Mitochondrial dysfunction contributes to podocyte injury, but normal podocyte bioenergetics have not been characterized. We measured oxygen consumption rates (OCR) and extracellular acidification rates (ECAR), using a transformed mouse podocyte cell line and the Seahorse Bioscience XF24 Extracellular Flux Analyzer. Basal OCR and ECAR were 55.2 ± 9.9 pmol/min and 3.1 ± 1.9 milli-pH units/min, respectively. The complex V inhibitor oligomycin reduced OCR to ∼45% of baseline rates, indicating that ∼55% of cellular oxygen consumption was coupled to ATP synthesis. Rotenone, a complex I inhibitor, reduced OCR to ∼25% of the baseline rates, suggesting that mitochondrial respiration accounted for ∼75% of the total cellular respiration. Thus ∼75% of mitochondrial respiration was coupled to ATP synthesis and ∼25% was accounted for by proton leak. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), which uncouples electron transport from ATP generation, increased OCR and ECAR to ∼360% and 840% of control levels. FCCP plus rotenone reduced ATP content by 60%, the glycolysis inhibitor 2-deoxyglucose reduced ATP by 35%, and 2-deoxyglucose in combination with FCCP or rotenone reduced ATP by >85%. The lactate dehydrogenase inhibitor oxamate and 2-deoxyglucose did not reduce ECAR, and 2-deoxyglucose had no effect on OCR, although 2-deoxyglucose reduced ATP content by 25%. Mitochondrial uncoupling induced by FCCP was associated with increased OCR with certain substrates, including lactate, glucose, pyruvate, and palmitate. Replication of these experiments in primary mouse podocytes yielded similar data. We conclude that mitochondria play the primary role in maintaining podocyte energy homeostasis, while glycolysis makes a lesser contribution.
- oxygen consumption rate
- extracellular acidification rate
progressive glomerulosclerosis of diverse etiologies, including focal segmental glomerulosclerosis (FSGS), diabetic nephropathy, IgA nephropathy, and lupus, is associated with injury and loss of podocytes. Multiple pathways of cellular injury probably contribute to podocyte depletion in these various disorders. A role for podocyte mitochondrial injury in FSGS is suggested by the fact that mutations in enzymes responsible for coenzyme Q generation [including para-hydroxybenzoate-polyprenyl transferase (COQ2) and decaprenyl diphosphate synthase subunit 2 (PDSS2)] and in the mitochondrial tRNALeu(UUR) are associated with FSGS, as well as disorders in other tissues (4, 9, 14). Although mitochondrial function has been investigated in various cell types, a role of mitochondrial dysfunction as a contributor to podocyte injury in other glomerulopathies remains largely unexplored. Recently, the Seahorse Bioscience XF24 Extracellular Flux Analyzer (Seahorse Bioscience, North Billerica, MA) has been used to monitor cell respiration together with extracellular pH as an indicator of anaerobic glycolysis (26). Therefore, we wanted to further characterize mitochondrial function in cultured podocytes with the Seahorse Bioscience XF24 Extracellular Flux Analyzer. In particular, we set out to develop estimates of the relative contributions of glycolysis and oxidative phosphorylation and to determine the preferred substrates for podocyte metabolism, using a transformed mouse podocyte cell line and primary cultures of mouse podocytes.
The mouse podocyte cell line AI was established from glomeruli of transgenic mice bearing the podocin/rtTA gene and the SV40 temperature-sensitive T antigen as described previously (10). For these studies, clone 1-1P4G5 was selected based on high expression of WT-1 and synaptopodin when cultured under nonpermissive conditions (37°C) for 5 days without medium change, suggesting some degree of differentiation (10). Cells were cultured in growth medium consisting of RPMI 1640, 10% fetal bovine serum, 100 U/ml of penicillin, and 100 μg/ml streptomycin (obtained from GIBCO, Rockville, MD) under permissive conditions (33°C). Cells were trypsinized and transferred to the nonpermissive temperature of 37°C when cells reached ∼90% confluence.
