia. Silencing APOC3 prevented diabetes-augmented albuminuria, renal glomerular hypertrophy, monocyte recruitment, and macrophage accumulation, partly driven by reduced ICAM1 expression. Furthermore, reduced levels of APOC3 suppressed atherosclerosis associated with diabetes. This suggests that targeting APOC3 might benefit both diabetes-accelerated atherosclerosis and kidney disease.

Apolipoprotein C3: form begets function

Increased circulating levels of apolipoprotein C3 (APOC3) predict cardiovascular disease (CVD) risk in humans, and APOC3 promotes atherosclerosis in mouse models. APOC3's mechanism of action is due in large part to its ability to slow the clearance of triglyceride-rich lipoproteins (TRLs) and their remnants when APOC3 is carried by these lipoproteins. However, different pools and forms of APOC3 exert distinct biological effects or associations with atherogenic processes. Thus, lipid-free APOC3 induces inflammasome activation in monocytes whereas lipid particle-bound APOC3 does not. APOC3-enriched LDL binds better to the vascular glycosaminoglycan biglycan than does LDL depleted of APOC3. Patterns of APOC3 glycoforms predict CVD risk differently. The function of APOC3 bound to HDL is largely unknown. There is still much to learn about the mechanisms of action of different forms and pools of APOC3 in atherosclerosis and CVD, and whether APOC3 inhibition would prevent CVD risk in patients on LDL-cholesterol lowering medications.

Regulatory mechanisms of the cAMP-responsive element binding protein 3 (CREB3) family in cancers

The CREB3 family of proteins, encompassing CREB3 and its four homologs (CREB3L1, CREB3L2, CREB3L3, and CREB3L4), exerts pivotal control over cellular protein metabolism in response to unfolded protein reactions. Under conditions of endoplasmic reticulum stress, activation of the CREB3 family occurs through regulated intramembrane proteolysis within the endoplasmic reticulum membrane. Perturbations in the function and expression of the CREB3 family have been closely associated with the development of diverse diseases, with a particular emphasis on cancer. Recent investigations have shed light on the indispensable role played by CREB3 family members in modulating the onset and progression of various human cancers. This comprehensive review endeavors to provide an in-depth examination of the involvement of CREB3 family members in distinct human cancer types, accentuating their significance in the pathogenesis of cancer and the manifestation of malignant phenotypes.

Quartet of APOCs and the Different Roles They Play in Diabetes

APOA1 and APOB are the structural proteins of high-density lipoprotein and APOB-containing lipoproteins, such as low-density lipoprotein and very low-density lipoprotein, respectively. The 4 smaller APOCs (APOC1, APOC2, APOC3, and APOC4) are exchangeable apolipoproteins; they are readily transferred among high-density lipoproteins and APOB-containing lipoproteins. The APOCs regulate plasma triglyceride and cholesterol levels by modulating substrate availability and activities of enzymes interacting with lipoproteins and by interfering with APOB-containing lipoprotein uptake through hepatic receptors. Of the 4 APOCs, APOC3 has been best studied in relation to diabetes. Elevated serum APOC3 levels predict incident cardiovascular disease and progression of kidney disease in people with type 1 diabetes. Insulin suppresses APOC3 levels, and accordingly, elevated APOC3 levels associate with insulin deficiency and insulin resistance. Mechanistic studies in a mouse model of type 1 diabetes have demonstrated that APOC3 acts in the causal pathway of diabetes-accelerated atherosclerosis. The mechanism is likely due to the ability of APOC3 to slow the clearance of triglyceride-rich lipoproteins and their remnants, thereby causing an increased accumulation of atherogenic lipoprotein remnants in lesions of atherosclerosis. Less is known about the roles of APOC1, APOC2, and APOC4 in diabetes.

Hematopoietic NLRP3 and AIM2 Inflammasomes Promote Diabetes-Accelerated Atherosclerosis, but Increased Necrosis Is Independent of Pyroptosis

