Proteasomal degradation of IRS-2, but not IRS-1 by calcineurin inhibition: Attenuation of insulin-like growth factor-I-induced GSK-3β and ERK pathways in adrenal chromaffin cells

DYRK1A aggravates β cell dysfunction and apoptosis by promoting the phosphorylation and degradation of IRS2

In this study, we aimed to investigate the role of dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1A (DYRK1A), which is one of the most important regulators of Alzheimer's disease development, in islet β cell dysfunction and apoptosis. We found significantly increased expression of DYRK1A in both the hippocampus and pancreatic islets of APPswe/PS1ΔE9 transgenic mice than in wild-type littermates. Furthermore, we observed that the overexpression of DYRK1A greatly aggravated β cell apoptosis. Most importantly, we found that DYRK1A directly interacted with insulin receptor substrate-2 (IRS2) and promoted IRS2 phosphorylation, leading to the proteasomal degradation of IRS2 and promotion of β cell dysfunction and apoptosis. These findings suggested that DYRK1A is a potential drug target in diabetes mellitus.

Nutritional intervention restores muscle but not kidney phenotypes in adult calcineurin Aα null mice

Mice lacking the α isoform of the catalytic subunit of calcineurin (CnAα) were first reported in 1996 and have been an important model to understand the role of calcineurin in the brain, immune system, bones, muscle, and kidney. Research using the mice has been limited, however, by failure to thrive and early lethality of most null pups. Work in our laboratory led to the rescue of CnAα−/− mice by supplemental feeding to compensate for a defect in salivary enzyme secretion. The data revealed that, without intervention, knockout mice suffer from severe caloric restriction. Since nutritional deprivation is known to significantly alter development, it is imperative that previous conclusions based on CnAα−/− mice are revisited to determine which aspects of the phenotype were attributable to caloric restriction versus a direct role for CnAα. In this study, we find that defects in renal development and function persist in adult CnAα−/− mice including a significant decrease in glomerular filtration rate and an increase in blood urea nitrogen levels. These data indicate that impaired renal development we previously reported was not due to caloric restriction but rather a specific role for CnAα in renal development and function. In contrast, we find that rather than being hypoglycemic, rescued mice are mildly hyperglycemic and insulin resistant. Examination of muscle fiber types shows that previously reported reductions in type I muscle fibers are no longer evident in rescued null mice. Rather, loss of CnAα likely alters insulin response due to a reduction in insulin receptor substrate-2 (IRS2) expression and signaling in muscle. This study illustrates the importance of re-examining the phenotypes of CnAα−/− mice and the advances that are now possible with the use of adult, rescued knockout animals.

Association of growth factors, HIF-1 and NF-κB expression with proteasomes in endometrial cancer

Insulin-like growth factors (IGFs), vascular endothelial growth factor (VEGF), hypoxia-inducible factor-1 (HIF-1), and nuclear factor kappa-B (NF-κB) are known to play an important role in endometrial cancer pathogenesis. However, the proteolytic regulation of these factors is still poorly understood. We studied the correlation between chymotrypsin-like activity of proteasomes and IGF-I, IGF-II, VEGF, HIF-1, and NF-κB levels in endometrial cancer tissues. It was shown that the total activity of proteasomes and the activity of the 20S and 26S proteasomes in malignant tumors were significantly higher than those observed in the normal endometrium. Negative relationships between the proteasome activity and IGF-I, HIF-1, and NF-κB p50 expressions were found. High 20S proteasome activity was associated with increase of HIF-1 level. Positive relationships between IGF-I expression and two classic forms of NF-κB p50 and p65 in endometrial cancer were revealed. The data obtained indicate the possible proteasomal regulation of growth and transcription factors. The major pool of IGF-I is located in the extracellular space, and it is likely that extracellular proteasomes also take part in the regulation of the IGF-I content. The present data show the evidence of proteasome regulation of growth and nuclear factors that can play an important role in cancer pathogenesis.

The expression of nicotinamide N-methyltransferase increases ATP synthesis and protects SH-SY5Y neuroblastoma cells against the toxicity of Complex I inhibitors

NNMT (nicotinamide N-methyltransferase, E.C. 2.1.1.1) catalyses the N-methylation of nicotinamide to 1-methylnicotinamide. NNMT expression is significantly elevated in a number of cancers, and we have previously demonstrated that NNMT expression is significantly increased in the brains of patients who have died of Parkinson's disease. To investigate the cellular effects of NNMT overexpression, we overexpressed NNMT in the SH-SY5Y cell line, a tumour-derived human dopaminergic neuroblastoma cell line with no endogenous expression of NNMT. NNMT expression significantly decreased SH-SY5Y cell death, which correlated with increased intracellular ATP content, ATP/ADP ratio and Complex I activity, and a reduction in the degradation of the NDUFS3 [NADH dehydrogenase (ubiquinone) iron-sulfur protein 3] subunit of Complex I. These effects were replicated by incubation of SH-SY5Y cells with 1-methylnicotinamide, suggesting that 1-methylnicotinamide mediates the cellular effects of NNMT. Both NNMT expression and 1-methylnicotinamide protected SH-SY5Y cells from the toxicity of the Complex I inhibitors MPP+ (1-methyl-4-phenylpyridinium ion) and rotenone by reversing their effects upon ATP synthesis, the ATP/ADP ratio, Complex I activity and the NDUFS3 subunit. The results of the present study raise the possibility that the increase in NNMT expression that we observed in vivo may be a stress response of the cell to the underlying pathogenic process. Furthermore, the results of the present study also raise the possibility of using inhibitors of NNMT for the treatment of cancer.

