Troglitazone inhibits glutamine metabolism in rat mesangial cells

Troglitazone suppresses glutamine metabolism through a PPAR-independent mechanism

Enhanced glutamine metabolism is required for tumor cell growth and survival, which suggests that agents targeting glutaminolysis may have utility within anti-cancer therapies. Troglitazone, a PPARγ agonist, exhibits significant anti-tumor activity and can alter glutamine metabolism in multiple cell types. Therefore, we examined whether troglitazone would disrupt glutamine metabolism in tumor cells and whether its action was reliant on PPARγ activity. We found that troglitazone treatment suppressed glutamine uptake and the expression of the glutamine transporter, ASCT2, and glutaminase. In addition, troglitazone reduced 13C-glutamine incorporation into the TCA cycle, decreased [ATP], and resulted in an increase in reactive oxygen species (ROS). Further, troglitazone treatment decreased tumor cell growth, which was partially rescued with the addition of the TCA-intermediate, α-ketoglutarate, or the antioxidant N-acetylcysteine. Importantly, troglitazone's effects on glutamine uptake or viable cell number were found to be PPARγ-independent. In contrast, troglitazone caused a decrease in c-Myc levels, while the proteasomal inhibitor, MG132, rescued c-Myc, ASCT2 and GLS1 expression, as well as glutamine uptake and cell number. Lastly, combinatorial treatment of troglitazone and metformin resulted in a synergistic decrease in cell number. Therefore, characterizing new anti-tumor properties of previously approved FDA therapies supports the potential for repurposing of these agents.

Molecular mechanisms of glutamine action

Glutamine is the most abundant free amino acid in the body and is known to play a regulatory role in several cell specific processes including metabolism (e.g., oxidative fuel, gluconeogenic precursor, and lipogenic precursor), cell integrity (apoptosis, cell proliferation), protein synthesis, and degradation, contractile protein mass, redox potential, respiratory burst, insulin resistance, insulin secretion, and extracellular matrix (ECM) synthesis. Glutamine has been shown to regulate the expression of many genes related to metabolism, signal transduction, cell defense and repair, and to activate intracellular signaling pathways. Thus, the function of glutamine goes beyond that of a simple metabolic fuel or protein precursor as previously assumed. In this review, we have attempted to identify some of the common mechanisms underlying the regulation of glutamine dependent cellular functions.

Troglitazone's rapid and sustained activation of ERK1/2 induces cellular acidosis in LLC-PK1-F+cells: physiological responses

We studied the signal pathway through which troglitazone (TRO) acts in inducing cellular acidosis in LLC-PK1-F+cells in relation to ammoniagenesis and DNA synthesis. Cells were grown to confluent monolayers in 30-mm chambers and monitored for intracellular pH (pHi) by the BCECF assay and activated ERK by phospo-ERK1/2 antibodies. TRO induces a severe cellular acidosis (pHi6.68 ± 0.10 vs. 7.28 ± 0.07 time control at 4 min, P < 0.01), whereas phospho-ERK1/2 to total ERK1/2 ratio increases 3.4-fold ( P < 0.01). To determine whether ERK1/2 was activated by cellular acidosis or TRO was acting via MEK1/2 to activate ERK1/2, cells were pretreated with specific inhibitors of MEK1/2 activity, PD-098059 and U-0126, followed by the addition of TRO or vehicle. With MEK1/2 activity inhibited, TRO treatment failed to activate ERK1/2. Preventing ERK1/2 activation abrogated the TRO-induced cellular acidosis and maintained the pHiwithin the low normal range (7.06 ± 0.11). To determine whether blocking ERK activation prevents TRO's inhibitory effect on NHE activity, cells were acid-loaded and the recovery response was monitored as ΔpHi/ t over a 4-min recovery period. TRO inhibited NHE activity by 85% ( P < 0.01), whereas blocking ERK activation restored the response. We measured activated ERK levels and pHiafter 3- and 18-h exposure to TRO or extracellular acidosis (pHe = 6.95) to determine whether ERK activation was sustained. Whereas both TRO and extracellular acidosis increased activated ERK and decreased pHiafter 3 h, only TRO sustained this response at 18 h. Furthermore, both enhanced ammoniagenesis and decreased DNA synthesis reflected the effect of TRO to induce and sustain a cellular acidosis.