Primary glomerular cells were obtained from isolated glomeruli from 4–6 wk old FVB/N mice with the Dynabeads methods (11, 23). Briefly, purified glomeruli were placed in culture in RPMI 1640 medium supplemented with 10% fetal bovine serum. When colonies were established, the cells were harvested with trypsin and used at passage 3 or 4. For immunostaining, cells were fixed with 2% paraformaldehye and 4% sucrose, exposed to primary antibody (mouse monoclonal anti-WT-1 antibody, Upstate, Lake Placid, NY; mouse anti-nestin antibody, Millipore, Billerica, MA), exposed to fluorescent secondary antibody, and examined under fluorescent microscopy. Most cells in these primary glomerular cell cultures exhibited strong nuclear staining for WT-1 and for nestin within cytoplasmic filaments (data not shown); for simplicity, we refer to these cultures as primary podocytes, although by their nature these cells are not a homogenous population.
Mice were cared for under a protocol that was approved in advance by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) Animal Care and Use Committee, and all animal care conformed to the National Institutes of Health Guide for the Care and Use of Laboratory Animals.
Sodium pyruvate, lactate, 2-deoxyglucose (2-DG), sodium oxamate, rotenone, carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), ouabain octahydrate, sodium palmitate, l-carnitine, and antimycin A were obtained from Sigma-Aldrich (St. Louis, MO), and oligomycin was obtained from Calbiochem (San Diego, CA). Concentrated stocks of 2-DG (1,000 mM) and oxamate (500 mM) were prepared in assay medium. Concentrated stocks of rotenone (1 mM) and oligomycin (1 mM) were prepared in DMSO. Concentrated stocks of FCCP (10 mM) and antimycin A (10 mM) were prepared in ethyl alcohol. Concentrated stocks of ouabain (50 mM) and l-carnitine (50 mM) were prepared in deionized water. Concentrated stocks of sodium palmitate (2 mM) were conjugated with 0.34 mM (2.267 g/dl) ultra-fatty acid-free bovine serum albumin (BSA). Ultra-fatty acid-free BSA was purchased from Roche Diagnostics (Indianapolis, IN).
Measurements of oxygen consumption rate and extracellular acidification rate.
A Seahorse Bioscience XF24-3 Extracellular Flux Analyzer was used to measure the rate change of dissolved O2 and pH in medium immediately surrounding adherent cells cultured in a collagen I-coated XF24 V7 cell culture microplate (Seahorse Bioscience).
Transformed mouse podocytes were seeded in XF24-well microplates at 2.0 × 104 cells per well (area 0.32 cm2) in 100 μl of growth medium and then incubated at 37°C with 5% CO2 overnight. The following day an additional 100 μl of growth medium was added, and 2 days later medium was replaced. After incubation for a total of 5 days, growth medium was removed and replaced with 675 μl of assay medium prewarmed to 37°C, composed of RPMI 1640 without bicarbonate containing (in mM) 10 KCl, 10 NaCl, 1 sodium pyruvate, and 3 lactate, and cultured at 37°C in room air. Primary mouse podocytes were seeded in XF24-well microplates at 2.0 × 104 cells per well in 200 μl of growth medium and then incubated at 37°C with 5% CO2 overnight. The following day, growth medium was replaced with 675 μl of assay medium.
To investigate coupling efficiency (defined as the reciprocal relationship between rates of oxidative phosphorylation and glycolysis, adjusted to meet cellular energy needs) and spare respiratory capacity, five different assay media were employed: RPMI 1640 without bicarbonate and glucose, RPMI 1640 without bicarbonate and glucose but containing 3 mM lactate, RPMI 1640 without bicarbonate containing 11 mM glucose, RPMI 1640 without bicarbonate and glucose but containing 10 mM pyruvate, and RPMI 1640 without bicarbonate containing 11 mM glucose and 10 mM pyruvate. All of these media included 2 mM l-glutamine (Table 1).
To assess β-oxidation in podocytes, RPMI 1640 containing 11 mM glucose and 0.5 mM carnitine was employed as an assay medium, and sodium palmitate was administered at the final concentration of 200 μM.
