Fasting upregulates adipose triglyceride lipase and hormone-sensitive lipase levels and phosphorylation in mouse kidney

Lipophagy and Mitophagy in Renal Pathophysiology

BACKGROUND

The lysosomal autophagic pathway plays a fundamental role in cellular and tissue homeostasis, and its deregulation is linked to human pathologies including kidney diseases. Autophagy can randomly degrade cytoplasmic components in a nonselective manner commonly referred to as bulk autophagy. In contrast, selective forms of autophagy specifically target cytoplasmic structures such as organelles and protein aggregates, thereby being important for cellular quality control and organelle homeostasis.

SUMMARY

Research during the past decades has begun to elucidate the role of selective autophagy in kidney physiology and kidney diseases.

KEY MESSAGES

In this review, we will summarize the knowledge on lipophagy and mitophagy, two forms of selective autophagy important in renal epithelium homeostasis, and discuss how their deregulations contribute to renal disease progression.

Renal tubule-specific Atgl deletion links kidney lipid metabolism to glucagon-like peptide 1 and insulin secretion independent of renal inflammation or lipotoxicity

OBJECTIVE

Lipotoxic injury from renal lipid accumulation in obesity and type 2 diabetes (T2D) is implicated in associated kidney damage. However, models examining effects of renal ectopic lipid accumulation independent of obesity or T2D are lacking. We generated renal tubule-specific adipose triglyceride lipase knockout (RT-SAKO) mice to determine if this targeted triacylglycerol (TAG) over-storage affects glycemic control and kidney health.

METHODS

Male and female RT-SAKO mice and their control littermates were tested for changes in glycemic control at 10-12 and 16-18 weeks of age. Markers of kidney health and blood lipid and hormone concentrations were analyzed. Kidney and blood lysophosphatidic acid (LPA) levels were measured, and a role for LPA in mediating impaired glycemic control was evaluated using the LPA receptor 1/3 inhibitor Ki-16425.

RESULTS

All groups remained insulin sensitive, but 16- to 18-week-old male RT-SAKO mice became glucose intolerant, without developing kidney inflammation or fibrosis. Rather, these mice displayed lower circulating insulin and glucagon-like peptide 1 (GLP-1) levels. Impaired first-phase glucose-stimulated insulin secretion was detected and restored by Exendin-4. Kidney and blood LPA levels were elevated in older male but not female RT-SAKO mice, associated with increased kidney diacylglycerol kinase epsilon. Inhibition of LPA-mediated signaling restored serum GLP-1 levels, first-phase insulin secretion, and glucose tolerance.

CONCLUSIONS

TAG over-storage alone is insufficient to cause renal tubule lipotoxicity. This work is the first to show that endogenously derived LPA modulates GLP-1 levels in vivo, demonstrating a new mechanism of kidney-gut-pancreas crosstalk to regulate insulin secretion and glucose homeostasis.

Palmitate reduces starvation-induced ER stress by inhibiting ER-phagy in hypothalamic cells

Palmitate is a saturated fatty acid that is well known to induce endoplasmic reticulum (ER) stress and autophagy. A high-fat diet increases the palmitate level in the hypothalamus, the main region of the brain regulating energy metabolism. Interestingly, hypothalamic palmitate level is also increased under starvation, urging the study to distinguish the effects of elevated hypothalamic palmitate level under different nutrient conditions. Herein, we show that ER-phagy (ER-targeted selective autophagy) is required for progress of ER stress and that palmitate decreases ER stress by inhibiting ER-phagy in hypothalamic cells under starvation. Palmitate inhibited starvation-induced ER-phagy by increasing the level of B-cell lymphoma 2 (Bcl-2) protein, which inhibits autophagy initiation. These findings suggest that, unlike the induction of ER stress under nutrient-rich conditions, palmitate protects hypothalamic cells from starvation-induced stress by inhibiting ER-phagy.

