Ichiro Tatsuno

We identified a p53 target gene, phosphate-activated mitochondrial glutaminase (GLS2), a key enzyme in conversion of glutamine to glutamate, and thereby a regulator of glutathione (GSH) synthesis and energy production. GLS2 expression is induced in response to DNA damage or oxidative stress in a p53-dependent manner, and p53 associates with the GLS2 promoter. Elevated GLS2 facilitates glutamine metabolism and lowers intracellular reactive oxygen species (ROS) levels, resulting in an overall decrease in DNA oxidation as determined by measurement of 8-OH-dG content in both normal and stressed cells. Further, siRNA down-regulation of either GLS2 or p53 compromises the GSH-dependent antioxidant system and increases intracellular ROS levels. High ROS levels following GLS2 knockdown also coincide with stimulation of p53-induced cell death. We propose that GLS2 control of intracellular ROS levels and the apoptotic response facilitates the ability of p53 to protect cells from accumulation of genomic damage and allows cells to survive after mild and repairable genotoxic stress. Indeed, overexpression of GLS2 reduces the growth of tumor cells and colony formation. Further, compared with normal tissue, GLS2 expression is reduced in liver tumors. Thus, our results provide evidence for a unique metabolic role for p53, linking glutamine metabolism, energy, and ROS homeostasis, which may contribute to p53 tumor suppressor function.

A novel bioactive peptide was recently isolated from ovine hypothalamus and was named PACAP (pituitary adenylate cyclase-activating polypeptide). PACAP was present in two bioactive, amidated forms, PACAP27 and PACAP38 (27 and 38 amino acids, respectively), and showed a 68% sequence homology with vasoactive intestinal peptide (VIP) in the N-terminal 28 residues. PACAP38 was at least 1000 times more potent than VIP in stimulating adenylate cyclase in pituitary cells, but both peptides exhibited comparable vasodepressor activity. Thus, we sought to determine whether PACAP acts on specific binding sites in the anterior pituitary or other tissues and whether these binding sites are different from those of VIP. Binding of [125I] PACAP27 to freshly prepared rat anterior pituitary membranes in the presence and absence of 212 nM unlabeled PACAP27 was specific, saturable, and more rapid at 22 C than at 4 C. Scatchard analysis of this binding site using increasing doses of unlabeled PACAP27 revealed a single high affinity site with a Kd of 446 +/- 141 pM and a maximum number of sites of 1312 +/- 182 fmol/mg protein. These results do not exclude the possibility of a second pituitary binding site with significantly lower affinity. Unlabeled PACAP38 and PACAP38OH exhibited significantly higher affinity binding (3- to 5-fold) than PACAP27 with a similar number of pituitary sites. A variable distribution of binding sites was observed between PACAP27 and VIP when binding to different tissue membranes was measured with 125I-labeled peptides. Very high specific binding of both PACAP27 and VIP was observed in lung membranes. An almost identical relative magnitude of binding was observed between PACAP27 and VIP in lung, liver, duodenum, ovary, and thymus. However, whereas PACAP27 binding to hypothalamic and pituitary membranes was great, VIP binding to these tissues was almost absent. To determine if VIP and PACAP might share a binding site in peripheral tissues, displacement curves were generated using [125I]PACAP27 binding to lung membranes and VIP, PACAP27, and PACAP38 as unlabeled ligands. VIP was highly potent in displacing [125I] PACAP27 binding in lung membrane, and the IC50 values for all three of these peptides were between 1-10 nM. These results suggest that 1) a saturable, high affinity binding site for PACAP is present on anterior pituitary membranes; 2) PACAP27 and PACAP38, but not VIP, share this binding site in the anterior pituitary and possibly the hypothalamus; and 3) PACAP27, PACAP38, and VIP share a similar or identical binding site on lung membranes and possibly other peripheral tissues.