Troglitazone Acts on Cellular pH and DNA Synthesis through a Peroxisome Proliferator-Activated Receptor γ-Independent Mechanism in Breast Cancer-Derived Cell Lines

PURPOSE

The purpose of this study was to assess whether troglitazone (TRO) would induce cellular acidosis by inhibiting Na(+)/H(+) exchanger (NHE) 1 in breast carcinoma-derived cell lines and, if so, whether cellular acidosis would be associated with a reduction in proliferation.

EXPERIMENTAL DESIGN

Intracellular pH (pH(i)) and acid extrusion capacity after an exogenous acid load were assayed using (2, 7)-biscarboxyethyl-5(6)-carboxyfluorescein in MCF-7 and MDA-MB-231 cells treated with TRO. Radiolabeled thymidine incorporation was used to assess DNA synthesis. Peroxisome proliferator-activated receptor (PPAR) gamma involvement was assessed using an antagonist and PPARgamma(-/-) NIH3T3 cells.

RESULTS

TRO induced a prompt (<4 minute) and severe cellular acidosis in both MCF-7 (7.54 +/- 0.23 to 6.77 +/- 0.06; P < 0.001) and MDA-MB-231 cells (7.38 +/- 0.18 to 6.89 +/- 0.25; P < 0.05) after 12 minutes, without increasing acid production. Acid extrusion as assessed by the response to an exogenous acid load (NH(4)Cl pulse) was markedly blunted (MDA-MB-231, P < 0.01) or eliminated (MCF-7, P < 0.001). Chronic exposure to TRO resulted in NHE1 activity reduction (P < 0.05) and a dose-dependent decrease in DNA synthesis (<75% inhibition at 100 micromol/L; P < 0.001 and P < 0.01 for MCF-7 and MDA-MB-231, respectively) associated with a decreased number of viable cells. TRO-mediated inhibition of proliferation was not reversed by the presence of the PPARgamma inhibitor GW9662 and was demonstrable in PPARgamma(-/-) NIH3T3 cells, consistent with a PPARgamma-independent mechanism.

CONCLUSIONS

TRO induces marked cellular acidosis in MCF-7 and MDA-MD-231 cells. Sustained acidosis is consonant with decreased proliferation and growth that is not reversed by a PPARgamma antagonist. Our results support a NHE-mediated action of TRO that exerts its effect independent of PPARgamma.

Troglitazone acts by PPARγ and PPARγ-independent pathways on LLC-PK1-F+acid-base metabolism

Troglitazone was studied in pH-sensitive LLC-PK1-F+cells to determine the effect on pHiand glutamine metabolism as well as the role of peroxisome proliferator-activated receptor (PPARγ)-dependent and PPARγ-independent signaling pathways. Troglitazone induces a dose-dependent cellular acidosis that occurs within 4 min and persists over 18 h as a result of inhibiting Na+/H+exchanger-mediated acid extrusion. Cellular acidosis was associated with glutamine-dependent augmented [15N]ammonium production and decreased [15N]alanine formation from15N-labeled glutamine. The shift in glutamine metabolism from alanine to ammoniagenesis appears within 3 h and is associated after 18 h with both a reduction in assayable alanine aminotransferase (ALT) activity as well as cellular acidosis. The relative contribution of troglitazone-induced cellular acidosis vs. the decrease in assayable ALT activity to alanine production could be demonstrated. The PPARγ antagonist bisphenol A diglycide ether (BADGE) reversed both the troglitazone-induced cellular acidosis and ammoniagenesis but enhanced the troglitazone reduction of assayable ALT activity; BADGE also blocked troglitazone induction of peroxisome proliferator response element-driven firefly luciferase activity. The protein kinase C (PKC) inhibitor chelerythrine mimics troglitazone effects, whereas phorbol ester reverses the effects on ammoniagenesis consistent with troglitazone negatively regulating the DAG/PKC/ERK pathway. Although functional PPARγ signaling occurs in this cell line, the major troglitazone-induced acid-base responses appear to be mediated by pathway(s) involving PKC/ERK.

