Individualizing antimetabolic treatment strategies for head and neck squamous cell carcinoma based on TP53 mutational status.

BACKGROUND: Mutations in the tumor protein 53 (TP53) tumor suppressor gene are common in head and neck squamous cell carcinoma (HNSCC) and correlate with radioresistance. Currently, there are no clinically available therapeutic approaches targeting p53 in HNSCC. In this report, the authors propose a strategy that uses TP53 mutational status to individualize antimetabolic strategies for the potentiation of radiation toxicity in HNSCC cells. METHODS: Glycolytic flux and mitochondrial respiration were evaluated in wild-type (wt) and mutant (mut) TP53 HNSCC cell lines. Sensitivity to external-beam radiation (XRT) was measured using a clonogenic assay. RESULTS: HNSCC cells that expressed mutTP53 demonstrated radioresistance compared with HNSCC cells that expressed wtTP53. Glycolytic inhibition potentiated radiation toxicity in mutTP53-expressing, but not wtTP53-expressing, HNSCC cells. The relative sensitivity of mutTP53 HNSCC cells to glycolytic inhibition was caused by a glycolytic dependence associated with decreased mitochondrial complex II and IV activity. The wtTP53-expressing cells maintained mitochondrial reserves and were relatively insensitive to glycolytic inhibition. Inhibition of respiration using metformin increased glycolytic dependence in wtTP53-expressing cells and potentiated the effects of glycolyic inhibition on radiation toxicity. CONCLUSIONS: TP53 mutation in HNSCC cells was correlated with a metabolic shift away from mitochondrial respiration toward glycolysis, resulting in increased sensitivity to the potentiating effects of glycolytic inhibition on radiation toxicity. In contrast, wtTP53-expressing cells required inhibition of both mitochondrial respiration and glycolysis to become sensitized to radiation. Therefore, the authors concluded that TP53 mutational status may be used as a marker of altered tumor cell metabolism to individualize HNSCC treatment selection of specific, targeted metabolic agents that can overcome cellular resistance to radiation therapy.

The catalytic subunit of Drosophila glutamate-cysteine ligase is a nucleocytoplasmic shuttling protein.

GSH concentration is considerably lower in the nucleus than in the cytoplasm; however, it is significantly elevated during active cell proliferation. The main purpose of this study was to understand the mechanism underlying these variations in nuclear/cytoplasmic distribution of GSH. The rate-limiting step in the de novo GSH biosynthesis pathway is catalyzed by glutamate cysteine ligase (GCL), a heterodimer, composed of a catalytic subunit (GCLc) and a modulatory subunit (GCLm). In Drosophila, GCLc, but not GCLm, contains a nuclear localization signal (NLS). Drosophila S2 cells, constitutively expressing regular GCLc protein or expressing GCLc protein with a mutated NLS motif, were generated by transfection. In quiescent S2 cells, GCLc is aggregated in the perinuclear cytosol and the nucleus, whereas GLCm resides solely in the cytosol. In actively proliferating S2 cells, expressing the normal NLS motif, GCLc migrates from the perinuclear cytoplasm into the nucleus, and the nuclear GSH level becomes elevated; in contrast, in proliferating cells, expressing the mutated NLS motif, neither does the GCLc migrate into the nucleus nor does the nuclear GSH amount rise. In S2 cells expressing wild type GCLc, perturbation of cellular redox state by exposure to cadmium resulted in the migration of GCLc into the nucleus but not in cells expressing GCLc with the mutated NLS motif. Overall, results indicated that GSH biosynthesis in the nucleus is associated with migration of only the GCLc subunit from the cytoplasm into the nucleus, and this migration requires the presence of an intact NLS.

Modulating longevity in Drosophila by over- and underexpression of glutamate-cysteine ligase.

A notable extension of life span (up to 50%) was achieved in Drosophila melanogaster when the catalytic subunit of glutamate-cysteine ligase (GCLc) was overexpressed in neuronal tissue, while a moderate increase (up to 24%) was observed when the modulatory subunit of GCL (GCLm) was overexpressed globally. We sought to identify specific tissue domains that are particularly sensitive to the beneficial effects of GCLc overexpression. Overexpression of GCLc using the mushroom body driver (OK107-GAL4) had a small but significant beneficial effect on longevity (approximately 12%) while overexpression in serotonergic (MZ360-GAL4) neurons or dopaminergic and serotonergic neurons (Ddc-GAL4) had small, nonsignificant effects on longevity. A significant beneficial effect (12-13%) was also observed using the C23-GAL4 transverse muscle driver. Finally, a low-level global driver (armadillo) was shown to increase life span significantly (15%). A series of mutant and knockdown studies were also carried out. Reduction of GCLm by > 95% had no discernable effect on longevity or resistance to oxidative stress. In contrast, knockdown of GCLc by 30-70% using an RNAi-hairpin strategy had a significant effect, resulting in greater sensitivity to H(2)O(2) and reduced survivorship under normal conditions varying from a 50% reduction in median life span to lethality. A GCLc null allele was identified and shown to be recessive lethal. Overall, this study demonstrates that the longevity effects of GCLc are dependent on dosage and that there are specific tissues (mushroom bodies, motor neurons, and transverse muscle cells) particularly sensitive to the benefits of GCLc overexpression.

Overexpression of glutamate-cysteine ligase extends life span in Drosophila melanogaster.

The hypothesis that overexpression of glutamate-cysteine ligase (GCL), which catalyzes the rate-limiting reaction in de novo glutathione biosynthesis, could extend life span was tested in the fruit fly, Drosophila melanogaster. The GAL4-UAS binary transgenic system was used to generate flies overexpressing either the catalytic (GCLc) or modulatory (GCLm) subunit of this enzyme, in a global or neuronally targeted pattern. The GCL protein content of the central nervous system was elevated dramatically in the presence of either global or neuronal drivers. GCL activity was increased in the whole body or in heads, respectively, of GCLc transgenic flies containing global or neuronal drivers. The glutathione content of fly homogenates was increased by overexpression of GCLc or GCLm, particularly in flies overexpressing either subunit globally, or in the heads of GCLc flies possessing neuronal drivers. Neuronal overexpression of GCLc in a long-lived background extended mean and maximum life spans up to 50%, without affecting the rate of oxygen consumption by the flies. In contrast, global overexpression of GCLm extended the mean life span only up to 24%. These results demonstrate that enhancement of the glutathione biosynthetic capability, particularly in neuronal tissues, can extend the life span of flies, and thus support the oxidative stress hypothesis of aging.