Cyclic AMP regulates basement membrane heparan sulfate proteoglycan, perlecan, metabolism in rat glomerular epithelial cells

Alteration of endothelial proteoglycan and heparanase gene expression by high glucose, insulin and heparin

Heparan sulfate proteoglycans (HSPGs) contain a core protein with glycosaminoglycans attached. Reduced glycosaminoglycan, in endothelial HSPGs syndecan and perlecan, is associated with diabetic cardiovascular complications but changes in core protein remain controversial. Since heparanase degrades heparan sulfate, we wished to determine if changes in endothelial heparanase mRNA, by high glucose (HG), correlate with changes in syndecan and perlecan core proteins, and to observe effects of heparin or insulin. RNA was isolated from cultured human aortic endothelial cells treated with HG (30mM), insulin (0.01 units/mL), heparin (0.5μg/mL), HG plus heparin and/or insulin for 24h. Real time PCR revealed that HG alone significantly increased heparanase, decreased syndecan with no effect on perlecan mRNA. Heparin or insulin significantly prevented the increase in heparanase but decreased perlecan mRNA while heparin, but not insulin, prevented the decrease in syndecan mRNA in HG treated cells. HG plus heparin and insulin increased heparanase and syndecan mRNA compared to all other treatments and decreased perlecan mRNA compared to control and HG alone. Heparin may protect endothelium from HG injury by reducing heparanase and increasing syndecan while insulin inhibits heparanase expression. Effects with insulin plus heparin suggest interference in transcriptional regulation of heparanase and syndecan genes.

Prostaglandins in cancer cell adhesion, migration, and invasion

Prostaglandins exert a profound influence over the adhesive, migratory, and invasive behavior of cells during the development and progression of cancer. Cyclooxygenase-2 (COX-2) and microsomal prostaglandin E₂ synthase-1 (mPGES-1) are upregulated in inflammation and cancer. This results in the production of prostaglandin E₂ (PGE₂), which binds to and activates G-protein-coupled prostaglandin E_1–4 receptors (EP_1–4). Selectively targeting the COX-2/mPGES-1/PGE₂/EP_1–4 axis of the prostaglandin pathway can reduce the adhesion, migration, invasion, and angiogenesis. Once stimulated by prostaglandins, cadherin adhesive connections between epithelial or endothelial cells are lost. This enables cells to invade through the underlying basement membrane and extracellular matrix (ECM). Interactions with the ECM are mediated by cell surface integrins by “outside-in signaling” through Src and focal adhesion kinase (FAK) and/or “inside-out signaling” through talins and kindlins. Combining the use of COX-2/mPGES-1/PGE₂/EP_1–4 axis-targeted molecules with those targeting cell surface adhesion receptors or their downstream signaling molecules may enhance cancer therapy.

Inhibition of cyclooxygenase-2 disrupts tumor vascular mural cell recruitment and survival signaling

Much evidence supports an important role for the inducible enzyme cyclooxygenase-2 (COX-2) in tumor angiogenesis. Previous studies have focused on the role of COX-2 in stimulating endothelial proliferation, with blockade of this enzyme impairing endothelial homeostasis. However, recent data suggest that COX-2 also regulates molecules implicated in endothelial trafficking with pericytes/vascular mural cells (VMC), an interaction crucial to vessel stability. We investigated the role of COX-2 in vascular assembly by testing the effect of the specific COX-2 inhibitor SC-236 in an orthotopic xenograft model of human Wilms' tumor. Tumor growth was significantly suppressed by SC-236 (78% at day 28, 55% at day 35). Perfusion studies and immunostaining showed a marked decrease in vasculature, particularly in small vessels. Specifically, SC-236 inhibited participation of VMC in xenograft vessels. SC-236-treated tumors developed segmentally dilated, architecturally erratic tumor vessels with decreased nascent pericytes and scant mature VMC. Although vascular endothelial growth factor expression was unchanged, expression of the chemokine receptor CXCR4 was decreased in tumor vessels, consistent with defective homing of vascular progenitor cells. Vascular expression of phosphorylated platelet-derived growth factor receptor-beta was also diminished, indicating impaired VMC-endothelial trafficking. Consistent with the key role of this interaction in vessel homeostasis, vascular cells in SC-236-treated tumors displayed markedly diminished phosphorylated Akt, indicating disrupted survival signaling. These results show that SC-236 causes defective vascular assembly by attenuating incorporation of VMC into tumor vessels, impairing endothelial survival, and raise the possibility that blockade of COX-2 may provide therapeutic synergies with antiangiogenic molecules that more selectively target endothelial cells.

