Diets high in calories can be used to model metabolic diseases, including obesity and its associated comorbidities, in animals. Drosophila melanogaster fed high-sugar diets (HSDs) exhibit complications of human obesity including hyperglycemia, hyperlipidemia, insulin resistance, cardiomyopathy, increased susceptibility to infection, and reduced longevity. We hypothesize that lipid storage in the high-sugar-fed fly’s fat body (FB) reaches a maximum capacity, resulting in the accumulation of toxic lipids in other tissues or lipotoxicity. We took two approaches to characterize tissue-specific lipotoxicity. Ultra-HPLC-MS/MS and MALDI-MS imaging enabled spatial and temporal localization of lipid species in the FB, heart, and hemolymph. Substituent chain length was diet dependent, with fewer odd chain esterified FAs on HSDs in all sample types. By contrast, dietary effects on double bond content differed among organs, consistent with a model where some substituent pools are shared and others are spatially restricted. Both di- and triglycerides increased on HSDs in all sample types, similar to observations in obese humans. Interestingly, there were dramatic effects of sugar feeding on lipid ethers, which have not been previously associated with lipotoxicity. Taken together, we have identified candidate endocrine mechanisms and molecular targets that may be involved in metabolic disease and lipotoxicity.

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Hypertension may develop before or after the onset of diabetes and it is known to increase the risk of developing diabetic nephropathy. Alpha-1 antitrypsin (AAT) is a multi-functional protein with beneficial effects in various diseases but its role in reducing blood pressure in the diabetic kidney has not been thoroughly studied. Like blood pressure, epithelial sodium channels (ENaC) and its adaptor protein myristoylated alanine-rich C-kinase substrate (MARCKS) are regulated by circadian rhythms. Our hypothesis is that administration of human AAT (hAAT) reduces blood pressure in hypertensive diabetic mice by attenuating membrane expression of ENaC and its association with the actin cytoskeleton. First, we show hAAT administration results in reduced blood pressure in diabetic db/db mice compared to vehicle treatment in both the inactive and active cycles. Western blotting and immunohistochemistry analyses showed a reduction of ENaC and the actin cytoskeleton protein, MARCKS in the kidneys of diabetic db/db mice treated with hAAT compared to vehicle. hAAT treatment resulted in elevated amounts of extracellular vesicles present in the urine of diabetic db/db mice compared to vehicle treatment both in the inactive and active cycles. Multiple hexosylceramides, among other lipid classes increased in urinary EVs released from hAAT treated hypertensive diabetic mice compared to vehicle treated mice. Taken together, these data suggest hAAT treatment could normalize blood pressure in the diabetic kidney in a mechanism involving attenuation of renal ENaC and MARCKS protein expression and possibly ceramide metabolism to hexosylceramide in kidney cells.

In addition to inhibiting renal glucose reabsorption and allowing for glucose excretion, the sodium/glucose cotransporter 2 (SGLT2) inhibitor dapagliflozin may be efficacious in treating various comorbidities associated with type 2 diabetes mellitus (T2DM). The molecular mechanisms by which dapagliflozin exerts its beneficial effects are largely unknown. We hypothesized dapagliflozin treatment in the diabetic kidney alters plasma membrane lipid composition, suppresses extracellular vesicle (EV) release from kidney cells, and disrupts lipid rafts in proximal tubule cells. In order to test this hypothesis, we treated diabetic db/db mice with dapagliflozin (N = 8) or vehicle (N = 8) and performed mass spectrometry-based lipidomics to investigate changes in the concentrations of membrane lipids in the kidney cortex. In addition, we isolated urinary EVs (uEVs) from urine samples collected during the active phase and the inactive phase of the mice and then probed for changes in membrane proteins enriched in the EVs. Multiple triacylglycerols (TAGs) were enriched in the kidney cortex membrane fractions of vehicle-treated diabetic db/db mice, while the levels of multiple phosphatidylethanolamines were significantly higher in similar mice treated with dapagliflozin. EV concentration and size were lesser in the urine samples collected during the inactive phase of dapagliflozin-treated diabetic mice. In cultured mouse proximal tubule cells treated with dapagliflozin, the lipid raft protein caveolin-1 shifted from less dense fractions to more dense sucrose density gradient fractions. Taken together, these results suggest dapagliflozin may regulate lipid-mediated signal transduction in the diabetic kidney.