Serum apolipoprotein C3 (APOC3) predicts incident cardiovascular events in people with type 1 diabetes, and silencing of APOC3 prevents both lesion initiation and advanced lesion necrotic core expansion in a mouse model of type 1 diabetes. APOC3 acts by slowing the clearance of triglyceride-rich lipoproteins, but lipid-free APOC3 has recently been reported to activate an inflammasome pathway in monocytes. We therefore investigated the contribution of hematopoietic inflammasome pathways to atherosclerosis in mouse models of type 1 diabetes. LDL receptor-deficient diabetes mouse models were transplanted with bone marrow from donors deficient in NOD, LRR and pyrin domain-containing protein 3 (NLRP3), absent in melanoma 2 (AIM2) or gasdermin D (GSDMD), an inflammasome-induced executor of pyroptotic cell death. Mice with diabetes exhibited inflammasome activation and consistently, increased plasma interleukin-1β (IL-1β) and IL-18. Hematopoietic deletions of NLRP3, AIM2, or GSDMD caused smaller atherosclerotic lesions in diabetic mice. The increased lesion necrotic core size in diabetic mice was independent of macrophage pyroptosis because hematopoietic GSDMD deficiency failed to prevent necrotic core expansion in advanced lesions. Our findings demonstrate that AIM2 and NLRP3 inflammasomes contribute to atherogenesis in diabetes and suggest that necrotic core expansion is independent of macrophage pyroptosis.

ARTICLE HIGHLIGHTS

The contribution of hematopoietic cell inflammasome activation to atherosclerosis associated with type 1 diabetes is unknown. The goal of this study was to address whether hematopoietic NOD, LRR, and pyrin domain-containing protein 3 (NLRP3), absent in melanoma 2 (AIM2) inflammasomes, or the pyroptosis executioner gasdermin D (GSDMD) contributes to atherosclerosis in mouse models of type 1 diabetes. Diabetic mice exhibited increased inflammasome activation, with hematopoietic deletions of NLRP3, AIM2, or GSDMD causing smaller atherosclerotic lesions in diabetic mice, but the increased lesion necrotic core size in diabetic mice was independent of macrophage pyroptosis. Further studies on whether inflammasome activation contributes to cardiovascular complications in people with type 1 diabetes are warranted.

Monocyte and macrophage foam cells in diabetes-accelerated atherosclerosis

Diabetes results in an increased risk of atherosclerotic cardiovascular disease. This minireview will discuss whether monocyte and macrophage lipid loading contribute to this increased risk, as monocytes and macrophages are critically involved in the progression of atherosclerosis. Both uptake and efflux pathways have been described as being altered by diabetes or conditions associated with diabetes, which may contribute to the increased accumulation of lipids seen in macrophages in diabetes. More recently, monocytes have also been described as lipid-laden in response to elevated lipids, including triglyceride-rich lipoproteins, the class of lipids often elevated in the setting of diabetes.

FACI is a novel clathrin adaptor protein 2-binding protein that facilitates low-density lipoprotein endocytosis

BACKGROUND

Cholesterol plays a vital role in multiple physiological processes. Cellular uptake of cholesterol is mediated primarily through endocytosis of low-density lipoprotein (LDL) receptor. New modifiers of this process remain to be characterized. Particularly, the role of fasting- and CREB-H-induced (FACI) protein in cholesterol homeostasis merits further investigation.

METHODS

Interactome profiling by proximity labeling and affinity purification - mass spectrometry was performed. Total internal reflection fluorescence microscopy and confocal immunofluorescence microscopy were used to analyze protein co-localization and interaction. Mutational analysis was carried out to define the domain and residues required for FACI localization and function. Endocytosis was traced by fluorescent cargos. LDL uptake in cultured cells and diet-induced hypercholesterolemia in mice were assessed.

RESULTS

FACI interacted with proteins critically involved in clathrin-mediated endocytosis, vesicle trafficking, and membrane cytoskeleton. FACI localized to clathrin-coated pits (CCP) on plasma membranes. FACI contains a conserved DxxxLI motif, which mediates its binding with the adaptor protein 2 (AP2) complex. Disruption of this motif of FACI abolished its CCP localization but didn't affect its association with plasma membrane. Cholesterol was found to facilitate FACI transport from plasma membrane to endocytic recycling compartment in a clathrin- and cytoskeleton-dependent manner. LDL endocytosis was enhanced in FACI-overexpressed AML12 cells but impaired in FACI-depleted HeLa cells. In vivo study indicated that hepatic FACI overexpression alleviated diet-induced hypercholesterolemia in mice.

CONCLUSIONS

FACI facilitates LDL endocytosis through its interaction with the AP2 complex.

