Interleukin-1 inhibits angiotensin II-stimulated protein kinase B pathway in renal mesangial cells via the inducible nitric oxide synthase

Insulin signaling in retinal neurons is regulated within cholesterol-enriched membrane microdomains

Neuronal cell death is an early pathological feature of diabetic retinopathy. We showed previously that insulin receptor signaling is diminished in retinas of animal models of diabetes and that downstream Akt signaling is involved in insulin-mediated retinal neuronal survival. Therefore, further understanding of the mechanisms by which retinal insulin receptor signaling is regulated could have therapeutic implications for neuronal cell death in diabetes. Here, we investigate the role of cholesterol-enriched membrane microdomains to regulate PKC-mediated inhibition of Akt-dependent insulin signaling in R28 retinal neurons. We demonstrate that PKC activation with either a phorbol ester or exogenous application of diacylglycerides impairs insulin-induced Akt activation, whereas PKC inhibition augments insulin-induced Akt activation. To investigate the mechanism by which PKC impairs insulin-stimulated Akt activity, we assessed various upstream mediators of Akt signaling. PKC activation did not alter the tyrosine phosphorylation of the insulin receptor or IRS-2. Additionally, PKC activation did not impair phosphatidylinositol 3-kinase activity, phosphoinositide-dependent kinase phosphorylation, lipid phosphatase (PTEN), or protein phosphatase 2A activities. Thus, we next investigated a biophysical mechanism by which insulin signaling could be disrupted and found that disruption of lipid microdomains via cholesterol depletion blocks insulin-induced Akt activation and reduces insulin receptor tyrosine phosphorylation. We also demonstrated that insulin localizes phosphorylated Akt to lipid microdomains and that PMA reduces phosphorylated Akt. In addition, PMA localizes and recruits PKC isotypes to these cholesterol-enriched microdomains. Taken together, these results demonstrate that both insulin-stimulated Akt signaling and PKC-induced inhibition of Akt signaling depend on cholesterol-enriched membrane microdomains, thus suggesting a putative biophysical mechanism underlying insulin resistance in diabetic retinopathy.

Activation of elastin transcription by transforming growth factor-beta in human lung fibroblasts

Elastin synthesis is essential for lung development and postnatal maturation as well as for repair following injury. Using human embryonic lung fibroblasts that express undetectable levels of elastin as assessed by Northern analyses, we found that treatment with exogenous transforming growth factor-beta (TGF-beta) induced rapid and transient increases in levels of elastin heterogeneous nuclear RNA (hnRNA) followed by increases of elastin mRNA and protein expression. In fibroblasts derived from transgenic mice, TGF-beta induced increases in the expression of a human elastin gene promoter fragment driving a chloramphenicol acetyl transferase reporter gene. The induction of elastin hnRNA and mRNA expression by TGF-beta was abolished by pretreatments with TGF-beta receptor I inhibitor, global transcription inhibitor actinomycin D, and partially blocked by addition of protein synthesis inhibitor cycloheximide, but was not affected by the p44/42 MAPK inhibitor U0126. Pretreatment with the p38 MAPK inhibitor SB-203580 also partially attenuated the levels of TGF-beta-induced elastin mRNA but not its hnRNA. Western analysis indicated that TGF-beta stimulated Akt phosphorylation. Inhibition of phosphatidylinositol 3-kinase and Akt phosphorylation by LY-294002 abolished TGF-beta-induced increases in elastin hnRNA and mRNA expression. Treatment of lung fibroblasts with interleukin-1beta or the histone deacetylase inhibitor trichostatin A inhibited TGF-beta-induced elastin mRNA and hnRNA expression by a mechanism that involved inhibition of Akt phosphorylation. Downregulation of Akt2 but not Akt1 expression employing small interfering RNA duplexes blocked TGF-beta-induced increases of elastin hnRNA and mRNA levels. Together, our results demonstrated that TGF-beta activates elastin transcription that is dependent on phosphatidylinositol 3-kinase/Akt activity.

Albumin endocytosis in proximal tubule cells is modulated by angiotensin II through an AT2 receptor-mediated protein kinase B activation