While the deposition of immune complexes in the glomeruli is thought to initiate lupus nephritis, the extent of tubular damage better predicts progression to end stage kidney disease than the glomerular injury. However, the mechanisms underlying tubular injury in lupus nephritis are incompletely understood. Iron accumulates in tubular epithelial cells of lupus nephritis patients and mice. Ferroptosis is a druggable, iron-dependent form of cell death that is characterized by lipid peroxidation but has received little attention in lupus nephritis. Here we identify intra-renal ferroptosis as a novel pathological feature in human and murine lupus nephritis of different etiologies. Kidneys of lupus nephritis patients and mice showed increased lipid peroxidation mainly in the tubular segments. Nephritic kidneys presented with an increase in Acyl-CoA synthetase long-chain family member 4, a pro-ferroptosis enzyme, an impaired glutathione synthesis pathway, and attenuated expression of glutathione peroxidase 4, a glutathione-dependent ferroptosis inhibitor. Semi targeted lipidomics of murine nephritic kidneys revealed increased esterification of the sn-2 chain of phosphatidylethanolamine with adrenic acid (P-18:0/22:4), the preferred lipid substrate for lipid peroxidation and ferroptosis. Using congenic mice and nephrotoxic serum-induced immune complex glomerulonephritis model, we show that conditional deletion of heavy chain ferritin (FtH1) in the proximal tubules exacerbates ferroptosis and tubular injury. These findings were recapitulated by knocking down FtH1 in human proximal tubular cells and underscore the critical role of iron and heavy chain ferritin in tubular injury during the evolution of glomerulonephritis. Of translational relevance, Liproxstatin-2, a novel second-generation ferroptosis inhibitor, prophylactically and therapeutically mitigated lupus nephritis patient serum-induced ferroptosis in human proximal tubular cells. Collectively, our findings identify intra-renal ferroptosis as a pathological feature and contributor to tubular injury in lupus nephritis.

Objective An appreciation of factors that lead to tubular injury in lupus nephritis is lacking. Iron accumulates in the kidney tubules of nephritic patients and lupus-prone nephritic mice. Ferroptosis is a druggable, iron-dependent form of cell death that has received little attention in lupus nephritis. This study investigated whether intra-renal ferroptosis is a target for intervention in lupus nephritis. Methods Kidneys of lupus nephritis patients and two spontaneous murine models of lupus nephritis were characterized for ferroptosis using protein, RNA, and lipidomics-based approaches. Susceptibility of heavy chain ferritin (FtH1; an essential iron sequestration protein) deficient proximal tubular epithelial cells (PTECs) was studied using nephrotoxic serum nephritis and FtH1 knockdown human PTECs. The benefit of Liproxstatin-2, a novel second-generation ferroptosis, was evaluated using human PTECs exposed to lupus nephritis patients serum. Results Human and murine nephritic kidneys have the characteristic markers of ferroptosis, such as 4-hydroxynonenal and acyl-CoA synthetase long-chain family member 4, mainly in the tubular segments. Murine kidneys showed impairment in the glutathione synthesis pathway, decreased expression of glutathione peroxidase 4, a glutathione-dependent ferroptosis inhibitor, and characteristic ferroptotic lipid signature. Loss of FtH1 increased PTEC pathology independent of glomerular injury. These findings were recapitulated in human PTECs. Of translational relevance, Liproxstatin-2 demonstrated a prophylactic and therapeutic benefit in mitigating lupus nephritis patient serum-induced PTEC ferroptosis. Conclusion Our findings highlight tubular cell ferroptosis as a pathological feature in human and murine lupus nephritis and identify ferroptosis inhibitors as potential novel adjunct therapeutics to treat lupus nephritis.

Cathpesin B is a multi-functional protease that plays numerous roles in physiology and pathophysiology. We hypothesized that actin cytoskeleton proteins that are substrates of cathepsin B, various lipids, and kinases that are regulated by lipids would be down-regulated in the kidney of cathepsin B knockout mice. Here, we show by Western blot and densitometric analysis that the expression and proteolysis of the actin cytoskeleton proteins myristoylated alanine-rich C-kinase substrate (MARCKS) and spectrin are significantly reduced in kidney cortex membrane fractions of cathepsin B knockout mice compared to C57B6 wild-type control mice. Lipidomic results show that specific lipids are increased while other lipids, including lysophosphatidylcholine (LPC) species LPC (16:0), LPC (18:0), LPC (18:1), and LPC (18:2), are significantly decreased in membrane fractions of the kidney cortex from Cathepsin B null mice. Protein Kinase C (PKC) activity is significantly lower in the kidney cortex of cathepsin B knockout mice compared to wild-type mice, while calcium/calmodulin-dependent protein kinase II (CaMKII) activity and phospholipase D (PLD) activity are comparable between the two groups. Together, these results provide the first evidence of altered actin cytoskeleton organization, membrane lipid composition, and PKC activity in the kidneys of mice lacking cathepsin B.