All-trans retinoic acid improves structure and function of diabetic rat skin in organ culture

Diabetes increases susceptibility to chronic ulceration. The cause of chronic wound formation in diabetic individuals is multifactorial but may be accelerated by changes in the structure and function of the skin secondary to impaired fibroblast proliferation, decreased collagen synthesis, and increased matrix metalloproteinase (MMP) expression. This study explored cellular and biochemical changes in organ cultures of skin from streptozotocin-diabetic (STZ-D) rats and the effects of all-trans retinoic acid (RA) on these changes. STZ-D rats were killed after 6 weeks. The skin was cut into 2-mm pieces and incubated in organ culture for 3 or 6 days in the absence or presence of 3 micromol/l RA. After organ culture incubation, control and RA-treated tissue was examined histologically after staining with hematoxylin and eosin. In parallel, organ culture-conditioned medium was assayed for MMPs. Additional organ cultures were examined for collagen synthesis using (3)H-proline incorporation into trichloroacetic acid-precipitable material and for glycosaminoglycan production based on interaction with the cationic dye 1,9-dimethylmethylene blue and by staining of tissue sections with periodic acid Schiff reagents. Skin from 6-week STZ-D rats demonstrated features of dermal atrophy including thinning and disorganization of connective tissue bundles and increased space between bundles. The addition of RA resulted in cellular reactivation and partially reversed the histological features of dermal atrophy. Levels of latent and active MMP-9 and MMP-13 were elevated 4- and 10-fold, respectively, in STZ-D skin and reduced by 50-75% (P < 0.05) by RA. Collagen synthesis was increased by 30% (P < 0.05) by RA, whereas glycosaminoglycan expression was increased by only 9% (NS). RA also increased proliferation of STZ-D skin fibroblasts (approximately threefold over control; P < 0.05). Together, these data suggest that RA has the capacity to improve structure and function of diabetic skin.

Glitazones regulate glutamine metabolism by inducing a cellular acidosis in MDCK cells

We studied the effect of the antihyperglycemic glitazones, ciglitazone, troglitazone, and rosiglitazone, on glutamine metabolism in renal tubule-derived Madin-Darby canine kidney (MDCK) cells. Troglitazone (25 microM) enhanced glucose uptake and lactate production by 108 and 92% (both P < 0.001). Glutamine utilization was not inhibited, but alanine formation decreased and ammonium formation increased (both P < 0.005). The decrease in net alanine formation occurred with a change in alanine aminotransferase (ALT) reactants, from close to equilibrium to away from equilibrium, consistent with inhibition of ALT activity. A shift of glutamine's amino nitrogen from alanine into ammonium was confirmed by using L-[2-(15)N]glutamine and measuring the [(15)N]alanine and [(15)N]ammonium production. The glitazone-induced shift from alanine to ammonium in glutamate metabolism was dose dependent, with troglitazone being twofold more potent than rosiglitazone and ciglitazone. All three glitazones induced a spontaneous cellular acidosis, reflecting impaired acid extrusion in responding to both an exogenous (NH) and an endogenous (lactic acid) load. Our findings are consistent with glitazones inducing a spontaneous cellular acidosis associated with a shift in glutamine amino nitrogen metabolism from predominantly anabolic into a catabolic pathway.

Troglitazone induces a cellular acidosis by inhibiting acid extrusion in cultured rat mesangial cells

We studied the effect of troglitazone on cellular acid-base balance and alanine formation in isolated rat mesangial cells. Mesangial cells were grown to confluency in RPMI 1640 media on 30-mm chambers used to monitor both cellular pH using the pH-sensitive dye 2'7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein and metabolic acid production as well as glutamine metabolism. Troglitazone (10 microM) induced a spontaneous cellular acidosis (6.95 +/- 0.02 vs. 7.47 +/- 0.04, respectively; P < 0.0001) but without an increase in lactic acid production. Alanine production was reduced 64% (P < 0.01) consistent with inhibition of the glutamate transamination. These findings pointed to a decrease in acid extrusion rather than an increase in acid production as the underlying mechanism leading to the cellular acidosis. To test their acid extrusion capabilities, mesangial cells were acid loaded with NH and then allowed to recover in Krebs-Henseleit media or in Krebs-Henseleit media minus bicarbonate (HEPES substituted), and the recovery response (Delta pH(i)/min) was monitored. In the presence of 10 microM troglitazone, the recovery response to the NH acid load was virtually eliminated in the bicarbonate-buffered media (0.00 +/- 0.001 vs. 0.06 +/- 0.02 pH(i)/min, P < 0.0001 vs. control) and reduced 75% in HEPES-buffered media (0.01 +/- 0.01 vs. 0.04 +/- 0.02 pH(i)/min, P < 0.002 vs. control). These results show that troglitazone induces a spontaneous cellular acidosis resulting from a reduction in cellular acid extrusion.