Albumin endocytosis in renal proximal tubule cells is a clathrin- and receptor-mediated mechanism that, in several pathophysiological conditions, is involved in initiating or promoting tubule-interstitial disease. Although much work has been done on this pathway, the regulation of albumin endocytosis in proximal tubule cells is not well understood. Here, we study the modulation by angiotensin II (Ang II) of albumin endocytosis in LLC-PK1, a model of proximal tubule cells. We observed that Ang II increases albumin endocytosis by approximately 100% at 10(-9) M. This effect is completely reversed by 10(-9) M PD123319, a specific AT(2) receptor antagonist, but not by losartan, a specific AT(1) receptor antagonist, at concentrations up to 10(-7) M. The Ang II effect on albumin endocytosis is also reversed by: phosphoinositide 3-kinase inhibitors LY294002 (2.5 x 10(-6) M) or wortmannin (10(-7) M), the protein kinase B inhibitor (2 x 10(-5) M), and staurosporine (2 x 10(-6) M), an inhibitor of 3'-phosphoinositide-dependent kinase 1. Ang II induced the selective phosphorylation of protein kinase B (PKB) at the Thr-308 residue without a change in Ser-473 phosphorylation, a combination that leads to an increase in PKB activity. These effects were completely abolished by 3 x 10(-6) M staurosporine or 10(-8) M PD123319. Our experiments also showed that PKB is present in the membrane fraction in overnight-starved LLC-PK1 cells. Taken together, these data show that Ang II increases albumin endocytosis through an AT(2) receptor mediated by activation of PKB in the plasma membrane, which depends on the basal activity of the phosphatidyl-inositol 3-kinase.

Hypotensive peptides from milk proteins

Hypertension is the major controllable risk factor associated with cardiovascular disease (CVD) events such as myocardial infarction, stroke, heart failure, and end-stage diabetes. A 5 mm Hg decrease in blood pressure has been equated with approximately 16% decrease in CVD. In the U.S. alone current annual antihypertensive drug costs are approximately dollars 15 billion. The renin-angiotensin-aldosterone system is a target for blood pressure control. Cleavage of angiotensinogen by renin produces angiotensin I which is subsequently hydrolyzed by angiotensin-I-converting enzyme (ACE) to angiotensin II (a potent vasoconstrictor). Various side effects are associated with the use of ACE inhibitory drugs in the control of blood pressure including hypotension, increased potassium levels, reduced renal function, cough, angioedema, skin rashes, and fetal abnormalities. Milk proteins, both caseins and whey proteins, are a rich source of ACE inhibitory peptides. Several studies in spontaneously hypertensive rats show that these casokinins and lactokinins can significantly reduce blood pressure. Furthermore, a limited number of human studies have associated milk protein-derived peptides with statistically significant hypotensive effects (i.e., lower systolic and diastolic pressures). The advent of effective milk protein based functional food ingredients/nutraceuticals for the prevention/control of blood pressure therefore has the potential to significantly reduce global healthcare cost.

Akt is a major downstream target of PI3-kinase involved in angiotensin II-induced proliferation

Different signal transduction cascades have been implicated in angiotensin II (Ang II)-mediated cell growth, such as the extracellular signal-regulated kinase 1/2 (ERK1/2) and the phosphatidylinositol 3-kinase (PI3K) pathways. To identify the downstream targets of PI3K involved in Ang II-induced proliferation, we used both rat aortic smooth muscle (RASM) cells and a CHO cell line stably expressing the rat AT1A receptor. The ERK1/2 and PI3K pathways are independently activated and implicated in Ang II-mediated DNA synthesis and cell number increase in these 2 cell lines. In addition, a specific inhibitor of Akt inhibited Ang II-induced Akt phosphorylation, DNA synthesis and proliferation in CHO-AT1A or RASM cells. A dominant-negative mutant of Akt was also found to selectively block Ang II-induced proliferation of CHO-AT1A cells. To further elucidate the signaling events leading to Akt activation, we used an AT1 receptor mutant (AT1AD74E), deficient for Gq protein coupling, and the intracellular calcium chelator BAPTA-AM. Although altered Akt and ERK1/2 activation was observed in the CHO-AT1AD74E cell line, blockade of intracellular calcium elevation did not affect phosphorylation of these kinases. These results provide the first evidence of a specific and necessary role of Akt in Ang II-induced proliferation through a Gq protein-dependent calcium-independent pathway.

Nitric Oxide Signalling with a Special Focus on Lipid-Derived Mediators

The ways in which cells communicate among each other concerns all aspects of biology, from developmental processes to diseases. Nitric oxide (NO) is one of the most remarkable and unusual regulatory molecules. It is a labile free radical gas that is not stored but generated on demand, and has been implicated in an extraordinarily diverse range of physiological and pathophysiological functions. The modulation of cell signalling by free radicals is an emerging area of research that provides insight into the orchestration of cell adaptation to a changing microenvironment. In a multicellular organism this serves to coordinate complex physiological responses, such as inflammation. Cell signalling is also accompanied by rapid remodelling of membrane lipids by activated lipases. The discovery that NO, which does not reversibly interact with membrane receptors like conventional hormones and growth factors, targets enzymes such as phospholipase A2, sphingomyelinases or ceramidases, has stimulated growing interest in the crosstalk between redox and lipid signalling.