Polyamines are required for entry and progression of the cell cycle. As such, augmentation of polyamine levels is essential for cellular transformation. Polyamines are autoregulated through induction of antizyme, which represses both the rate-limiting polyamine biosynthetic enzyme ornithine decarboxylase and cellular polyamine transport. In the present study we demonstrate that agmatine, a metabolite of arginine via arginine decarboxylase (an arginine pathway distinct from that of the classical polyamines), also serves the dual regulatory functions of suppressing polyamine biosynthesis and cellular polyamine uptake through induction of antizyme. The capacity of agmatine to induce antizyme is demonstrated by: (a) an agmatine-dependent translational frameshift of antizyme mRNA to produce a fulllength protein and (b) suppression of agmatine-dependent inhibitory activity by either anti-antizyme IgG or antizyme inhibitor. Furthermore, agmatine administration depletes intracellular polyamine levels to suppress cellular proliferation in a transformed cell line. This suppression is reversible with polyamine supplementation. We propose a novel regulatory pathway in which agmatine acts as an antiproliferative molecule and potential tumor suppressor by restricting the cellular polyamine supply required to support growth.Polyamines (putrescine, spermidine, and spermine) are required for DNA replication, proliferation, and cell homeostasis (1-3). Ornithine decarboxylase (ODC) 1 is the first rate-limiting enzyme of polyamine biosynthesis and one of the most highly regulated eukaryotic enzymes. Cellular polyamine transporters are stimulated by many of the same factors that induce ODC activity, and similarly, enhanced cellular polyamine uptake occurs both in normal but rapidly proliferating cells (4) and in tumor cell lines (5-8). Cells in vivo can acquire polyamines released into the circulation by other cells, dietary sources, and gut flora. Polyamines have been demonstrated to play an important role in the transformation process. Conversely, polyamine depletion results in growth arrest (9, 10).Intracellular polyamine concentrations are autoregulated by the induction of the protein antizyme (11). Antizyme is the only known endogenous protein that binds to ODC, inhibiting activity and accelerating its degradation (12). In addition to inhibiting polyamine biosynthesis, antizyme has recently been shown to concurrently suppress polyamine transporter(s) (13,14). Pharmacological inhibition of ODC activity, however, has been shown to result in compensatory cellular polyamine uptake (6). Beneficial therapeutic intervention must therefore address both polyamine transport as well as biosynthesis (for review see Ref. 15).The metabolism of arginine to agmatine by ADC has only recently been demonstrated in mammals (16). As agmatine and polyamines are structurally analogous polycationic molecules derived from distinct arginine-dependent pathways (6), we speculated that the ADC metabolite agmatine may play a role in regulating intracellul...

Until recently, conversion of arginine to agmatine by arginine decarboxylase (ADC) was considered important only in plants and bacteria. In the following, we demonstrate ADC activity in the membrane-enriched fraction of brain, liver, and kidney cortex and medulla by radiochemical assay. Diamine oxidase, an enzyme shown here to metabolize agmatine, was localized by immunohistochemistry in kidney glomeruli and other nonrenal cells. Production of labeled agmatine, citrulline, and ornithine from [3 H]arginine was demonstrated and endogenous agmatine levels (10 Ϫ 6 M) in plasma ultrafiltrate and kidney were measured by HPLC. Microperfusion of agmatine into renal interstitium and into the urinary space of surface glomeruli of Wistar-Frömter rats produced reversible increases in nephron filtration rate (SNGFR) and absolute proximal reabsorption (APR). Renal denervation did not alter SNGFR effects but prevented APR changes. Yohimbine (an ␣ 2 antagonist) microperfusion into the urinary space produced opposite effects to that of agmatine. Microperfusion of urinary space with BU-224 ( M), a synthetic imidazoline 2 (I 2 ) agonist, duplicated agmatine effects on SNGFR but not APR whereas an I 1 agonist had no effect. Agmatine effects on SNGFR and APR are not only dissociable but appear to be mediated by different mechanisms. The production and degradation of this biologically active substance derived from arginine constitutes a novel endogenous regulatory system in the kidney. ( J. Clin. Invest. 1996. 97:413-420.)

In patients with chronic kidney disease, metabolic acidosis can occur as a result of insufficient ammoniagenesis within the damaged kidney. This, in turn, can bring about a variety of sequella that have their basis in hormonal and cellular abnormalities that effect stunted growth, loss of muscle and bone mass, and negative nitrogen balance. Cellular mechanisms accounting for these findings are reviewed. In bone, metabolic acidosis causes direct dissolution of bone; ostoeclastic activity is increased while osteoblastic activity is suppressed. In muscle, branched-chain amino acid oxidation is increased and the ubiquitin-proteasome pathway is activated: muscle wasting results. Even a modest degree of metabolic acidosis can be harmful and can initiate a series of maladaptive responses that are not easily reversed, although there is evidence that alkali therapy can be beneficial in reversing these responses.

Choline acetyltransferase (ChAT), the enzyme that synthesizes the neurotransmitter acetylcholine (ACh), is thought to be present in kinetic excess in cholinergic neurons. The rate-limiting factor in ACh production is the provision of choline to ChAT. Cholinergic neurons are relatively unique in their expression of the choline transporter 1 (CHT1), which exhibits high-affinity for choline and catalyzes its uptake from the extracellular space to the neuron. Multiple lines of evidence indicate that the activity of CHT1 is a key determinant of choline supply for ACh synthesis. We examined the interaction of ChAT and ChT activity using mice heterozygous for a null mutation in the Chat gene (Chatϩ/Ϫ). In these mice, brain ChAT activity was reduced by 40 -50% relative to the wild type, but brain ACh levels as well as ACh content and depolarization-evoked ACh release in hippocampal slices were normal. However, the amount of choline taken up by CHT1 and ACh synthesized de novo from choline transported by CHT1 in hippocampal slices, as well as levels of CHT1 mRNA in the septum and CHT1 protein in several regions of the CNS, were 50 -100% higher in Chatϩ/Ϫ than in Chatϩ/ϩ mice. Thus, haploinsufficiency of ChAT leads to an increased expression of CHT1. Increased ChT activity may compensate for the reduced ChAT activity in Chatϩ/Ϫ mice, contributing to the maintenance of apparently normal cholinergic function as reflected by normal performance of these mice in several behavioral assays.

Pizzo DP, Ientile R, Colombatto S, Toninello A, Satriano J. The arginine metabolite agmatine protects mitochondrial function and confers resistance to cellular apoptosis.