Differential p70S6k and 4E-BP1 regulation by insulin and amino acids in vascular endothelial and smooth muscle cells

Interactive contribution of hyperinsulinemia, hyperglycemia, and mammalian target of rapamycin signaling to valvular interstitial cell differentiation and matrix remodeling

Diabetes and its major key determinants insulin resistance and hyperglycemia are known risk factors for calcific aortic valve disease (CAVD). The processes leading to molecular and structural alterations of the aortic valve are yet not fully understood. In previous studies, we could show that valvular interstitial cells (VIC) display canonical elements of classical insulin signaling and develop insulin resistance upon hyperinsulinemia and hyperglycemia accompanied by impaired glucose metabolism. Analyses of cultured VIC and aortic valve tissue revealed extracellular matrix remodeling and degenerative processes. Since PI3K signaling through mammalian target of rapamycin (mTOR) is involved in fibrotic processes of the heart, we aim at further functional investigation of this particular Akt-downstream signaling pathway in the context of diabetes-induced CAVD. Primary cultures of VIC were treated with hyperinsulinemia and hyperglycemia. Phosphorylation of mTOR(Ser²⁴⁴⁸) was determined by Western blot analysis after acute insulin stimulus. Inhibition of mTOR phosphorylation was performed by rapamycin. Phosphorylation of mTOR complex 1 (MTORC1) downstream substrates 4E-BP1(Thr^37/46) and P70S6K(Thr³⁸⁹), and MTORC2 downstream substrate Akt(Ser⁴⁷³) as well as the PDK1-dependent phosphorylation of Akt(Thr³⁰⁸) was investigated. Markers for extracellular matrix remodeling, cell differentiation and degenerative changes were analyzed by Western blot analysis, semi-quantitative real-time PCR and colorimetric assays. Hyperinsulinemia and hyperglycemia lead to alterations of VIC activation, differentiation and matrix remodeling as well as to an abrogation of mTOR phosphorylation. Inhibition of mTOR signaling by rapamycin leads to a general downregulation of matrix molecules, but to an upregulation of α-smooth muscle actin expression and alkaline phosphatase activity. Comparison of expression patterns upon diabetic conditions and rapamycin treatment reveal a possible regulation of particular matrix components and key degeneration markers by MTORC1 downstream signaling. The present findings broaden the understanding of mitogenic signaling pathways in VIC triggered by hyperinsulinemia and hyperglycemia, supporting the quest for developing strategies of prevention and tailored treatment of CAVD in diabetic patients.

mTOR-FABP4 signal is activated in brain arteriovenous malformations in humans

Arteriovenous malformations (AVMs) are the most common types of cerebral vascular malformations, which are dynamic lesions with de novo growth potentials. The dysfunction of endothelial cells has been postulated to play a role in the pathogenesis of brain AVMs. mTOR-FABP4 signal enhances the angiogenic responses of endothelial cells and is not activated in the normal cerebral vasculature. Herein, we investigated the hypothesis that the mTOR-FABP4 signal may be activated in brain AVMs. The abundance of molecules in mTOR-FABP4 signal expression was detected by immunohistochemistry and Western blotting; special expressing cells were further characterized by double immunofluorescence using antibodies against various cell-specific markers. Next, several functional assays were performed to analyze the influence of the mTOR-FABP4 signal on proliferation, apoptosis, migration, and vascular tube formation of endothelial cells in human umbilical vein endothelial cells (HUVECs) using rapamycin and L-leucine. The expression of mTOR, p-mTOR, and FABP4 was increased in endothelial cells of human brain AVMs. Endothelial cell mTOR and p-mTOR expression were present in 70% and 55% of brain AVMs, respectively. Moreover, a population of FABP4-positive endothelial cells was detected in 80% of brain AVMs. The mTOR-FABP4 signal was activated and inhibited by L-leucine and rapamycin in HUVECs. The proliferation, apoptosis, migration, and vascular tube formation of endothelial cells could be inhibited by rapamycin. The mTOR-FABP4 signal was activated in human brain AVMs, and the mTOR-FABP4 signal was involved in proliferation, apoptosis, migration, and the vascular tube formation of endothelial cells. Taken together, whether rapamycin has therapeutic potential for treating human brain AVMs is worthy of further study. KEY MESSAGES : We confirmed that the mTOR- FABP4 pathway is activated in human brain arteriovenous malformations. We confirmed that mTOR signaling pathway affects endothelial cell function by regulating proliferation, migration, apoptosis, and tube formation of endothelial cell. Our study can provide theoretical support for mTOR pathway inhibitors in the treatment of human brain arteriovenous malformations.