Measurements of oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) were performed after equilibration in assay medium for 1 h. Briefly, the Seahorse analyzer uses a cartridge with 24 optical fluorescent O2 and pH sensors that are embedded in a sterile disposable cartridge, 1 for each well. Before each rate measurement, the plungers mix assay media in each well for 8 min to allow the oxygen partial pressure to reach equilibrium. For measurements of rates, the plungers gently descend into the wells, forming a chamber that entraps the cells in an ∼7-μl volume. Measurements of O2 concentration and pH are periodically made over 4 min, and OCR and ECAR are obtained from the slopes of concentration change in these parameters vs. time. After the rate measurements, the plungers ascend and the plate is gently agitated to reequilibrate the medium. OCR is reported in the unit of picomoles per minute and ECAR is reported in milli-pH units (mpH) per minute. Baseline rates are measured four times. One or more testing chemicals are preloaded in the reagent delivery chambers of the sensor cartridge and then pneumatically injected into the wells to reach the desired final working concentration. After 2 min of mixing, postexposure OCR and ECAR measurements are made four to six times. The averages of four baseline rates and up to six test rates were used for data analysis.
Podocytes were seeded in collagen I-coated 96-well microplates (Discovery Labware, Becton Dickinson, Franklin Lakes, NJ) at 2.0 × 104 cells per well (0.32 cm2) in 100 μl of growth medium. Cells were incubated at 37°C overnight, with addition of another 100 μl of growth medium on the next day and replacement of medium 2 days later. After incubation for 5 days, assays were initiated by removing the growth medium from each well and replacing it with 100 μl of assay medium. Cells were exposed to vehicle or compound for 45 min in the 37°C incubator before the ATP assay started. The quantity of ATP present in the test cells in each well was measured by CellTiter-Glo Luminescent Cell Viability Assay (Promega, Madison, WI). The ATP assay was performed according to the manufacturer's instruction. Luminescence intensity from each well was measured with a FLUOstar Optima plate reader (BMG Labtech, Offenberg, Germany).
Calcein acetoxymethyl ester cell viability assay.
Cell viability after 30-min exposure to compounds or vehicle was determined with the calcein acetoxymethyl ester (AM) assay. Podocytes were seeded in collagen I-coated 96-well black microplates (Discovery Labware, Becton Dickinson) at 2.0 × 104 cells per well (0.32 cm2) in 100 μl of growth medium. Cells were incubated at 37°C overnight, with addition of another 100 μl of growth medium on the next day and replacement of medium 2 days later. After incubation for 5 days, assays were initiated by removing the growth medium from each well and replacing it with 100 μl of assay medium. Cells were exposed to vehicle or compound for 30 min at 37°C. Calcein AM (Invitrogen, Carlsbad, CA) staining solution was prepared by diluting to 1 μM in Hanks' balanced salt solution (HBSS) immediately before use. The assay was performed by first removing the assay medium containing the compounds and then washing each well with 200 μl of HBSS; 100 μl of 1 μM calcein AM staining solution was then added to each well. After incubation at 37°C for 30 min, the fluorescence intensity of each well was measured with a FLUOstar Optima plate reader (BMG Labtech).
Statistical analysis was performed with Prism (GraphPad, San Diego, CA). Data are presented as means ± SD, unless stated otherwise. For OCR and ECAR measurements over time, the area under the curve was generated by an algorithm executed by the Seahorse device and these values were compared between experimental and control groups. Analyses included Student t-test and ANOVA. A P value of <0.05 was considered significant.
We first investigated respiration in transformed mouse podocytes, assessed as OCR, and glycolytic lactic acid production, assessed as ECAR. Basal cellular OCR and ECAR were found to be 55.2 ± 9.88 pmol/min per 20 × 103 cells and 3.1 ± 1.9 mpH/min per 20 × 103 cells (initial cell count), respectively (Fig. 1A). We wanted to investigate whether the bioenergetic profile was similar in primary podocyte cultures to what we found in transformed podocytes. Basal cellular OCR and ECAR were found to be 99.6 ± 15.2 pmol/min per 20 × 103 cells and 3.1 ± 2.5 mpH/min per 20 × 103 cells (initial cell count), respectively (Fig. 1A; each data point represents mean ± SD, n = 20).