A hepatokine derived from the ER protein CREBH promotes triglyceride metabolism by stimulating lipoprotein lipase activity

The endoplasmic reticulum (ER)-tethered, liver-enriched stress sensor CREBH is processed in response to increased energy demands or hepatic stress to release an amino-terminal fragment that functions as a transcription factor for hepatic genes encoding lipid and glucose metabolic factors. Here, we discovered that the carboxyl-terminal fragment of CREBH (CREBH-C) derived from membrane-bound, full-length CREBH was secreted as a hepatokine in response to fasting or hepatic stress. Phosphorylation of CREBH-C mediated by the kinase CaMKII was required for efficient secretion of CREBH-C through exocytosis. Lipoprotein lipase (LPL) mediates the lipolysis of circulating triglycerides for tissue uptake and is inhibited by a complex consisting of angiopoietin-like (ANGPTL) 3 and ANGPTL8. Secreted CREBH-C blocked the formation of ANGPTL3-ANGPTL8 complexes, leading to increased LPL activity in plasma and metabolic tissues in mice. CREBH-C administration promoted plasma triglyceride clearance and partitioning into peripheral tissues and mitigated hypertriglyceridemia and hepatic steatosis in mice fed a high-fat diet. Individuals with obesity had higher circulating amounts of CREBH-C than control individuals, and human CREBH loss-of-function variants were associated with dysregulated plasma triglycerides. These results identify a stress-induced, secreted protein fragment derived from CREBH that functions as a hepatokine to stimulate LPL activity and triglyceride homeostasis.

The Role and Mechanism of CREBH Regulating SIRT3 in Metabolic Associated Fatty Liver Disease

AIMS

To investigate the effect of cAMP response element-binding protein H (CREBH) on metabolic associated fatty liver disease by regulating sirtuin 3 (SIRT3).

MAIN METHODS

Two mouse models of fatty liver induced by a methionine-choline deficient (MCD) diet and a high-fat (HF) diet and an in vitro model of palmitic acid (PA) induced lipid-overloaded hepatocytes were constructed to detect the expression of CREBH, SIRT3, total acetylation, and downstream protein interactions and lipid metabolism phenotype, which were further validated in CREBH^-/- mice and lentivirus-overexpressing CREBH hepatocytes.

KEY FINDINGS

In fatty liver and lipid overload models, the expressions of CREBH and SIRT3 were down-regulated and their expression was positively correlated, accompanied by an increase in the level of total protein acetylation. Overexpression of CREBH alleviated excess lipid accumulation, impaired viability, and the ability to metabolize energy through the fatty acid oxidation pathway in hepatocytes in vitro. Furthermore, overexpression of CREBH restored the interaction of the deacetylase SIRT3 with the molecules carnitine palmitoyl-transferase 2 (CPT2) and long-chain acyl CoA dehydrogenase (ACADL) involved in the fatty acid oxidation pathway and their deacetylation status. However, CREBH^-/- aggravated the damage of lipid metabolism in the liver tissue of mice.

SIGNIFICANCE

CREBH increased the enzymatic activity of downstream factors by positively regulating the expression of SIRT3, which promoted the oxidative decomposition of fatty acids in hepatocytes and played an important role in fatty acid oxidation in MAFLD.

The Remnant Lipoprotein Hypothesis of Diabetes-Associated Cardiovascular Disease

Both type 1 and type 2 diabetes are associated with an increased risk of atherosclerotic cardiovascular disease (CVD). Research based on human-first or bedside-to-bench approaches has provided new insights into likely mechanisms behind this increased risk. Although both forms of diabetes are associated with hyperglycemia, it is becoming increasingly clear that altered lipoprotein metabolism also plays a critical role in predicting CVD risk in people with diabetes. This review examines recent findings indicating that increased levels of circulating remnant lipoproteins could be a missing link between diabetes and CVD. Although CVD risk associated with diabetes is clearly multifactorial in nature, these findings suggest that we should increase efforts in evaluating whether remnant lipoproteins or the proteins that govern their metabolism are biomarkers of incident CVD in people living with diabetes and whether reducing remnant lipoproteins will prevent the increased CVD risk associated with diabetes.

Recruiting a transcription factor in the liver to prevent atherosclerosis

Hypertriglyceridemia is associated with obesity, diabetes, and atherosclerosis. While lipoprotein lipase (LPL) hydrolyzes triglyceride (TG) cargo into remnant lipoproteins with atherogenic properties, how remnant lipoprotein clearance relates to atherosclerosis in people with diabetes remains unclear. In this issue of the JCI, Shimizu-Albergine et al. examined the effects of the basic leucine zipper transcription factor CREBH, which induces genes that activate LPL in mouse models of type I diabetes. Overexpression of a CREBH fragment reduced apolipoprotein C3 (APOC3) levels, which reduced plasma TGs. Notably, the TGs were lowered by a mechanism that was independent of LPL, and atherosclerosis was alleviated by enhanced lipoprotein remnant clearance as opposed to increased lipolysis of TG-rich lipoprotein precursors. A proinflammatory mechanism likely underlies the atherogenicity of remnant lipoproteins. These findings suggest that modifying CREBH expression in the liver may ameliorate atherosclerosis and, perhaps, other diabetes complications.