ABSTRACT
Excessive fat intake can lead to cellular injury and inflammation (termed lipotoxicity), but studies on lipid metabolism in the kidney have been scarce. We recently identified a novel mechanism of lipotoxicity in the kidney proximal tubules especially focusing on the effect of lipid overload on lysosomal function and autophagic activity. Lipid overload basically stimulates macroautophagy/autophagy for renovation of the plasma and organelle membranes, which plays an essential role in maintaining the integrity of proximal tubules. However, this autophagic activation is inevitably accompanied with lysosomal stress and consequent downstream suppression of autophagy, which manifest as phospholipid accumulation in the lysosome. Stagnation of autophagy can enhance vulnerability to additional stress such as ischemic injury. Pharmacological correction of phospholipid accumulation that restores autophagic flux will be a novel therapeutic option for obesity-related kidney diseases.
KEYWORDS: autophagy, high fat diet, lipid overload, mitophagy, palmitic acid
The β-oxidation of free fatty acids (FAs) is a major source of renal ATP synthesis, particularly in proximal tubular cells (PTCs), which have a high-energy demand. Although PTCs consistently uptake FAs from circulation and glomerular filtrate, studies on lipid metabolism (especially on lipotoxicity) in the kidney are insufficient compared with organs that have evolved specialized lipid handling functions, such as adipose tissue and liver. In a recently published paper, we identified a novel mechanism of lipotoxicity in the kidney especially focusing on autophagy: 1) lipid overload basically stimulates autophagy, which redistributes phospholipids from the plasma and organelle membranes into lysosomes for renovation; 2) long-term lipid overload places a burden on the lysosomal system, resulting in phospholipid accumulation and inadequate acidification; 3) lysosomal dysfunction in turn stagnates autophagic flux, which can lead to vulnerability to additional stress such as ischemic injury; 4) genetic autophagy ablation during lipid overload leads to mitochondrial dysfunction, inflammasome activation, and fibrosis in the kidney; and 5) some of these findings are recapitulated in the kidneys of obese patients.
It has been demonstrated that several saturated and unsaturated free FAs “activate” or “inactivate” autophagic activity. The significant variation and conflicting results may be attributed to differences in cell types or FAs, observation period, and more likely difficulty in the assessment of autophagic flux in vivo. To monitor lipid-mediated alteration in autophagic flux, we investigated the effects of palmitic acid (PA) overload on autophagic activity in cultured PTCs and demonstrated that PA treatment basically stimulates autophagic activity and subsequently induces downstream suppression of autophagic flux. Next, we evaluated the FA-induced alteration of autophagic activity in vivo by counting the number of GFP-positive puncta in the PTCs of high fat diet (HFD)- or normal diet (ND)-fed GFP-MAP1LC3 transgenic mice before or 6 h after administration of chloroquine. We found high basal autophagic activity in HFD-fed mice and low basal autophagy in ND-fed mice during the fed state; however, in response to 24 h of starvation, ND-fed mice exhibit high autophagic flux, while HFD-fed mice exhibit stagnated autophagic flux. Furthermore, we investigated the HFD-mediated changes in demand and activity of basal autophagy by using tamoxifen-inducible PTC-specific autophagy-deficient mice (KO mice). ND- and HFD-fed mice received tamoxifen to induce genetic ablation of Atg5 3 wk before euthanasia. Three-wk ablation of autophagy triggers increased accumulation of SQSTM1/p62- and ubiquitin-positive protein aggregates in HFD-fed KO mice, indicating that HFD-fed mice augment autophagic activity for the degradation of increasing substrates. Analysis using a pH-sensitive probe elucidated that progressive stagnation of autophagic degradation activity during PA overload is due to the impaired lysosomal acidification and consequent excessive phospholipid accumulation.
What is the substrate of autophagy during lipid overload? What is the origin of phospholipids accumulated in the lysosome? To answer these questions, we performed an FA pulse-chase assay using phosphatidylcholine fluorescently tagged at its FA tail (FL HPC). Incubation overnight with FL HPC in cultured PTCs leads to its integration into the plasma and organelle membranes, including the endoplasmic reticulum, mitochondria, and Golgi apparatus. Subsequent PA overload induces redistribution of phospholipids from cellular membranes into lysosomes, which is inhibited by autophagy deficiency, suggesting that PA overload decreases the quality of cellular membranes, which requires renovation by autophagy. Moreover, we found that, among the organelle, mitochondria can be a main substrate of autophagy during lipid overload by demonstrating that PA significantly increases mitochondrial reactive oxygen species and decreases mitochondrial membrane potential in autophagy-deficient PTCs, both of which can trigger mitophagy. Moreover, prolonged lipid overload in PTC-specific ATG5-deficient mice results in massive aggregation of degenerated mitochondria and mitochondrial dysfunction, macrophage infiltration, and fibrosis in the kidney, whereas phospholipid accumulation in the lysosomes is almost completely resolved, suggesting that autophagy protects kidney PTCs from HFD-induced reduction in mitochondrial function, suppresses inflammasome activation and slows HFD-induced renal fibrosis. Finally, phospholipid accumulation in the enlarged lysosomes and impaired autophagic flux are recapitulated in the kidneys of obese patients.
Similar to this study, we have previously observed that the basal autophagic activity is unexpectedly higher in the aged kidney than that in young kidney to cope with increasing substrate with age, while autophagic flux in response to additional metabolic stress is blunted with aging partly through lysosomal dysfunction (e.g., lipofuscin). Thus, aging and lipid overload appear to have a common mechanistic platform, a significant increase in demand of autophagy, lysosomal dysfunction and consequent impaired autophagic flux. With regard to this point, therapeutic effort to restore lysosomal function and thereby autophagic flux will pave the way to improve the clinical outcome of kidney injury related to aging or lipid overload.
Knowledge on renal lipid physiology and pathophysiology has been scarce and the available evidence is less amenable to systematic study especially in humans. Renal lipid accumulation has been described in many kidney diseases including hypertensive nephrosclerosis, focal segmental glomerulosclerosis, minimal change disease, and diabetic nephropathy, many of which have excessively focused on the accumulation of lipid droplets. Our study provides an incentive for further exploration by identifying stagnation of autophagy (manifested as phospholipid accumulation) due to the lysosomal dysfunction as a novel mechanism of renal lipotoxicity. In addition, our study explains a multi-hit mechanism of lipotoxicity in the kidney: even if lipid overload per se is not sufficient to lead to kidney injury, it contributes to kidney injury in combination with other factors. Stagnation of autophagy derived from lipid overload can jeopardize the function of PTCs especially when proper activation of autophagy should exert a protective role (e.g., during ischemic injury). Alternatively, bad lipid(s) accumulation in cellular membranes or cytosol that escapes from the autophagic renovation can be deleterious for proper cellular function. Although it remains to be determined, ceramide and diacylglycerol will be putative culprits.
Although our observation is based on the data from HFD-fed mice or cultured PTCs treated with PA, it is quite possible that different lipid not only has a different effect on autophagic activity, but also even alleviates HFD (PA)-induced stagnation of autophagy. Identifying lipids (or chemical compounds) that can restore lysosomal function and thereby alleviate autophagic numbness will lead to a clue to a novel treatment of obesity-related kidney diseases.
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.