Contribution of ATP turnover, proton leak, and nonmitochondrial respiration to total cellular respiration.
We next carried out titration studies for oligomycin (which blocks the mitochondrial complex V, where the electron chain is coupled to ATP synthesis) and rotenone (which blocks complex I, thereby eliminating mitochondrial respiration) and assessed OCR, ECAR, and ATP generation. In the presence of increasing doses of oligomycin and rotenone for 45 min, OCR was reduced, while ECAR was simultaneously increased (Fig. 2, A–D), indicating that the cells shifted mitochondrial respiration to glycolysis. In contrast to cancer cells, however, in podocytes the shift to glycolysis was unable to completely fulfill cellular energy demand, and consequently ATP charge fell (Fig. 2, E and F).
Mitochondrial function comprises coupled and uncoupled respiration. Coupled respiration generates ATP, while uncoupled respiration involves the futile cycle of proton pumping and proton leak back across the inner mitochondrial membrane. Using maximally effective doses of oligomycin and rotenone, we found that oligomycin reduced OCR to ∼47% of baseline rates, indicating that ∼53% of cellular oxygen consumption was related to ATP synthesis. Rotenone reduced OCR to ∼23% of the baseline rates, suggesting that mitochondrial respiration accounted for ∼77% of the total cellular respiration. Thus in transformed podocytes ∼69% (53%/77%) of mitochondrial respiration was coupled to ATP synthesis, and ∼31% of mitochondrial respiration was accounted for by proton leak (Fig. 3). The rotenone-resistant rate reflects the nonmitochondrial respiration rate, which includes substrate oxidation and cell surface oxygen consumption (7).
With regard to primary podocytes, oligomycin and rotenone reduced OCR to ∼41% and ∼23%, respectively. These results indicate that ∼59% of cellular oxygen consumption was related to ATP synthesis and mitochondrial respiration accounted for ∼77% of the total cellular respiration. Thus in primary podocytes ∼77% (59%/77%) of mitochondrial respiration was coupled to ATP synthesis, and ∼23% of mitochondrial respiration was accounted for by proton leak (Fig. 3).
To evaluate whether the effect of oligomycin to inhibit OCR was via inhibition of Na+-K+-transporting ATPase, titration study was performed with ouabain, a specific inhibitor of Na+-K+-transporting ATPase. Ouabain (from 0.03 to 5 mM) had no effect on OCR (data not shown). These findings suggest that the effect of oligomycin is not due to inhibition of the plasma membrane Na+-K+-ATPase and that OCR is strongly coupled to ATP usage.
Contribution of mitochondrial respiration and of glycolysis to ATP charge.
We characterized the relative contribution of glycolysis and mitochondrial function to ATP charge in transformed podocytes. We measured cellular ATP concentration in cells exposed for 45 min to eight conditions: vehicle, the glycolysis inhibitor 2-DG, the ionophore and mitochondrial uncoupler FCCP, the mitochondrial complex I inhibitor rotenone, FCCP plus rotenone, 2-DG plus FCCP, 2-DG plus rotenone, and 2-DG plus FCCP plus rotenone (Fig. 1B). 2-DG, FCCP, and rotenone treatment each reduced ATP content by 20–35%. Of those three treatments, 2-DG was the most effective at reducing ATP (35% reduction). FCCP plus rotenone reduced ATP content by 60%, and 2-DG in combination with FCCP or rotenone reduced ATP by >85%. Finally, the combination of 2-DG, FCCP, and rotenone essentially eliminated ATP production.
Thus 2-DG decreased ATP charge more than FCCP or rotenone did, and 2-DG plus FCCP (as well as 2-DG plus rotenone) decreased ATP charge more than FCCP plus rotenone did. These data indicate that ATP generation shifts from oxidative phosphorylation to glycolysis when mitochondrial function is impaired (Fig. 1B). Although 1 mM pyruvate was included in the assay medium, administering 2-DG before FCCP injection had no additional effect on OCR and ECAR but administering 2-DG after FCCP injection decreased ECAR (Fig. 1C). These data indicate that FCCP can activate glycolysis but transformed podocytes do not depend on anaerobic glycolysis to generate ATP. Furthermore, oxidative phosphorylation cannot be activated by 2-DG, and ATP charge is not fully maintained after inhibition of either glycolysis or mitochondrial function.
