Chronic insulin treatment suppresses PTP1B function, induces increased PDGF signaling, and amplifies neointima formation in the balloon-injured rat artery

The Role of Cdc42 in the Insulin and Leptin Pathways Contributing to the Development of Age-Related Obesity

Age-related obesity significantly increases the risk of chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension, and certain cancers. The insulin-leptin axis is crucial in understanding metabolic disturbances associated with age-related obesity. Rho GTPase Cdc42 is a member of the Rho family of GTPases that participates in many cellular processes including, but not limited to, regulation of actin cytoskeleton, vesicle trafficking, cell polarity, morphology, proliferation, motility, and migration. Cdc42 functions as an integral part of regulating insulin secretion and aging. Some novel roles for Cdc42 have also been recently identified in maintaining glucose metabolism, where Cdc42 is involved in controlling blood glucose levels in metabolically active tissues, including skeletal muscle, adipose tissue, pancreas, etc., which puts this protein in line with other critical regulators of glucose metabolism. Importantly, Cdc42 plays a vital role in cellular processes associated with the insulin and leptin signaling pathways, which are integral elements involved in obesity development if misregulated. Additionally, a change in Cdc42 activity may affect senescence, thus contributing to disorders associated with aging. This review explores the complex relationships among age-associated obesity, the insulin-leptin axis, and the Cdc42 signaling pathway. This article sheds light on the vast molecular web that supports metabolic dysregulation in aging people. In addition, it also discusses the potential therapeutic implications of the Cdc42 pathway to mitigate obesity since some new data suggest that inhibition of Cdc42 using antidiabetic drugs or antioxidants may promote weight loss in overweight or obese patients.

PTPN1 Deficiency Modulates BMPR2 Signaling and Induces Endothelial Dysfunction in Pulmonary Arterial Hypertension

Bone morphogenic protein receptor 2 (BMPR2) expression and signaling are impaired in pulmonary arterial hypertension (PAH). How BMPR2 signaling is decreased in PAH is poorly understood. Protein tyrosine phosphatases (PTPs) play important roles in vascular remodeling in PAH. To identify whether PTPs modify BMPR2 signaling, we used a siRNA-mediated high-throughput screening of 22,124 murine genes in mouse myoblastoma reporter cells using ID1 expression as readout for BMPR2 signaling. We further experimentally validated the top hit, PTPN1 (PTP1B), in healthy human pulmonary arterial endothelial cells (PAECs) either silenced by siRNA or exposed to hypoxia and confirmed its relevance to PAH by measuring PTPN1 levels in blood and PAECs collected from PAH patients. We identified PTPN1 as a novel regulator of BMPR2 signaling in PAECs, which is downregulated in the blood of PAH patients, and documented that downregulation of PTPN1 is linked to endothelial dysfunction in PAECs. These findings point to a potential involvement for PTPN1 in PAH and will aid in our understanding of the molecular mechanisms involved in the disease.

The role of protein tyrosine phosphatase 1B (PTP1B) in the pathogenesis of type 2 diabetes mellitus and its complications

Insulin resistance, the most important characteristic of the type 2 diabetes mellitus (T2DM), is mostly caused by impairment in the insulin receptor (IR) signal transduction pathway. Protein tyrosine phosphatase 1B (PTP1B), one of the main negative regulators of the IR signaling pathway, is broadly expressed in various cells and tissues. PTP1B decreases the phosphorylation of the IR resulting in insulin resistance in various tissues. The evidence for the physiological role of PTP1B in regulation of metabolic pathways came from whole-body PTP1B-knockout mice. Whole-body and tissue-specific PTP1B-knockout mice showed improvement in adiposity, insulin resistance, and glucose tolerance. In addition, the key role of PTP1B in the pathogenesis of T2DM and its complications was further investigated in mice models of PTP1B deficient/overexpression. In recent years, targeting PTP1B using PTP1B inhibitors is being considered an attractive target to treat T2DM. PTP1B inhibitors improve the sensitivity of the insulin receptor and have the ability to cure insulin resistance-related diseases. We herein summarized the biological functions of PTP1B in different tissues in vivo and in vitro. We also describe the effectiveness of potent PTP1B inhibitors as pharmaceutical agents to treat T2DM.

