**7. Glitazones accelerate GDH flux via the push/pull mechanism: A strategy for therapeutic intervention**

Fortuitously there are agents that can be employed to impose this push/pull mechanism on the GDH flux in cancer cells and thereby present a window of vulnerability (targeted inhibitors). The antihyperglycemic agents, troglitazone (Rezulin) and rosiglitazone (Avandia) block pyruvate entrance into the TCA cycle(25,33] lowering αKG(glutamate,7,25] and accelerating GDH flux via this "pull" mechanism (**Figure 2**). Simultaneously, both troglitazone and rosiglitazone directly inhibit NHE [25,34] lowering pHi and driving GDH via the "push" mechanism (**Figure 2**). Noteworthy the glutaminase flux (glutamine disappearance) remains unchanged while the NH4+ production increases as the result of the increase in deamination flux (**GDH Figure 1, Rxn1**). Although resembling glucose deprivation ("pull" mechanism), troglitazone further increases the NH4+ production (additive "push+pull" mechanism) exceeding the fall in alanine production ("pull" mechanism alone). More specifically the accelerated GDH flux ("push+pull") induced by troglitazone can be demonstrated using 15N amino labeled glutamine as shown in **Figure 5**; in contrast, another glitazone pioglitazone(Actos) activates GDH flux [34] solely by reducing cell pH("push" mechanism) and consequently does not reduce alanine production [34]. Noteworthy troglitazone acutely inhibits GDH flux (0-3hrs) as the result of a fall in mitochondrial membrane potential( Ѱm) requiring accelerated GDH flux(3-24hrs) to fully restore the Ѱm [7], a response that is PPARγ independent [7,25] and possibly mitoNEET [35] dependent. Little recognized is the direct inhibition of NHE[20,34] by both troglitazone and rosiglitazone as well as indirect inhibition mediated through PPARγ suppression of NHE gene expression[20,36]; in contrast, pioglitazone does not inhibit NHE directly[34] rather acts indirectly through PPARγ[36]lowering cell pH[34] and accelerating GDH flux("push" mechanism). In combination, TRO +PIO together exert an additive effect on GDH flux

presumably reflecting both TRO's "pull" action and PIO's PPARγ- mediated downregulation of NHE gene expression. Significantly, the dual effect of glitazones to increase GDH flux while reducing NHE activity decreases proliferation(NHE) but increases cell survival(GDH) resulting in only a slight decrease in cell number[26]. Nor does adding troglitazone to glucose deleted cells induce massive cell death since the effect on GDH flux is additive (further reducing TCA intermediates and cell pH, **Figure 2**) and although proliferative rates are decreased, survival is enhanced [26]. Under these conditions, e.g. cell survival mechanisms, inhibition of GDH is most effective in causing massive cell death as occurs with the GDH inhibitor EGCG [5] combined with troglitazone [26]. Although rescue of cells is partly possible by restoring anaplerosis with methyl pyruvate, failure to restore NHE activity and the cellular acidosis preclude full recovery underlining the importance of both GDH and NHE in cell survival[26].

Role of Glutamate Dehydrogenase in Cancer Growth and Homeostasis 37

**Author details** 

*Center, Shreveport, LA, USA* 

*Center, Shreveport, LA, USA* 

**Acknowledgement** 

Fancesco Turturro

**8. References** 

*Department of Medicine- Feist- Weiller Cancer Center* 

Robert Oliver III and Tomas Welbourne

Natl Acad Sci. 104:19345-19350.

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cellular function. News Physiol Sci.16:157-160.

*Department of Cellular and Molecular Physiology, Louisiana State University Health Sciences* 

*Department of Cellular and Molecular Physiology, Louisiana State University Health Sciences* 

The authors would like to acknowledge the support from the Feist-Weiller Cancer Center

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*Department of Lymphoma; Myeloma, MD Anderson Cancer Center, Houston, TX, USA* 

(EF and FT) and The Southern Arizona Research Foundation (TW).

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Ellen Friday

**Figure 5. Ammonium production from amino nitrogen of glutamine.** Cells were incubated for 18 hours in [2-15N] glutamine. TRO was used at 20 uM, PIO 10 uM. Results are from 3 experiments.