Maximal mitochondrial respiratory capacity and cellular response to suppression of mitochondrial respiration.
We examined the effect on OCR, ECAR, and intracellular ATP levels of uncoupling respiration from oxidative phosphorylation. In the presence of increasing doses of FCCP, OCR and ECAR were both increased to ∼360% and 840% of control levels (Fig. 4). Compared with blocking by the combination of 2-DG, rotenone, and FCCP (Fig. 1B), intracellular ATP content was lowered by ∼70% in the presence of FCCP alone compared with untreated controls (Fig. 4). These data suggest that podocytes, faced with partial suppression of mitochondrial function, fail to upregulate glycolysis sufficiently to sustain cellular ATP levels. After a maximal response was reached, higher FCCP concentrations (>5 μM) were noted to be toxic to podocytes, manifested as decreased OCR (data not shown). We extended these findings to primary mouse podocytes, in which FCCP increased both OCR and ECAR, as was observed in transformed podocytes (Fig. 4, B and D).
Coupling efficiency and spare respiratory capacity.
To assess the optimal energy substrate for podocytes, we investigated coupling efficiency and spare respiratory capacity. Coupling efficiency is assessed by the administration of oligomycin, and then spare respiratory capacity is assessed by the administration of FCCP. Pyruvate and pyruvate plus glucose increased the OCR in each phase (Table 2). In addition to increasing OCR, it is apparent that exogenous pyruvate is additive in enhancing spare respiratory capacity in the presence of FCCP in both transformed podocytes and primary podocytes (Fig. 5A). Interestingly, 3 mM lactate could also enhance spare respiratory capacity, and did so to a greater extent even than glucose in primary podocytes. Furthermore, FCCP with 2 mM glutamine could increase OCR especially in primary podocytes. Although calculated ATP turnover was not altered, after FCCP, proton leak (expressed as % of basal rates) and mitochondrial respiration (also expressed as % of basal rates) decreased in the presence of lactate, glucose, pyruvate, and glucose plus pyruvate (Fig. 5B).
To investigate the mechanisms for this reduced mitochondrial respiration and reduced proton leak, rotenone was added in the presence and absence of FCCP, using glucose plus pyruvate medium (Fig. 5C). In the absence of FCCP, rotenone completely inhibited OCR, but when FCCP was added first and rotenone added later, rotenone was not able to completely inhibit OCR. By contrast, when the complex III inhibitor antimycin was used in place of rotenone, OCR was completely blocked whether FCCP was present or absent (Fig. 5C). In this experiment, mitochondrial respiration and proton leak were lowest in the rotenone group in the presence of FCCP (Fig. 5D).
We next investigated the effects of various glycolytic substrates, using the substrates listed in Table 1. After addition of oligomycin there was a modest numerical increase in podocyte ECAR with glucose medium (reflecting increased glycolysis as compensation for inhibited mitochondrial ATP synthesis) but little change with pyruvate medium and with glucose plus pyruvate medium (Fig. 5E). After the addition of FCCP, podocyte ECAR increased with all three media; the peak ECAR was highest in glucose plus pyruvate medium, next highest in glucose medium, and third highest in pyruvate medium. By contrast, podocyte ECAR showed no increase following addition of oligomycin and addition of FCCP in control medium (containing only glutamine), consistent with the lack of glycolytic substrate in this medium (data not shown). These data suggest that podocytes utilize glucose and to a lesser extent pyruvate in the tricarboxylic acid (TCA) cycle, leading to CO2 production and extracellular acidification.
Cellular responses to suppression of glycolysis.