Local insulin application on the carotid artery inhibits neointima formation

Anti-mitogenic agents currently used to prevent restenosis in drug-eluting stents delay re-endothelialization. Delayed re-endothelialization is now considered as the main cause of late stent thrombosis with drug-eluting stents, which emphasizes the need for new treatments. We have shown that systemic insulin treatment decreases neointimal growth and accelerates re-endothelialization after arterial injury in a rat model of restenosis. However, systemic insulin treatment cannot be given to non-diabetic individuals because of the risk of hypoglycemia. Thus, we investigated whether local insulin treatment is also effective in reducing neointimal growth after arterial injury. Rats were given local vehicle or local insulin delivered via Pluronic gel applied around the carotid artery immediately following balloon injury. Plasma glucose and systemic insulin levels were not affected by local insulin treatment. Insulin decreased intimal area at 28 days (P < 0.05) and also inhibited vascular smooth muscle cell migration by 60% at 4 days (P < 0.05). NPH (a longer-lasting insulin) also decreased neointimal area. These results indicate that local insulin treatment can lead to decreased restenosis, suggesting a protective vascular effect of insulin in vivo and that local insulin treatment, possibly via insulin-eluting stents, may be clinically relevant.

Adipose weight gain during chronic insulin treatment of mice results from changes in lipid storage without affecting de novo synthesis of palmitate

Insulin treatment is associated with increased adipose mass in both humans and mice. However, the underlying dynamic basis of insulin induced lipid accumulation in adipose tissue remains elusive. To assess this, young female C57BL6/J mice were fed a low fat diet for 3 weeks, treated subsequently with 7 days of constant subcutaneous insulin infusion by osmotic minipumps and compared to mice with only buffer infused. To track changes in lipid deposition during insulin treatment, metabolic labeling was conducted with heavy water for the final 4 days. Blood glucose was significantly lowered within one hour after implantation of insulin loaded mini pumps and remained lower throughout the study. Insulin treated animals gained significantly more weight during treatment and the mean weight of the subcutaneous adipose depots was significantly higher with the highest dose of insulin. Surprisingly, de novo palmitate synthesis within the subcutaneous and the gonadal depots was not affected significantly by insulin treatment. In contrast insulin treatment caused accumulation of triglycerides in both depots due to either deposition of newly synthesised triglycerides (subcutaneous depot) or inhibition of lipolysis (gonadal depot).

Emerging drugs for the treatment of diabetic ulcers

INTRODUCTION

Diabetic ulcers are chronic nonhealing ulcerations that despite the available medical tools still result in high amputation rates. Growing evidence suggests that alteration of the biochemical milieu of the chronic wound plays a significant role in impaired diabetic wound healing.

AREAS COVERED

The basic pathophysiology and the conventional treatment strategy of diabetic foot ulcers have been reviewed in the first section. In the second part, the most up-to-date bench and translational research in the field are described. The third section focuses on the drugs currently under development and the ongoing clinical trials evaluating their safety and efficacy. Finally, the major drug development issues and the possible scientific approaches to overcome them are analyzed.

EXPERT OPINION

Significant strides in understanding the chronic wound development have led to the development of topical therapies to address aberrant expression of growth factors and overexpression of inflammatory cytokines. Current research in the laboratory suggests that while decreased growth factor expression occurs at the local wound level, increased systemic serum levels of growth factors suggest growth factor resistance.

PDGF-induced vascular smooth muscle cell proliferation is associated with dysregulation of insulin receptor substrates

In vascular smooth muscle cells (VSMCs), platelet-derived growth factor (PDGF) plays a major role in inducing phenotypic switching from contractile to proliferative state. Importantly, VSMC phenotypic switching is also determined by the phosphorylation state/expression levels of insulin receptor substrate (IRS), an intermediary signaling component that is shared by insulin and IGF-I. To date, the roles of PDGF-induced key proliferative signaling components including Akt, p70S6kinase, and ERK1/2 on the serine phosphorylation/expression of IRS-1 and IRS-2 isoforms remain unclear in VSMCs. We hypothesize that PDGF-induced VSMC proliferation is associated with dysregulation of insulin receptor substrates. Using human aortic VSMCs, we demonstrate that prolonged PDGF treatment led to sustained increases in the phosphorylation of protein kinases such as Akt, p70S6kinase, and ERK1/2, which mediate VSMC proliferation. In addition, PDGF enhanced IRS-1/IRS-2 serine phosphorylation and downregulated IRS-2 expression in a time- and concentration-dependent manner. Notably, phosphoinositide 3-kinase (PI 3-kinase) inhibitor (PI-103) and mammalian target of rapamycin inhibitor (rapamycin), which abolished PDGF-induced Akt and p70S6kinase phosphorylation, respectively, blocked PDGF-induced IRS-1 serine phosphorylation and IRS-2 downregulation. In contrast, MEK1/ERK inhibitor (U0126) failed to block PDGF-induced IRS-1 serine phosphorylation and IRS-2 downregulation. PDGF-induced IRS-2 downregulation was prevented by lactacystin, an inhibitor of proteasomal degradation. Functionally, PDGF-mediated IRS-1/IRS-2 dysregulation resulted in the attenuation of insulin-induced IRS-1/IRS-2-associated PI 3-kinase activity. Pharmacological inhibition of PDGF receptor tyrosine kinase with imatinib prevented IRS-1/IRS-2 dysregulation and restored insulin receptor signaling. In conclusion, strategies to inhibit PDGF receptors would not only inhibit neointimal growth but may provide new therapeutic options to prevent dysregulated insulin receptor signaling in VSMCs in nondiabetic and diabetic states.