Reducing 14-3-3ζ expression influences adipocyte maturity and impairs function

One of the primary metabolic functions of a mature adipocyte is to supply energy via lipolysis, or the catabolism of stored lipids. Adipose triacylglycerol lipase (ATGL) and hormone-sensitive lipase (HSL) are critical lipolytic enzymes, and their phosphorylation generates phospho-binding sites for 14-3-3 proteins, a ubiquitously expressed family of molecular scaffolds. Although we previously identified essential roles of the 14-3-3ζ isoform in murine adipogenesis, the presence of 14-3-3 protein binding sites on ATGL and HSL suggests that 14-3-3ζ could also influence mature adipocyte processes like lipolysis. Here we demonstrate that 14-3-3ζ is necessary for lipolysis in male mice and fully differentiated 3T3-L1 adipocytes, as depletion of 14-3-3ζ significantly impaired glycerol and free fatty acid (FFA) release. Unexpectedly, reducing 14-3-3ζ expression was found to significantly impact adipocyte maturity, as observed by reduced abundance of peroxisome proliferator-activated receptor (PPAR)γ2 protein and expression of mature adipocyte genes and those associated with de novo triglyceride synthesis and lipolysis. The impact of 14-3-3ζ depletion on adipocyte maturity was further examined with untargeted lipidomics, which revealed that reductions in 14-3-3ζ abundance promoted the acquisition of a lipidomic signature that resembled undifferentiated preadipocytes. Collectively, these findings reveal a novel aspect of 14-3-3ζ in adipocytes, as reducing 14-3-3ζ was found to have a negative effect on adipocyte maturity and adipocyte-specific processes like lipolysis.

Intermittent Fasting: Physiological Implications on Outcomes in Mice and Men

Intermittent fasting (IF) is a widely practiced dietary method that encompasses periodic restriction of food consumption. Due to its protective benefits against metabolic diseases, aging, and cardiovascular and neurodegenerative diseases, IF continues to gain attention as a preventative and therapeutic intervention to counteract these chronic diseases. Although numerous animal studies have reported positive health benefits of IF, its feasibility and efficacy in clinical settings remain controversial. Importantly, since dietary interventions such as IF have systemic effects, thoroughly investigating the tissue-specific changes in animal models is crucial to identify IF's mechanism and evaluate its potential adverse effects in humans. As such, we will review and compare the outcomes and underlying mechanisms of IF in both animal and human studies. Moreover, the limitations of IF and inconsistencies between preclinical and clinical studies will be discussed to provide insight into the gaps between translating research from bench to bedside.

The Lipolysome-A Highly Complex and Dynamic Protein Network Orchestrating Cytoplasmic Triacylglycerol Degradation

The catabolism of intracellular triacylglycerols (TAGs) involves the activity of cytoplasmic and lysosomal enzymes. Cytoplasmic TAG hydrolysis, commonly termed lipolysis, is catalyzed by the sequential action of three major hydrolases, namely adipose triglyceride lipase, hormone-sensitive lipase, and monoacylglycerol lipase. All three enzymes interact with numerous protein binding partners that modulate their activity, cellular localization, or stability. Deficiencies of these auxiliary proteins can lead to derangements in neutral lipid metabolism and energy homeostasis. In this review, we summarize the composition and the dynamics of the complex lipolytic machinery we like to call "lipolysome".

Liraglutide attenuates renal tubular ectopic lipid deposition in rats with diabetic nephropathy by inhibiting lipid synthesis and promoting lipolysis

Liraglutide is a new hypoglycemic drug. The previous studies have shown that liraglutide can improve the renal outcomes of patients with type 2 diabetes. Recently, it was approved by the U.S. FDA for used as a weight-loss drugs. However, the mechanism of its improvements of renal function in diabetic nephropathy patients is unclear. In addition, the effect of liraglutide on lipid metabolism is also not clear. The purpose of this study was to investigate the effects and mechanisms of liraglutide in alleviating ectopic lipid deposition (ELD) in rats with diabetic nephropathy (DN). Male Sprague-Dawley (SD) rats were treated with high-fat diet + unilateral nephrectomy + low-dose STZ combined to establish a DN rat model to evaluate the lipid-lowering effect of liraglutide. Liraglutide at 0.4 mg/kg/d were subcutaneous injected into for 12 weeks (DN + liraglutide group). After the DN rat model was established, body weight loss, 24-h urine volume increasing, serum triglycerides (TG) and serum total cholesterol (TCh) increasing, ectopic lipid droplet deposition in renal tubular increasing, mesangial proliferation in renal tissue were observed in DN rats. The treatment with liraglutide could reduce the body weight and the average daily food intake of the rats, as well as TG, TCh, and ectopic lipid droplet deposition in renal tubular. Metabolomics result showed that serum differential metabolites between the DN - vehicle control group and the DN + liraglutide group mainly included serine, threonine, phenylalanine, oxyproline, threonine, sorbitol, glyceryl monostearate, glycerol monostearate, and β-d-glucuronic acid. Moreover, liraglutide can reduce plasma lipid levels in DN rats by increasing the products of lipolysis including 1-monopalmitin and 1-monoostearin. Immunohistochemistry and Western blot showed that the expression levels of lipid synthesis-related sterol regulatory element binding protein 1 (SREBP-1) and fatty acid synthase (FAS) were significantly increased, and lipolysis-related adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) were significantly decreased both in the renal tissue of DN rats and PA-induced HK-2 cells (lipid droplet accumulation model). However, liraglutide can attenuate renal tubular ectopic lipid deposition in DN rats by inhibiting SREBP-1, FAS and increasing ATGL, HSL protein expression level, and also ameliorated PA-induced lipid accumulation in renal tubular epithelial cells. These lipid metabolism changes were attributed to liraglutide by upregulating AMP-activated protein kinase (AMPK) phosphorylation in the kidney of DN rats. Collectively, these findings confirm that liraglutide inhibits lipid synthesis and promotes lipolysis to attenuate renal ectopic lipid deposition in DN rats by promoting AMPK phosphorylation.