As podocytes appeared to have limited glycolytic reserve, we next examined glycolytic rates in greater detail. The contribution of lactic acid production from glycolysis to ECAR was assessed with two inhibitors. Oxamate inhibits lactate dehydrogenase, which converts pyruvate to lactate during the last step of glycolysis. 2-DG is a glucose analog and inhibits hexokinase, the first enzyme in the glycolysis pathway, which converts glucose to glucose-6-phosphate. Neither inhibitor affected OCR (Fig. 6, A and B), but, interestingly, both agents increased ECAR (Fig. 6, C and D). ATP synthesis was only modestly affected by oxamate, as might be expected since glycolysis is intact (Fig. 6E). By contrast, ATP synthesis dropped by ∼25% with high concentrations of 2-DG. These data indicate that mitochondrial function, while the dominant contributor to the energy budget, cannot fully compensate for impaired glycolysis (Fig. 6F).
Alternative mitochondrial substrates: fatty acid oxidation.
We examined the ability of transformed podocytes to utilize fatty acids as a metabolic fuel source in the presence of a maximally effective dose of the uncoupling agent FCCP. Palmitate with a BSA carrier significantly increased OCR and ATP charge more than basal medium containing BSA (Fig. 7), while ECAR was increased to a similar extent under both conditions (data not shown). These data suggest that transformed podocytes can use palmitate as a fuel source. However, these findings were not seen in primary podocytes (data not shown).
We have studied the bioenergetic profile of cultured mouse podocytes, including both transformed cells and primary cells. We measured OCR and ECAR, using a Seahorse extracellular flux analyzer, and cellular ATP levels in response to various inhibitors of mitochondrial function and of glycolysis and in response to provision of various energy substrates. Previous studies using a similar technique to characterize cellular bioenergetics have studied cancer lines, isolated synaptosomes, and myocytes (3, 21, 22, 26). To our knowledge, this is the first study using transformed cell lines and primary cells derived from the mouse kidney.
Our principal findings are as follows. First, mitochondrial respiration accounts for ∼77% of cellular respiration and 75% of this coupled to ATP synthesis. Second, inhibition of glycolysis or of oxidative phosphorylation each reduced ATP levels, suggesting that podocytes have limited ability to increase oxidative phosphorylation or glycolysis to preserve energy homeostasis under these conditions. Third, cultured podocytes utilize a range of substrates, including glucose, pyruvate, palmitate, and lactate.
Podocytes differ from previously studied cancer cells in two ways: podocytes do not depend on glycolysis for generation of ATP, and inhibition of glycolysis does not lead to an increase in oxidative phosphorylation, leading to a fall in cellular ATP content. In the human non-small cell carcinoma cell lines H460 and A549, Wu et al. (26) found that there was an effective compensatory upregulation of glycolysis following the administration of oligomycin to block oxidative phosphorylation, and this response was able to sustain ATP level. By contrast, we have shown that podocytes lack this ability to increase glycolysis when mitochondrial function is blocked (Table 3). This finding has important implications for podocyte biology, underscoring the importance of the mitochondria to podocyte homeostasis.
Cultured podocytes demonstrated additional bioenergetic differences from cancer cells. In cultured human non-small cell carcinoma cell, oxamate reduced ECAR by 80% and 2-DG reduced ECAR and increased OCR (26). The oxamate-sensitive portion of ECAR reflects the glycolysis rate, while the oxamate-insensitive ECAR is due to nonglycolytic acidification, mostly acidification by CO2. CO2 is the end product of the TCA cycle in mitochondria, and after generation of carbonic acid this contributes to extracellular acidification. Somewhat surprisingly, in podocytes, oxamate had little effect on OCR or cellular ATP, suggesting that, unlike what is seen in many tumors and cell lines, podocytes primarily oxidize their glycolytic pyruvate, and thus it appears that little lactic acid is produced from glycolysis, while oxamate increased ECAR. This increase in ECAR may be due to increased amounts of pyruvate being metabolized to CO2 within mitochondria, ultimately generating protons, rather than being converted to lactate, which generates only one proton per molecule compared with three protons generated per molecule of pyruvate that is metabolized to CO2. On the other hand, 2-DG, which blocks glycolysis, reduced cellular ATP and was not associated with an increase in OCR. This indicates that when glycolysis is inhibited podocytes do not increase mitochondrial activity to make up the energy deficit. These findings suggest that cultured podocytes have limited ability to increase mitochondrial function when the need arises.
