Effects of taurine on signal transduction steps induced during hypertrophy of rat heart myocytes

The taurine transporter: mechanisms of regulation

Taurine transport undergoes an adaptive response to changes in taurine availability. Unlike most amino acids, taurine is not metabolized or incorporated into protein but remains free in the intracellular water. Most amino acids are reabsorbed at rates of 98-99%, but reabsorption of taurine may range from 40% to 99.5%. Factors that influence taurine accumulation include ionic environment, electrochemical charge, and post-translational and transcriptional factors. Among these are protein kinase C (PKC) activation and transactivation or repression by proto-oncogenes such as WT1, c-Jun, c-Myb and p53. Renal adaptive regulation of the taurine transporter (TauT) was studied in vivo and in vitro. Site-directed mutagenesis and the oocyte expression system were used to study post-translational regulation of the TauT by PKC. Reporter genes and Northern and Western blots were used to study transcriptional regulation of the taurine transporter gene (TauT). We demonstrated that (i) the body pool of taurine is controlled through renal adaptive regulation of TauT in response to taurine availability; (ii) ionic environment, electrochemical charge, pH, and developmental ontogeny influence renal taurine accumulation; (iii) the fourth segment of TauT is involved in the gating of taurine across the cell membrane, which is controlled by PKC phosphorylation of serine 322 at the post-translational level; (iv) expression of TauT is repressed by the p53 tumour suppressor gene and is transactivated by proto-oncogenes such as WT1, c-Jun, and c-Myb; and (v) over-expression of TauT protects renal cells from cisplatin-induced nephrotoxicity.

Adjuvant strategies for prevention of glomerulosclerosis

The glomerulosclerosis which frequently complicates diabetes and severe hypertension is mediated primarily by increased mesangial production and activation of transforming growth factor-beta (TGF-beta), which acts on mesangial cells to boost their production of matrix proteins while suppressing extracellular proteolytic activity. Hyperglycemia and glomerular hypertension work in various complementary ways to stimulate superoxide production via NADPH oxidase in mesangial cells; the resulting oxidant stress results in the induction and activation of TFG-beta. Nitric oxide, generated by glomerular capillaries and by mesangial cells themselves, functions physiologically to oppose mesangial TGF-beta overproduction; however, NO bioactivity is compromised by oxidant stress. In addition to low-protein diets and drugs that suppress angiotensin II activity, a variety of other agents and measures may have potential for impeding the process of glomerulosclerosis. These include vitamin E, which blunts the rise in mesangial diacylglycerol levels induced by hyperglycemia; statins and (possibly) policosanol, which down-regulate NADPH oxidase activity by diminishing isoprenylation of Rac1; lipoic acid, whose potent antioxidant activity antagonizes the impact of oxidant stress on TGF-beta expression; pyridoxamine, which inhibits production of advanced glycation endproducts; arginine, high-dose folate, vitamin C, and salt restriction, which may support glomerular production of nitric oxide; and estrogen and soy isoflavones, which may induce nitric oxide synthase in glomerular capillaries while also interfering with TGF-beta signaling. Further research along these lines may enable the development of complex nutraceuticals which have important clinical utility for controlling and preventing glomerulosclerosis and renal failure. Most of these measures may likewise reduce risk for left ventricular hypertrophy in hypertensives, inasmuch as the signaling mechanisms which mediate this disorder appear similar to those involved in glomerulosclerosis.

DNA damage and expression of checkpoint genes p21(WAF1/CIP1) and 14-3-3 sigma in taurine-deficient cardiomyocytes

OBJECTIVE

Taurine depletion is associated with development of cardiomyopathy. Further, oxidative stress is advanced as a critical factor mediating the effect of taurine deficiency on target organs. However, the molecular mechanism(s) linking taurine deficiency with the development of cardiomyopathy remains elusive. Since transition between apoptotic degeneration and cell proliferation in stress conditions is regulated at cell cycle checkpoints, we determined the expression of two such genes, namely p21(WAF1/CIP1) and 14-3-3 sigma as well as p53 that are responsible for oxidative stress and DNA damage. We also carried out quantitative determination of DNA damage.

METHODS

Cardiomyocytes from beta-alanine-induced taurine-depleted (TD) rats were used for this investigation. Single- and double-stranded DNA damage was quantified using comet assay analysis. Western blot and two-dimensional polyacrylamide gel electrophoresis with immunoblotting analysis were applied for protein analysis.

