In both type 1 and type 2 diabetes, the late diabetic complications in nerve, vascular endothelium, and kidney arise from chronic elevations of glucose and possibly other metabolites including free fatty acids (FFA). Recent evidence suggests that common stress-activated signaling pathways such as nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases underlie the development of these late diabetic complications. In addition, in type 2 diabetes, there is evidence that the activation of these same stress pathways by glucose and possibly FFA leads to both insulin resistance and impaired insulin secretion. Thus, we propose a unifying hypothesis whereby hyperglycemia and FFA-induced activation of the nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases stress pathways, along with the activation of the advanced glycosylation end-products/receptor for advanced glycosylation end-products, protein kinase C, and sorbitol stress pathways, plays a key role in causing late complications in type 1 and type 2 diabetes, along with insulin resistance and impaired insulin secretion in type 2 diabetes. Studies with antioxidants such as vitamin E, alpha-lipoic acid, and N-acetylcysteine suggest that new strategies may become available to treat these conditions.

/pdf/oxidative-stress-and-stress-activated-signaling-pathways-a-3134ynljka.pdf

Oxidative Stress and Stress-Activated Signaling Pathways: A Unifying Hypothesis of Type 2 Diabetes

The ability of naive T cells to clonally expand and acquire effector functions depends on the strength of signals received by the T-cell receptor (TCR) and by an array of co-stimulatory receptors — the most prominent of which is CD28. In this review, we discuss recent genetic, biochemical and biophysical data that indicate a modified view of the molecular mechanism by which ligation of CD28 amplifies TCR-mediated T-cell activation. These studies indicate that the commonly held notion of a qualitative signalling role of CD28 in T-cell activation should be revised.

CD28-mediated co-stimulation: a quantitative support for TCR signalling

Up-regulation of the IKCa1 Potassium Channel during T-cell Activation MOLECULAR MECHANISM AND FUNCTIONAL CONSEQUENCES

Insulin receptor substrate (IRS) has been suggested as a molecular target of free fatty acids (FFAs) for insulin resistance. However, the signaling pathways by which FFAs lead to the inhibition of IRS function remain to be established. In this study, we explored the FFA-signaling pathway that contributes to serine phosphorylation and degradation of IRS-1 in adipocytes and in dietary obese mice. Linoleic acid, an FFA used in this study, resulted in a reduction in insulin-induced glucose uptake in 3T3-L1 adipocytes. This mimics insulin resistance induced by high-fat diet in C57BL/6J mice. The reduction in glucose uptake is associated with a decrease in IRS-1, but not IRS-2 or GLUT4 protein abundance. Decrease in IRS-1 protein was proceeded by IRS-1 (serine 307) phosphorylation that was catalyzed by serine kinases inhibitor κB kinase (IKK) and c-JUN NH2-terminal kinase (JNK). IKK and JNK were activated by linoleic acid and inhibition of the two kinases led to prevention of IRS-1 reduction. We demonstrate tha...

https://pubag.nal.usda.gov/pubag/downloadPDF.xhtml?id=8925&content=PDF

Inhibition of Insulin Sensitivity by Free Fatty Acids Requires Activation of Multiple Serine Kinases in 3T3-L1 Adipocytes

A region of the interleukin-2 (IL-2) promoter known as the RE/AP element is activated in concert by signals that originate from the T cell antigen receptor and the CD28 coreceptor. We show here that the serine-threonine kinase Akt can provide a costimulatory signal for RE/AP activation that is indistinguishable from the signal provided by CD28. This includes the ability of Akt, like antibodies to CD28, to synergize with protein kinase C theta (PKC-theta) in the induction of RE/AP. Retrovirus-mediated expression of activated Akt in primary T cells from CD28-deficient mice is capable of selectively restoring production of IL-2 and interferon gamma, but not IL-4 or IL-5. Our results provide evidence that CD28 costimulation of different cytokines is mediated by discrete signaling pathways, one of which includes Akt.

Akt provides the CD28 costimulatory signal for up-regulation of IL-2 and IFN-gamma but not TH2 cytokines.

Protein kinase C-θ (PKCθ) is a Ca2+-independent member of the PKC family that is selectively expressed in skeletal muscle and T lymphocytes and plays an important role in T cell activation. However, the molecular basis for the important functions of PKCθ in T cells and the manner in which it becomes coupled to the T cell receptor-signaling machinery are unknown. We addressed the functional relationship between PKCθ and CD28 costimulation, which plays an essential role in T cell receptor-mediated IL-2 production. Here, we provide evidence that PKCθ is functionally coupled to CD28 costimulation by virtue of its selective ability to activate the CD28RE/activator protein-1 (AP-1) element in the IL-2 gene promoter. First, CD28 costimulation enhanced the membrane translocation and catalytic activation of PKCθ. Second, among several PKC isoforms, PKCθ was the only one capable of activating NF-κB or CD28RE/AP-1 reporters in T cells (but not in 293T cells). Third, wild-type PKCθ synergized with CD28/CD3 signals to activate CD28RE/AP-1. In addition, PKCθ selectively synergized with Tat to activate a CD28RE/AP-1 reporter. Fourth, CD3/CD28-induced CD28RE/AP-1 activation and NF-κB nuclear translocation were blocked by a selective PKCθ inhibitor. Last, PKCθ-mediated activation of the same reporter was inhibited by the proteasome inhibitor MG132 (which blocks IκB degradation) and was found to involve IκB-kinase β. These findings identify a unique PKCθ-mediated pathway for the costimulatory action of CD28, which involves activation of the IκB-kinase β/IκB/NF-κB-signaling cascade.

https://www.pnas.org/content/pnas/97/7/3394.full.pdf

Nathan Englund

Papers

NF-κB activation induced by T cell receptor/CD28 costimulation is mediated by protein kinase C-θ