In the present study, exogenous lactate increased OCR to the same degree as glucose. Movement of lactate occurs via two types of transporters selective for monocarboxylates, the monocarboxylate transporters (MCTs) and sodium-coupled monocarboxylate transporters (SMCTs) (5, 6, 13, 27). The MCT family now comprises 14 members, of which 4 (MCT1–MCT4) have been demonstrated experimentally to catalyze the proton-linked transport of metabolically important monocarboxylates. SMCTs comprise SMCT1 (SLC5A8) and SMCT2 (SLC5A12). MCT1, MCT2, SMCT1, and SMCT2 are expressed in tubular cells. To our knowledge, there are no reports that both of these are expressed in the podocytes. In the present study we confirmed mRNA expression of MCT1 in both transformed podocytes and primary podocytes (data not shown). Further investigation will be required to investigate how podocytes take up exogenous lactate and to define the importance of this pathway. Furthermore, FCCP plus glutamine increased OCR, especially in primary podocytes. In the present study, assay media were added 1 h before the measurement. One possible explanation is that endogenous glycogen or glutamine is mobilized to provide a substrate for glycolysis (17).
Nonmitochondrial oxygen consumption is composed chiefly of cell surface oxygen consumption, peroxisomal oxygen consumption, and substrate oxidation. In this study, nonmitochondrial respiration was 23% in transformed podocytes and primary podocytes. Herst and Berridge (7) studied 19 cancer cell lines and found wide variability (1% to 96%) in the extent to which mitochondrial respiration accounts for cellular oxygen consumption, and the extent of cell surface oxygen consumption varied among those cell lines, ranging from 1% to 80%. Thus, while there is much variability in the rates of nonmitochondrial oxygen consumption among cell types, the results from transformed podocytes and primary podocytes are quite similar to each other.
Proton leak dissipates ∼20% of the energy derived from mitochondrial function in cells and tissues derived from many animal species (1). The physiological function of proton leak may include heat production and prevention of oxidative stress caused by reactive oxygen species (ROS) (1). Indeed, we found that proton leak accounts for the loss of 31% and 23% of total energy generated in transformed and primary podocytes, respectively.
Rotenone, a complex I inhibitor, suppresses mitochondrial respiration. In the presence of lactate, glucose, pyruvate, and glucose plus pyruvate, the fraction of OCR that was inhibited by rotenone fell compared with the fraction of OCR that was inhibited by rotenone in basal medium. The reasons for this decrease in mitochondrial respiration when these energy substrates are present are uncertain. It may be that these metabolic substrates stimulate glycolysis and thereby reduce mitochondrial respiration; such coupling has been shown previously (3). Since all these measurements were made after the administration of the uncoupler FCCP, it is also possible that these findings in some way reflect an artifact. For example, FCCP uncoupling might alter complex I function or might affect or the uptake of rotenone. We are not aware of other investigations of rotenone-inhibitable mitochondrial respiration in other cell types in the presence of the range of substrates we used, and so we cannot determine whether these findings extend to other cell types. Finally, the increase in OCR that we noted with certain substrates before oligomycin administration could be due to an increase of nonmitochondrial respiration including substrate oxidation and/or cell surface oxygen consumption.
Choi et al. (3) studied synaptosomes isolated from mouse cerebral cortex and showed that exogenous glucose and pyruvate were additive in enhancing spare respiratory capacity following FCCP (as we found in podocytes) and that pyruvate increased the proton leak conductance estimated from oligomycin-insensitive respiration (as we found in podocytes). Since proton leak rate is highly dependent upon the mitochondrial membrane potential (18) and pyruvate causes further hyperpolarization of the intrasynaptosomal mitochondria even in the presence of glucose (12), these authors suggested that the increased proton leak might be accounted for by this increase in mitochondrial membrane potential and further that pyruvate selectively energized a population of synaptosomes that had impaired glucose utilization (3). These authors did not use rotenone to measure mitochondrial respiration. Our data, derived from podocytes rather than isolated synaptosomes, indicate that proton leak is actually decreased by glucose, by pyruvate, and especially by a combination of these substrates. Exposure of cells to elevated glucose concentrations induces mitochondrial hyperpolarization and generation of mitochondrial superoxide, which is coupled with an accelerated electron flux across the renal mitochondria (16, 24). Proton leak has been shown to decrease ROS generation, whereas ROS have been shown to induce proton leak. This suggests the existence of a feedback loop between proton leak and ROS (2). Although high ROS generation is expected in the presence of elevated glucose concentrations, the calculated proton leak in podocytes was lowest in glucose plus pyruvate medium. Thus our data cannot be readily explained by a link between ROS and proton leak. At this time, we do not have a hypothesis to explain these findings in podocytes.