ZO-1 expression and phosphorylation in diabetic nephropathy

Cellular mechanisms responsible for the loss of capillary wall permselectivity in diabetic nephropathy are not well characterized. ZO-1 is a junctional protein involved in the assembly and proper function of a number of tight junctions and is also expressed at the junction of podocytes with the slit diaphragm. We investigated the effect of diabetes and high glucose concentration on the expression of ZO-1 in animal models of both type 1 and 2 diabetes and in rat glomerular epithelial cells. In diabetic animals, immunohistochemistry and Western blotting showed decreased expression of ZO-1 in glomeruli. Immunogold electron microscopy revealed redistribution of ZO-1 from the podocyte membrane to the cytoplasm in the diabetic animals. Exposure of rat glomerular epithelial cells to high glucose resulted in a decrease in the intensity of ZO-1 staining and redistribution of ZO-1 from the membrane to the cytoplasm, changes that are attenuated by blockade of the angiotensin II type 1 receptor. ZO-1 protein expression and serine and tyrosine phosphorylation of ZO-1 were also decreased in cells exposed to high glucose. These findings suggest that alterations in the content and localization of ZO-1 may be relevant to the pathogenesis of proteinuria in diabetes.

Regulation of glomerular endothelial cell proteoglycans by glucose

The presence of heparan sulfate proteoglycan (HSPG) in anionic sites in the lamina rara interna of glomerular basement membrane suggests that the proteoglycan may be deposited by the glomerular endothelial cells (GEndo). We have previously demonstrated that bovine GEndo in vitro synthesize perlecan, a species of glomerular basement membrane HSPG. In this study we examined whether high glucose medium regulates the GEndo metabolism of glycopeptides including perlecan. Metabolic labeling of glycoconjugates with 35S-SO4, sequential ion exchange and Sepharose CL-4B chromatography of labeled glycoconjugates, and northern analysis were performed. Incubation of GEndo for 8 to 14 weeks (but not for 1-2 weeks) in medium containing 30 mM glucose resulted in nearly 50% reduction in the synthesis of cell layer and medium 35SO4-labeled low anionic glycoproteins and proteoglycans, including that of basement membrane HSPG (Kav 0.42) compared to GEndo grown in 5 mM glucose medium; no changes in anionic charge density or hydrodynamic size of proteoglycans were noted. Northern analysis demonstrated that the mRNA abundance of perlecan was reduced by 47% in cells incubated with 30 mM glucose. Our data suggest that high glucose medium reduces the GEndo synthesis of perlecan by regulating its gene expression. Reduced synthesis of perlecan by GEndo may contribute to proteinuria seen in diabetic nephropathy.

cAMP pathway in podocytes

Podocytes possess receptors for a variety of hormones. The following receptors whose stimulation results in increased cAMP levels have been detected in podocytes: adrenergic beta(2) receptor, dopamine D(1) receptor, prostaglandin IP and EP(4) receptors, and parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor. Besides activating protein kinase A, increased levels of cAMP depolarize podocytes via opening of chloride channels. Relatively little is known about the impact of the cAMP pathway on podocyte function. Results obtained in a limited number of studies indicate that cAMP in podocytes may regulate cell morphology, actin assembly, and matrix production. In addition, cAMP seems to attenuate the action of hormones, which activate the Ca(2+)/protein kinase C pathway. Effects of the cAMP pathway on further aspects of podocyte biology, such as contractility, phosphorylation state of slit membrane-associated proteins, glomerular permeability, cell cycle control, and synthesis of reactive oxygen species can be anticipated from studies on other cell types and from studies on isolated glomeruli. In summary, the data available indicate that the cAMP pathway affects several aspects of podocyte biology in an overall glomerulo-protective manner.

Heparan sulfate proteoglycans in experimental models of diabetes: a role for perlecan in diabetes complications

Proteoglycans are ubiquitous extracellular proteins that serve a variety of functions throughout the organism. Unlike other glycoproteins, proteoglycans are classified based on the structure of the glycosaminoglycan carbohydrate chains, not the core proteins. Perlecan, a member of the heparan sulfate proteoglycan (HSPG) family, has been implicated in many complications of diabetes. Decreased levels of perlecan have been observed in the kidney and in other organs, both in patients with diabetes and in animal models. Perlecan has an important role in the maintenance of the glomerular filtration barrier. Decreased perlecan in the glomerular basement membrane has a central role in the development of diabetic albuminuria. The involvement of this proteoglycan in diabetic complications and the possible mechanisms underlying such a role have been addressed using a variety of models. Due to the importance of nephropathy among diabetic patients most of the studies conducted so far relate to diabetes effects on perlecan in different types of kidney cells. The various diabetic models used have provided information on some of the mechanisms underlying perlecan's role in diabetes as well as on possible factors affecting its regulation. However, many other aspects of perlecan metabolism still await full elucidation. The present review provides a description of the models that have been used to study HSPG and in particular perlecan metabolism in diabetes and some of the factors that have been found to be important in the regulation of perlecan.