Mechanisms related to NO-induced motility in differentiated rat aortic smooth muscle cells

Nitric oxide (NO) is thought to play an important role as an inhibitor of vascular cell proliferation, motility, and neointima formation. This effect is mediated, in part, via the upregulation of protein tyrosine phosphatase (PTP)1B. Conversely, studies have reported that in presumably hyperinsulinemic mice fed a high-fat diet, NO enhances vascular remodeling, whereas a deficit of NO attenuates vascular remodeling. We have reported that in differentiated cultured smooth muscle cells treated with insulin, NO induces a motogenic effect that is dependent on Src homology-2 domain PTP 2 (SHP2) upregulation. In the present study, we describe novel mechanisms relevant to the motogenic effect of NO. Treatment of cultured cells with the selective angiontensin type 1 receptor antagonist losartan, but not with the selective angiotensin type 2 receptor antagonist PD-123319, blocked the comotogenic capacity of NO and insulin. Insulin and NO increased the secretion of ANG II into the culture media by 2- and 2.5-fold (P < 0.05), respectively, whereas treatment of cells with ANG II uncovered the motogenic effect of NO (1.4-fold above control, P < 0.05) and decreased the levels of PTP1B to 45% of control (P < 0.05). Suppression of PTP1B function was sufficient to uncover the motogenic effect of NO. The capacity of insulin to suppress PTP1B activity was blocked by losartan, implicating ANG II function in mediating this effect. Both insulin and ANG II induced the upregulation of phosphatidyl inositol 3-kinase (PI3K)-δ by two- to threefold (P < 0.05), and this effect was both necessary and sufficient to uncover NO-induced motogenesis. Finally, suppression of PTP1B function potentiated, whereas overexpression of PTP1B inhibited, SHP2-induced motogenesis. These results support the hypothesis that the comotogenic effect of insulin and NO occurs via an ANG II-mediated effect involving the suppression of PTP1B and upregulation of PI3K-δ and SHP2.

Long-term high glucose concentration influences Akt, ERK1/2, and PTP1B protein expression in human aortic smooth muscle cells

Hyperglycemia stimulates a plethora of intracellular signaling pathways within the cells of the vascular wall resulting in dysfunction-associated pathologies. Most of the studies reported so far explored the effect of rather short-time exposure of smooth muscle cells to high glucose concentrations. To mimic situation in Type 2 diabetes in which vascular wall is constantly exposed to circulating hyperglycemia, we report here the long-term (7days) effect of high glucose concentration on human media artery smooth muscle cells. This consists in up-regulation of PTP1B protein expression, down-regulation of basal Akt phosphorylation, and elevation of basal ERK1/2 activation. Acute stimulation of cells in high glucose with insulin down-regulated PTP1B expression, slightly decreased ERK1/2 activity, and activated Akt, whereas oxidative stress up-regulated Akt and ERK1/2 phosphorylation. In conclusion, long-term high glucose and acute oxidative stress and insulin stimulation imbalance the expression of activated kinases Akt and ERK1/2 and of dephosphorylating PTP1B in the insulin signaling pathway.

Insulin increases reendothelialization and inhibits cell migration and neointimal growth after arterial injury

OBJECTIVE

Insulin has both growth-promoting and protective vascular effects in vitro, however the predominant effect in vivo is unclear. We investigated the effects of insulin in vivo on neointimal growth after arterial injury.

METHODS AND RESULTS

Rats were given subcutaneous control (C) or insulin implants (3U/d;I) 3 days before arterial (carotid or aortic) balloon catheter injury. Normoglycemia was maintained by oral glucose and, after surgery, by intraperitoneal glucose infusion (saline in C). Insulin decreased intimal area (P<0.01) but did not change intimal cell proliferation or apoptosis. However, insulin inhibited cell migration into the intima (P<0.01) and increased expression of smooth muscle cell (SMC) differentiation markers (P<0.05). Insulin also increased reendothelialization (P<0.01) and the number of circulating progenitor cells (P<0.05).

CONCLUSIONS

These results are the first demonstration that insulin has a protective effect on both SMC and endothelium in vivo, resulting in inhibition of neointimal growth after vessel injury.