We were somewhat surprised to see that rotenone and antimycin had slightly different quantitative effects on OCR inhibition. Antimycin, a complex III inhibitor, completely inhibited OCR both in the presence and in the absence of FCCP. It is known that the combination of carbonyl cyanide m-chlorophenylhydrazone with either oligomycin or antimycin gives rise to the elevated levels of ROS in both HeLa cells and HeEB1 cells, while each compound alone has a much smaller effect (8); thus the combination of two drugs could increase mitochondrial membrane potential. However, if the generation of mitochondrial membrane potential is inhibited by FCCP before antimycin, a part of the flow of energy can generate proton leak, since proton leak is mediated by electron flow via complexes I, III, and IV (Fig. 8) (2). If the generation of mitochondrial membrane potential is inhibited by FCCP before rotenone, the flow of energy or electrons cannot generate proton leak. Otherwise, since it is reported that high glucose exposure leads to reduced complex III activity in rat renal proximal cells (16), the same amount of antimycin could inhibit OCR more effectively than rotenone did.
Mitochondrial uncoupling induced by FCCP was associated with increased OCR with some but not all substrates. In transformed podocytes these substrates included lactate, glucose, pyruvate, and palmitate, while in primary podocytes these substrates included lactate, glucose, and pyruvate. These data are in general similar to those of other cell types, in which uncoupling leads to increase OCR with pyruvate and fatty acid (octanoic acid) (3, 26). To the extent that the response to mitochondrial uncoupling can provide a valid model of cellular response to increased energy demands, these findings may shed light on the utilization of various substrates under conditions of cell stress. Furthermore, it is known that mitochondrial uncoupling leads to stimulation of fatty acid β-oxidation in several studies with other cell types (19, 20, 25). With regard to fatty acid utilization in podocytes, Mayrhofer et al. (15) showed that mouse podocytes express CD36, which is a key cell surface molecule that promotes the uptake of long-chain fatty acids, and that disrupted fatty acid metabolism in concert with an impaired antioxidant defense mechanism may play a role in puromycin aminonucleoside nephrosis in rat podocytes.
Our study has certain limitations. First, much of our data derived from podocyte cell lines, which have the advantage of providing a stable cell phenotype over time. It is possible that transformation alters cellular bioenergetics. To address this possibility, we confirmed key aspects of our work in primary mouse podocytes. Second, the timescale of our experimental manipulations was necessarily fairly short, on the order of 3 h, and so the effect of particular interventions over a period of days or weeks could not be assessed. Third, these data may or may not reflect the bioenergetic profile of podocytes in vivo, where these cells receive cues from other cells and a specialized extracellular matrix and are bathed in a highly complex plasma milieu, with lower oxygen tension than is typical in cell culture.
In summary, we have developed an initial bioenergetic profile of cultured mouse podocytes and conclude that mitochondria play the primary role in maintaining energy homeostasis, while glycolysis makes a lesser contribution. Podocytes can use a diverse set of substrates, including glucose, lactate, pyruvate and, in transformed podocytes but not primary podocytes, palmitate.
This study was supported by the NIDDK Intramural Research Program, under project ZO1-DK-043308.
No conflicts of interest, financial or otherwise, are declared by the author(s).
We are grateful to Hideko Takahashi and Huiyan Lu for technical assistance with animal care, to Dr. Hisashi Hasumi and Dr. Masaya Baba for valuable advice, and to Dr. Kevin Bittman and Dr. Min Wu, both of Seahorse Bioscience, for valuable advice in the design and interpretation of particular experiments. We thank Dr. Michael Sack for critical review of the manuscript.