Scaffold Proteins: From Coordinating Signaling Pathways to Metabolic Regulation

Among their pleiotropic functions, scaffold proteins are required for the accurate coordination of signaling pathways. It has only been within the past 10 years that their roles in glucose homeostasis and metabolism have emerged. It is well appreciated that changes in the expression or function of signaling effectors, such as receptors or kinases, can influence the development of chronic diseases such as diabetes and obesity. However, little is known regarding whether scaffolds have similar roles in the pathogenesis of metabolic diseases. In general, scaffolds are often underappreciated in the context of metabolism or metabolic diseases. In the present review, we discuss various scaffold proteins and their involvement in signaling pathways related to metabolism and metabolic diseases. The aims of the present review were to highlight the importance of scaffold proteins and to raise awareness of their physiological contributions. A thorough understanding of how scaffolds influence metabolism could aid in the discovery of novel therapeutic approaches to treat chronic conditions, such as diabetes, obesity, and cardiovascular disease, for which the incidence of all continue to increase at alarming rates.

The PNPLA3 variant associated with fatty liver disease (I148M) accumulates on lipid droplets by evading ubiquitylation

A sequence variation (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) is strongly associated with fatty liver disease, but the underlying mechanism remains obscure. In this study, we used knock-in (KI) mice (Pnpla3^148M/M ) to examine the mechanism responsible for accumulation of triglyceride (TG) and PNPLA3 in hepatic lipid droplets (LDs). No differences were found between Pnpla3^148M/M and Pnpla3^+/+ mice in hepatic TG synthesis, utilization, or secretion. These results are consistent with TG accumulation in the Pnpla3^148M/M mice being caused by impaired TG mobilization from LDs. Sucrose feeding, which is required to elicit fatty liver in KI mice, led to a much larger and more persistent increase in PNPLA3 protein in the KI mice than in wild-type (WT) mice. Inhibition of the proteasome (bortezomib), but not macroautophagy (3-methyladenine), markedly increased PNPLA3 levels in WT mice, coincident with the appearance of ubiquitylated forms of the protein. Bortezomib did not increase PNPLA3 levels in Pnpla3^148M/M mice, and only trace amounts of ubiquitylated PNPLA3 were seen in these animals.

CONCLUSION

These results are consistent with the notion that the 148M variant disrupts ubiquitylation and proteasomal degradation of PNPLA3, resulting in accumulation of PNPLA3-148M and impaired mobilization of TG from LDs. (Hepatology 2017;66:1111-1124).