Cyclin-dependent kinase (Cdk) enzymes are activated for entry into the S phase of the cell cycle. Elimination of Cdk inhibitor protein p27Kip1 during the G1 to S phase is required for the activation process. An inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase prevents its elimination and leads to G1 arrest. Mevalonate and its metabolite, geranylgeranyl pyrophosphate, but not farnesyl pyrophosphate, restore the inhibitory effect of pravastatin on the degradation of p27 and allow Cdk2 activation. By the addition of geranylgeranyl pyrophosphate, Rho small GTPase(s) are geranylgeranylated and translocated to membranes during G1/S progression. The restoring effect of geranylgeranyl pyrophosphate is abolished with botulinum C3 exoenzyme, which specifically inactivates Rho. These results indicate (i) among mevalonate metabolites, geranylgeranyl pyrophosphate is absolutely required for the elimination of p27 followed by Cdk2 activation; (ii) geranylgeranylated Rho small GTPase(s) promote the degradation of p27 during G1/S transition in FRTL-5 cells. Cyclin-dependent kinase (Cdk) enzymes are activated for entry into the S phase of the cell cycle. Elimination of Cdk inhibitor protein p27Kip1 during the G1 to S phase is required for the activation process. An inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase prevents its elimination and leads to G1 arrest. Mevalonate and its metabolite, geranylgeranyl pyrophosphate, but not farnesyl pyrophosphate, restore the inhibitory effect of pravastatin on the degradation of p27 and allow Cdk2 activation. By the addition of geranylgeranyl pyrophosphate, Rho small GTPase(s) are geranylgeranylated and translocated to membranes during G1/S progression. The restoring effect of geranylgeranyl pyrophosphate is abolished with botulinum C3 exoenzyme, which specifically inactivates Rho. These results indicate (i) among mevalonate metabolites, geranylgeranyl pyrophosphate is absolutely required for the elimination of p27 followed by Cdk2 activation; (ii) geranylgeranylated Rho small GTPase(s) promote the degradation of p27 during G1/S transition in FRTL-5 cells.

Interleukin 6 (IL-6) production was shown to be stimulated by vasoactive intestinal peptide via cAMP dependent signal transduction pathway in the pituitary. We were interested in whether other hypothalamic neuropeptides, which activate adenylate cyclase in the pituitary, also stimulate pituitary IL-6 production. Whereas vasoactive intestinal peptide was effective in stimulating pituitary IL-6 production only at concentrations of 10(-6) M or higher, pituitary adenylate cyclase activating polypeptide with 38 residues (PACAP38) and calcitonin gene-related peptide (CGRP) at concentrations from 10(-10) to 10(-9) M significantly stimulated IL-6 production. Similar effective concentrations of each peptide were required for activating adenylate cyclase, as measured by extracellular cAMP accumulation. H89, a specific inhibitor of cAMP dependent protein kinase (protein kinase A), inhibited IL-6 production stimulated by PACAP38, CGRP, and (Bu)2cAMP. However, H89 failed to inhibit the IL-6 production stimulated by lipopolysaccharide, a ligand which enhanced IL-6 production in the absence of cAMP accumulation. Two other peptides which are known to activate pituitary adenylate cyclase, corticotropin-releasing factor and GRF failed to stimulate IL-6 production in pituitary cells. Using discontinuous Percoll gradients to fractionate the pituitary cells, the greatest PACAP38-stimulated IL-6 secretion was observed in the low density fraction 1 (F1). This fraction also contained the highest percentage of folliculo-stellate (FS) cells, one of the nonhormone secreting pituitary cells. However, the largest PACAP38-induced accumulation of cAMP was observed in F4. These results suggest that the production of IL-6 stimulated by PACAP and CGRP is mediated by the adenylate cyclase/protein kinase A signal transduction system. FS cells appear to be the most likely target cell type for PACAP-induced IL-6 production. However, IL-6 producing FS cells may not be an exclusive target for PACAP in the pituitary.