Effect of diabetes on protein synthesis rate and eukaryotic initiation factor activities in the liver of virgin and pregnant rats

The N-terminal domain of the human eIF2beta subunit and the CK2 phosphorylation sites are required for its function

CK2 (protein kinase CK2) is known to phosphorylate eIF2 (eukaryotic translation initiation factor 2) in vitro; however, its implication in this process in living cells has remained to be confirmed. The combined use of chemical inhibitors (emodin and apigenin) of CK2 together with transfection experiments with the wild-type of the K68A kinase-dead mutant form of CK2alpha evidenced the direct involvement of this protein kinase in eIF2beta phosphorylation in cultured HeLa cells. Transfection of HeLa cells with human wild-type eIF2beta or its phosphorylation site mutants showed Ser2 as the main site for constitutive eIF2beta phosphorylation, whereas phosphorylation at Ser67 seems more restricted. In vitro phosphorylation of eIF2beta also pointed to Ser2 as a preferred site for CK2 phosphorylation. Overexpression of the eIF2beta S2/67A mutant slowed down the rate of protein synthesis stimulated by serum, although less markedly than the overexpression of the Delta2-138 N-terminal-truncated form of eIF2beta (eIF2beta-CT). Mutation at Ser2 and Ser67 did not affect eIF2beta integrating into the eIF2 trimer or being able to complex with eIF5 and CK2alpha. The eIF2beta-CT form was also incorporated into the eIF2 trimer but did not bind to eIF5. Overexpression of eIF2beta slightly decreased HeLa cell viability, an effect that was more evident when overexpressing the eIF2beta S2/67A mutant. Cell death was particularly marked when overexpressing the eIF2beta-CT form, being detectable at doses where eIF2beta and eIF2beta S2/67A were ineffective. These results suggest that Ser2 and Ser67 contribute to the important role of the N-terminal region of eIF2beta for its function in mammals.

Regulation of elongation factor-1 expression by vitamin E in diabetic rat kidneys

Translation elongation factor-1 (EF-1) forms a primary site of regulation of protein synthesis and has been implicated amongst others in tumorigenesis, diabetes and cell death. To investigate whether diabetes-induced oxidative stress affects EF-1 gene expression, we used a free radical scavenger, vitamin E. The following groups of rats (5/group) were studied: control, vitamin E control, diabetic and diabetic treated with vitamin E. Markers of hyperglycemia, kidney function, oxidative stress, and kidney hypertrophy were elevated in diabetic rats. Increased urinary protein excretion indicated early signs of glomerular and tubular dysfunction. The mRNA and protein levels of the three EF-1 subunits (A, Balpha, and Bgamma) were determined in renal cortex extracts using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR), northern blot analysis and western blotting. EF-1A mRNA expression in renal cortex extracts was significantly increased by at least 2-fold (p < 0.002) in diabetic rats; however, there was no change in the mRNA levels of EF-1Balpha and EF-1Bgamma subunits. Similar results were observed at the protein level. Treatment of diabetic rats with vitamin E for 10 days suppressed both glycemic and oxidative stresses in renal cortex and kidney hypertrophy. EF-1A mRNA and protein levels were also reduced to control levels. In conclusion, EF-1A but not EF-1Balpha and EF-1Bgamma gene expression is significantly enhanced in the renal cortex of diabetic rats. Normalization of enhanced EF-1A expression by vitamin E treatment suggests a role for EF-1A during diabetes-induced oxidative stress.

Prevention by L-arginine and polyamines of delayed development and embryotoxicity caused by chemically-induced diabetes in rats

Diabetes mellitus induction with alloxan at a dose of 110 mg/kg i.p. in rats on Day 4 of pregnancy causes delayed development and resorptions as signs of embryotoxicity. In the present study, the administration of human NPH insulin at doses of 1 to 5 U/d to rats or 1.0 mL of 10 mM L-arginine for 8 d, starting the day following diabetes induction, prevented embryotoxicity and delayed development. Similar results were obtained when the polyamines putrescine, spermidine, or spermine were administered at doses of 1.0 mL of a 10 microM solution to each rat daily. However, even though L-arginine and polyamines prevented adverse effects of severe diabetes on the conceptus, and caused normalization of glucose, beta-hydroxybutyrate levels remained elevated. These results support the hypothesis that the mechanisms of normal and altered development could be mediated by the action of polyamines.