Sodium-dependent co-transported analogues of glucose stimulate ornithine decarboxylase mRNA expression in LLC-PK1 cells

Increased expression of ornithine decarboxylase in distal tubules of early diabetic rat kidneys: are polyamines paracrine hypertrophic factors?

Polyamines are small biogenic molecules that are essential for cell cycle entry and progression and proliferation. They can also contribute to hypertrophy. The activity of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, increases in the early diabetic kidney to enable renal hypertrophy. Inhibition of ODC in early diabetes attenuates diabetic renal hypertrophy and glomerular hyperfiltration. The current studies examine the temporal profile of renal ODC protein expression and localization, intrarenal polyamine levels, and sites of proliferation in kidneys of rats during the first 7 days of streptozotocin diabetes. ODC mRNA and protein content were increased in diabetic kidneys. High-performance liquid chromatography analysis showed increased intrarenal polyamine concentrations peaking after 24 h of diabetes. A subsequent increase in the number of proliferating proximal tubular cells was detected by in vivo 5-bromodeoxyuridine (BrdU) incorporation on day 3. Surprisingly, immunohistochemical studies revealed that increased ODC protein was apparent only in distal nephrons, whereas the main site of diabetic kidney hypertrophy is the proximal tubule. These findings raise the possibility that polyamines produced in the distal nephron may mediate the early diabetic kidney growth of the proximal tubules via a paracrine mechanism.

Antizyme-dependent and -independent mechanisms are responsible for increased spermidine transport in amino acid-restricted human cancer cells

Amino acid deprivation can inhibit tumour cell proliferation. Since polyamines are required for cell growth, we hypothesised that their regulatory pathways can respond to amino acid restriction. We report here that exposure of human colon adenocarcinoma Caco-2 cells to a medium restricted for a single amino acid, but not for D-glucose, activates spermidine transport. The increase was rapid and seemed transient with a maximum 4-6 hr after amino acid removal. Kinetics showed that the maximal velocity of transport was solely increased in L-methionine- or L-leucine-deprived cells, indicating increased number of transporters. The intracellular level of complex of ornithine decarboxylase (ODC) with antizyme, a negative regulator of polyamine transport, was decreased by 16-29% in amino acid-deprived cells. However, exposure to limited amounts of amino acid increased transport without altering the ODC-antizyme complex level. We propose that antizyme-independent mechanisms, sensitive to the amino acid concentration, also participate to the control of spermidine transport.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Transcription-dependent and -independent regulation of hepatic ornithine decarboxylase activity by CNS beta-endorphin in rat pups

We have previously shown that intracerebroventricular administration of relatively low doses of beta-endorphin suppresses basal levels of hepatic ODC activity as well as tissue ODC responsiveness to administered insulin in developing rats. Using Northern blotting analysis, the current studies examine whether these effects of CNS beta-endorphin may be mediated by changes in ODC gene expression. Subcutaneous administration of insulin (20 IU/kg body weight) rapidly and profoundly increased liver ODC activity. The time course of the response was characterized by proportionally increased levels of ODC mRNA, suggesting that insulin-induced stimulation of ODC activity is due to an increased transcription of ODC mRNA. Pretreatment with actinomycin D (2 mg/kg body weight, intraperitoneally) completely prevented the insulin-induced increase in ODC activity, confirming the requirement for the de novo synthesis of ODC mRNA for the effect. More importantly, intracerebroventricular but not subcutaneous injection of beta-endorphin (1 microgram) markedly diminished the stimulatory effect of insulin on hepatic ODC mRNA accumulation. The time course and magnitude of the inhibition of mRNA accumulation essentially mirrored that of the peptide on ODC activity. On the other hand, contrary to the inhibitory effect of beta-endorphin on basal ODC activity, the peptide did not lower basal ODC mRNA levels when given alone. Taken together, the results from these studies provide evidence for the existence of at least two separate mechanisms through which CNS beta-endorphin might downregulate ODC activity in peripheral organs of rat pups. The peptide can suppress insulin-induced ODC activity in the liver tissue by decreasing the rate of transcription of the ODC gene, whereas the inhibition of basal ODC activity appears to involve posttranscriptional mechanisms.