Role of calcineurin signaling in membrane potential-regulated maturation of cerebellar granule cells

At the early postnatal period, cerebellar granule cells proliferate, differentiate, migrate, and finally form refined synaptic connections with mossy fibers. During this period, the resting membrane potential of immature granule cells is relatively depolarized, but it becomes hyperpolarized in mature cells. This investigation was conducted to examine the role of this alteration in membrane potential and its downstream signaling mechanism in development and maturation of granule cells. Experiments were designed to precisely characterize the ontogenic processes of developing granule cells by combining organotypic cerebellar cultures with the specific expression of EGFP (enhanced green fluorescent protein) in granule cells by use of DNA transfection. Multiple approaches using morphology, electrophysiology, and immunohistochemistry demonstrated that granule cells developed and matured at the physiological KCl concentration in organotypic cultures in a temporally regulated manner. We addressed how persistent membrane depolarization influences the developmental and maturation processes of granule cells by depolarizing organotypic cultures with high KCl. Depolarization preserved the developmental processes of granule cells up to the stage of formation of immature dendrites but prevented the maturation processes for synaptic formation by granule cells. Importantly, this blockade of the terminal maturation of granule cells was reversed by inactivation of calcineurin with its specific inhibitor. This investigation has demonstrated that alteration of the membrane potential and its downstream calcineurin signaling play a pivotal role in triggering the maturation program for the synaptic organization of postnatally developing granule cells.

Nav1.7 sodium channel-induced Ca2+ influx decreases tau phosphorylation via glycogen synthase kinase-3beta in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells expressing Na(v)1.7 sodium channel isoform, veratridine increased Ser(473)-phosphorylation of Akt and Ser(9)-phosphorylation of glycogen synthase kinase-3beta by approximately 217 and approximately 195%, while decreasing Ser(396)-phosphorylation of tau by approximately 36% in a concentration (EC(50)=2.1 microM)- and time (t(1/2)=2.7 min)-dependent manner. These effects of veratridine were abolished by tetrodotoxin or extracellular Ca(2+) removal. Veratridine (10 microM for 5 min) increased translocation of Ca(2+)-dependent conventional protein kinase C-alpha from cytoplasm to membranes by 47%; it was abolished by tetrodotoxin, extracellular Ca(2+) removal, or Gö6976 (an inhibitor of protein kinase C-alpha), and partially attenuated by LY294002 (an inhibitor of phosphatidylinositol 3-kinase). LY294002 (but not Gö6976) abrogated veratridine-induced Akt phosphorylation. In contrast, either LY294002 or Gö6976 alone attenuated veratridine-induced glycogen synthase kinase-3beta phosphorylation by 65 or 42%; however, LY294002 plus Gö6976 completely blocked it. Veratridine (10 microM for 5 min)-induced decrease of tau phosphorylation was partially attenuated by LY294002 or Gö6976, but completely blocked by LY294002 plus Gö6976; okadaic acid or cyclosporin A (inhibitors of protein phosphatases 1, 2A, and 2B) failed to alter tau phosphorylation. These results suggest that Na(+) influx via Na(v)1.7 sodium channel and the subsequent Ca(2+) influx via voltage-dependent calcium channel activated (1) Ca(2+)/protein kinase C-alpha pathway, as well as (2) Ca(2+)/phosphatidylinositol 3-kinase/Akt and (3) Ca(2+)/phosphatidylinositol 3-kinase/protein kinase C-alpha pathways; these parallel pathways converged on inhibitory phosphorylation of glycogen synthase kinase-3beta, decreasing tau phosphorylation.

Drug development targeting the glycogen synthase kinase-3beta (GSK-3beta)-mediated signal transduction pathway: the role of GSK-3beta in the maintenance of steady-state levels of insulin receptor signaling molecules and Na(v)1.7 sodium channel in adrenal chromaffin cells

Glycogen synthase kinase-3 (GSK-3) is constitutively active in nonstimulated cells, where the majority of its substrates undergo inactivation/proteolysis by phosphorylation. Extracellular stimuli (e.g., insulin) catalyze inhibitory Ser(9)-phosphorylation of GSK-3beta, turning on signaling and causing other biological consequences otherwise constitutively suppressed by GSK-3beta. Regulated and dysregulated activities of GSK-3beta are pivotal to health, disease, and therapeutics (e.g., insulin resistance, neurodegeneration, tumorigenesis, inflammation); however, the underlying mechanisms of multifunctional GSK-3beta remain elusive. In cultured bovine adrenal chromaffin cells, 1) constitutive and negatively-regulated activities of GSK-3beta up- and down-regulated insulin receptor, insulin receptor substrate-1 (IRS-1), IRS-2, and Akt levels via controlling proteasomal degradation and protein synthesis; 2) nicotinic receptor/protein kinase C-alpha (PKC-alpha)/extracellular signal-regulated kinase (ERK) pathway up-regulated IRS-1 and IRS-2 levels, enhancing insulin-induced the phosphoinositide 3-kinase (PI3K)/Akt/GSK-3beta pathway; 3) inhibition of calcineurin by cyclosporin A or FK506 down-regulated IRS-2 level, attenuating insulin-like growth factor-I (IGF-I)-induced ERK and GSK-3beta pathways; and 4) insulin, IGF-I or therapeutics (e.g., lithium) up-regulated the voltage-dependent Na(v)1.7 sodium channel.