RESULTS

Comet assay analysis indicated that the extent of double-stranded DNA damage was greater in TD than in control cardiomyocytes. Whereas only traces of both p53 and p21(WAF1/CIP1) and no detectable expression of 14-3-3 sigma were found in cardiomyocytes of control animals, the TD cardiomyocytes expressed all three genes.

CONCLUSIONS

DNA damage and the consequent up-regulation of checkpoint proteins observed in TD cardiomyocytes indicate the involvement of cell cycle control mechanisms in the effect of taurine deficiency on cardiomyocytes. Single- and double-stranded DNA damage and the consequent arrest of cell proliferation in both G(1) and G(2) phases of the cell cycle induced by checkpoint proteins may trigger the cardiomyopathy that is associated with taurine deficiency.

Alcohol-induced reductions in cardiac protein synthesis in vivo are not ameliorated by treatment with the dihydropyridine calcium channel blocker amlodipine

BACKGROUND

Various studies have indicated that acute ethanol dosage perturbs cardiac function and/or structure with concomitant reductions in protein synthesis. Cellular calcium homeostasis is also perturbed, which may contribute to altered protein synthesis. This is supported by the observation that calcium channel blockers can prevent numerous features of alcohol-induced pathology. However, many of these studies have been carried out in vitro, employing supraphysiological levels of alcohol, or have failed to address whether their results obtained in isolated systems have direct relevance in vivo. The aim of the present investigation was to examine the response of cardiac protein synthesis in vivo due to a physiologically relevant dose of ethanol and determine whether a calcium channel antagonist could prevent these effects.

METHODS

Changes in cardiac protein synthesis rates in vivo were assessed by measuring the fractional rates of protein synthesis (i.e., ks) using a "flooding dose" of [3H]phenylalanine. Rats were treated either acutely (10 mg/kg body weight, 3 hr) or chronically (10 mg/kg body weight/day, 30 days) with amlodipine, a dihydropyridine-type calcium channel blocker, before dosing with ethanol (75 mmol/kg body weight, 2.5 hr).

RESULTS

Ethanol (75 mmol/kg body weight) inhibited cardiac protein synthesis after 1 hr. Similar responses were recorded at 2.5 and 6 hr after ethanol dosage. At 24 hr, ethanol decreased food intakes. However, a direct comparison between pair-fed controls and alcohol-dosed rats also showed a decrease in cardiac protein synthesis after 24 hr. Acute alcohol dosage reduced cardiac protein synthesis in mixed, myofibrillary, and sarcoplasmic protein fractions. Similar results were obtained when data were expressed relative to ribonucleic acid (i.e., kRNA). Neither acute nor chronic treatments with the calcium antagonist amlodipine ameliorated the deleterious actions of ethanol on protein synthesis.

CONCLUSIONS

Ethanol may affect cardiac protein synthesis independently of altered calcium entry.

Ser-322 is a critical site for PKC regulation of the MDCK cell taurine transporter (pNCT)

Previous studies have shown that the Madin-Darby canine kidney cell taurine transporter (pNCT) is downregulated by protein kinase C (PKC) activation. In this study, it is hypothesized that the highly conserved serine-322 (Ser-322) located in the fourth intracellular segment (S4) may play an important role in the function of taurine transporter, which is modulated by PKC phosphorylation. It is demonstrated that Ser-322 is the critical site of PKC phosphorylation, as determined by site-directed mutagenesis. When Ser-322 of pNCT was changed to alanine (S322A) and this mutant was evaluated in an oocyte expression system, taurine transport activity increased threefold compared with control (wild-type pNCT). Activation of PKC by the active phorbol ester 12-myristate 13-acetate did not influence taurine transport by mutant S322A. Kinetic analysis showed that the mutation of Ser-322 essentially changed the Vmax, rather than the Km, of the transporter. Mutation of all other PKC consensus sites did not affect transporter activity when expressed in the oocyte system. Western blot analysis showed that expression of taurine transporter protein was similar in oocytes injected with either wild-type or mutant pNCT cRNA, indicating that the enhanced taurine transport activity by mutant S322A was not caused by a greater amount of transporter expressed in the oocyte. Furthermore, this study demonstrated that the taurine transporter was phosphorylated after PKC activation, and this effect was not observed in mutant S322A. In conclusion, Ser-322 is critical in PKC regulation of taurine transporter activity. The steady-state taurine transporter activity is tightly controlled by endogenous PKC phosphorylation of Ser-322, which is located in the fourth intracellular segment of the taurine transporter.