Promoter regulatory elements and DNase I-hypersensitive sites involved in serglycin proteoglycan gene expression in human erythroleukemia, CHRF 288-11, and HL-60 cells

We have compared regulation of the serglycin gene in human erythroleukemia (HEL) and CHRF 288-11 cells, which have megakaryocytic characteristics, with promyelocytic HL-60 cells. Deletion constructs were prepared from the region -1123/+42 to -20/+42, and putative regulatory sites were mutated. In all three cell lines, the two major regulatory elements for constitutive expression were the (-80)ets site and the cyclic AMP response element (CRE) half-site at -70. A protein from HEL and CHRF, but not HL60, nuclear extracts bound to the (-80)ets site. Another protein from all three cell lines bound to the (-70)CRE. Phorbol 12-myristate 13-acetate (PMA) and dibutyryl cyclic AMP (dbcAMP) increased expression of the reporter in HEL cells 2.5-3- and 4.5-fold, respectively, from all constructs except those with (-70)CRE mutations. PMA virtually eliminated expression of serglycin mRNA and promoter constructs, but dbcAMP increased expression in HL-60 cells. The effects of PMA and dbcAMP on promoter expression correlated with mRNA expression. The strengths of two DNase I-hypersensitive sites in the 5'-flanking region and the first intron in all three cells correlated with relative endogenous serglycin mRNA expression. An additional DNase I-hypersensitive site in HL60 DNA in the first intron may be related to the high serglycin expression in HL60 relative to HEL or CHRF cells.

Transcriptional and posttranscriptional regulation of proteoglycan gene expression

Proteoglycans are among the most complex and sophisticated molecules of mammalian systems in terms of their protein and carbohydrate moieties. These macromolecules are in a continuous interplay with each other and the cell surface signal-transducing pathways, some of which are beginning to be elucidated. Because of their domain structure, catalytic potential, and diversity, these molecules appear to be designed for integrating numerous signaling events. For example, some proteoglycans interact with hyaluronan and lectins, thereby linking cell surfaces and distant matrix molecules. Some interact with collagen during the complex process of fibrillogenesis and regulate this biological process fundamental to animal life. Others interact with growth factors and serve as depot available during growth or tissue remodeling. In this review, we center on the most recent developments of proteoglycan biology, focusing primarily on genomic organization and transcriptional and posttranscriptional control. We discuss only those proteoglycans whose gene and promoter elements have been characterized and proved to be functional. When possible, we correlate the effects of growth factors and cytokines on proteoglycan gene expression with the topology of cis-acting elements in their genomic control regions. The analysis leads to a comprehensive critical appraisal of the principles that underlie the regulation of proteoglycan gene expression and to the delineation of common regulatory mechanisms.

Matrix proteoglycans: from molecular design to cellular function

The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

Regulation of proteoglycan and hyaluronan synthesis by elevated level of intracellular cyclic adenosine monophosphate in peritubular cells from immature rat testis

The effects of an increase in intracellular cAMP concentration on proteoglycan (PG) synthesis by peritubular (PT) cells from immature rat testis were investigated. In the presence of dBcAMP for 72 h, the [3H]-hexosamine incorporation in secreted PG and in cell-associated PG was reduced, whereas [35S]-sulfate radioactivity was enhanced in secreted PG and not affected in cell-associated PG. Cholera toxin and IBMX, known to generate high intracellular cAMP levels, induced similar changes. Cyclic AMP did not alter PG protein moiety synthesis but enhanced PG turnover. Cholera toxin and dBcAMP profoundly modified PG characteristics: (1) Apparent molecular weight of PG was increased. (2) This was due to an increase in glycosaminoglycans (heparan sulfate (HS) and chondroitin sulfate (CS)) length. (3) The number of glycosaminoglycan chains was presumably reduced. (4) Heparan sulfate and chondroitin sulfate chains of medium and cell layer-associated PG appeared oversulfated. (5) The pattern of cell layer associated PG was modified with a decrease in HSPG and a correlative increase in CSPG. Cholera toxin and dBcAMP also dramatically stimulated hyaluronan synthesis by possible phosphorylation induced activation of hyaluronan synthase(s).