Lipophagy maintains energy homeostasis in the kidney proximal tubule during prolonged starvation

Macroautophagy/autophagy is a self-degradation process that combats starvation. Lipids are the main energy source in kidney proximal tubular cells (PTCs). During starvation, PTCs increase fatty acid (FA) uptake, form intracellular lipid droplets (LDs), and hydrolyze them for use. The involvement of autophagy in lipid metabolism in the kidney remains largely unknown. Here, we investigated the autophagy-mediated regulation of renal lipid metabolism during prolonged starvation using PTC-specific Atg5-deficient (atg5-TSKO) mice and an in vitro serum starvation model. Twenty-four h of starvation comparably induced LD formation in the PTCs of control and atg5-TSKO mice; however, additional 24 h of starvation reduced the number of LDs in control mice, whereas increases were observed in atg5-TSKO mice. Autophagic degradation of LDs (lipophagy) in PTCs was demonstrated by electron microscopic observation and biochemical analysis. In vitro pulse-chase assays demonstrated that lipophagy mobilizes FAs from LDs to mitochondria during starvation, whereas impaired LD degradation in autophagy-deficient PTCs led to decreased ATP production and subsequent cell death. In contrast to the in vitro assay, despite impaired LD degradation, kidney ATP content was preserved in 48-h starved atg5-TSKO mice, probably due to increased utilization of ketone bodies. This compensatory mechanism was accompanied by a higher plasma FGF21 (fibroblast growth factor 21) level and its expression in the PTCs; however, this was not essential for the production of ketone bodies in the liver during prolonged starvation. In conclusion, lipophagy combats prolonged starvation in PTCs to avoid cellular energy depletion.

Kidney triglyceride accumulation in the fasted mouse is dependent upon serum free fatty acids

Lipid accumulation is a pathological feature of every type of kidney injury. Despite this striking histological feature, physiological accumulation of lipids in the kidney is poorly understood. We studied whether the accumulation of lipids in the fasted kidney are derived from lipoproteins or NEFAs. With overnight fasting, kidneys accumulated triglyceride, but had reduced levels of ceramide and glycosphingolipid species. Fasting led to a nearly 5-fold increase in kidney uptake of plasma [¹⁴C]oleic acid. Increasing circulating NEFAs using a β adrenergic receptor agonist caused a 15-fold greater accumulation of lipid in the kidney, while mice with reduced NEFAs due to adipose tissue deficiency of adipose triglyceride lipase had reduced triglycerides. Cluster of differentiation (Cd)36 mRNA increased 2-fold, and angiopoietin-like 4 (Angptl4), an LPL inhibitor, increased 10-fold. Fasting-induced kidney lipid accumulation was not affected by inhibition of LPL with poloxamer 407 or by use of mice with induced genetic LPL deletion. Despite the increase in CD36 expression with fasting, genetic loss of CD36 did not alter fatty acid uptake or triglyceride accumulation. Our data demonstrate that fasting-induced triglyceride accumulation in the kidney correlates with the plasma concentrations of NEFAs, but is not due to uptake of lipoprotein lipids and does not involve the fatty acid transporter, CD36.

Marks K, Duncan RE. Data on hepatic lipolysis, adipose triglyceride lipase, and hormone-sensitive lipase in fasted and non-fasted C57BL/6J female mice

Liver homogenates produced from fasted and non-fasted C57BL/6J female mice were assayed for total lipolytic activity measured as hydrolysis of [9,10-³H(N)]-triolein into [³H] free fatty acids (FFA). Liver homogenates were also used for immunoblotting to determine levels of the lipolytic enzymes adipose-triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL), as well as site specific phosphorylation at the 14-3-3 binding site of ATGL and the serine 565 and serine 660 sites of HSL. Significantly higher triolein hydrolysis activity was observed in fasted liver samples, as well as a significant increase in total ATGL and a significant decrease in HSL phosphorylation at the S565 site.

14-3-3ζ: A numbers game in adipocyte function?

Molecular scaffolds are often viewed as passive signaling molecules that facilitate protein-protein interactions. However, new evidence gained from the use of loss-of-function or gain-of-function models is dispelling this notion. Our own recent discovery of 14-3-3ζ as an essential regulator of adipogenesis highlights the complex roles of this member of the 14-3-3 protein family. Depletion of the 14-3-3ζ isoform affected parallel pathways that drive adipocyte development, including pathways controlling the stability of key adipogenic transcription factors and cell cycle progression. Going beyond adipocyte differentiation, this study opens new avenues of research in the context of metabolism, as 14-3-3ζ binds to a variety of well-established metabolic proteins that harbor its canonical phosphorylation binding motifs. This suggests that 14-3-3ζ may contribute to key metabolic signaling pathways, such as those that facilitate glucose uptake and fatty acid metabolism. Herein, we discuss these novel areas of research, which will undoubtedly shed light onto novel roles of 14-3-3ζ, and perhaps its related family members, on glucose homeostasis.