Intrauterine growth restriction decreases pulmonary alveolar and vessel growth and causes pulmonary artery endothelial cell dysfunction in vitro in fetal sheep

Intrauterine growth restriction (IUGR) increases the risk for bronchopulmonary dysplasia (BPD). Abnormal lung structure has been noted in animal models of IUGR, but whether IUGR adversely impacts fetal pulmonary vascular development and pulmonary artery endothelial cell (PAEC) function is unknown. We hypothesized that IUGR would decrease fetal pulmonary alveolarization, vascular growth, and in vitro PAEC function. Studies were performed in an established model of severe placental insufficiency and IUGR induced by exposing pregnant sheep to elevated temperatures. Alveolarization, quantified by radial alveolar counts, was decreased 20% (P < 0.005) in IUGR fetuses. Pulmonary vessel density was decreased 44% (P < 0.01) in IUGR fetuses. In vitro, insulin increased control PAEC migration, tube formation, and nitric oxide (NO) production. This response was absent in IUGR PAECs. VEGFA stimulated tube formation, and NO production also was absent. In control PAECs, insulin increased cell growth by 68% (P < 0.0001). Cell growth was reduced in IUGR PAECs by 29% at baseline (P < 0.01), and the response to insulin was attenuated (P < 0.005). Despite increased basal and insulin-stimulated Akt phosphorylation in IUGR PAECs, endothelial NO synthase (eNOS) protein expression as well as basal and insulin-stimulated eNOS phosphorylation were decreased in IUGR PAECs. Both VEGFA and VEGFR2 also were decreased in IUGR PAECs. We conclude that fetuses with IUGR are characterized by decreased alveolar and vascular growth and PAEC dysfunction in vitro. This may contribute to the increased risk for adverse respiratory outcomes and BPD in infants with IUGR.

Genetic polymorphisms of the enzymes involved in DNA methylation and synthesis in elite athletes

Physical exercise induces adaptive changes leading to a muscle phenotype with enhanced performance. We first investigated whether genetic polymorphisms altering enzymes involved in DNA methylation, probably responsible of DNA methylation deficiency, are present in athletes' DNA. We determined the polymorphic variants C667T/A1298C of 5,10-methylenetetrahydrofolate reductase (MTHFR), A2756G of methionine synthase (MTR), A66G of methionine synthase reductase (MTRR), G742A of betaine:homocysteine methyltransferase (BHMT), and 68-bp ins of cystathionine β-synthase (CBS) genes in 77 athletes and 54 control subjects. The frequency of MTHFR (AC), MTR (AG), and MTRR (AG) heterozygous genotypes was found statistically different in the athletes compared with the control group (P=0.0001, P=0.018, and P=0.0001), suggesting a reduced DNA methylating capacity. We therefore assessed whether DNA hypomethylation might increase the expression of myogenic proteins expressed during early (Myf-5 and MyoD), intermediate (Myf-6), and late-phase (MHC) of myogenesis in a cellular model of hypomethylated or unhypomethylated C2C12 myoblasts. Myogenic proteins are largely induced in hypomethylated cells [fold change (FC)=Myf-5: 1.21, 1.35; MyoD: 0.9, 1.47; Myf-6: 1.39, 1.66; MHC: 1.35, 3.10 in GMA, DMA, respectively] compared with the control groups (FC=Myf-5: 1.0, 1.38; MyoD: 1.0, 1.14; Myf-6: 1.0, 1.44; MHC: 1.0, 2.20 in GM, DM, respectively). Diameters and length of hypomethylated myotubes were greater then their respective controls. Our findings suggest that DNA hypomethylation due to lesser efficiency of polymorphic MTHFR, MS, and MSR enzymes induces the activation of factors determining proliferation and differentiation of myoblasts promoting muscle growth and increase of muscle mass.

In Human Endothelial Cells Amino Acids Inhibit Insulin-induced Akt and ERK1/2 Phosphorylation by an mTOR-dependent Mechanism

In several cellular systems, amino acids synergize with insulin in promoting protein synthesis through the activation of the protein kinases p70/S6-K and PHAS-1. Such activations are mediated by the upstream kinase: mammalian target of rapamycin (mTor). In this work we have investigated the intracellular pathways involved in insulin-induced and amino acid-induced p70/S6-K activations in human endothelial cells. In human umbilical vein endothelial cells, insulin induces the phosphorylation of p70/S6-K at 5 minutes decreasing thereafter, whereas amino acids alone or associated with insulin phosphorylate p70/S6-K at all the time points analyzed (60 minutes). Insulin and amino acids phosphorylate p70/S6-K by mTor-dependent and phosphotidylinositol 3-kinase-dependent mechanisms, whereas the mitogen-activated protein kinase pathway is involved only when p70/S6-K is activated by insulin. Insulin induces the phosphorylation of Akt and extracellular signal-regulated protein kinase (ERK) 1/2, whereas amino acids did not. Moreover, amino acids suppress the phosphorylations induced by insulin. The inhibitory effects of amino acids are reverted by the mTor inhibitor rapamycin. Insulin-induced phosphorylation of Akt (at 15 and 30 minutes) is not accompanied by the phosphorylation of the downstream kinase p70/S6-K, indicating the existence of a negative feedback at this level. Our data demonstrate that at the level of human endothelial cells, amino acids synergize with insulin in the phosphorylation of the kinase that lies downstream mTor, as p70/S6-K, whereas they inhibit the upstream kinases Akt and extracellular signal-regulated protein kinase 1/2 when activated by insulin, by an mTor-dependent mechanism.