Platelet-derived growth factor-D expression in developing and mature human kidneys

PDGF-D Is Dispensable for the Development and Progression of Murine Alport Syndrome

Alport syndrome is an inherited kidney disease, which can lead to glomerulosclerosis and fibrosis, as well as end-stage kidney disease in children and adults. Platelet-derived growth factor-D (PDGF-D) mediates glomerulosclerosis and interstitial fibrosis in various models of kidney disease, prompting investigation of its role in a murine model of Alport syndrome. In vitro, PDGF-D induced proliferation and profibrotic activation of conditionally immortalized human parietal epithelial cells. In Col4a3-/- mice, a model of Alport syndrome, PDGF-D mRNA and protein were significantly up-regulated compared with non-diseased wild-type mice. To analyze the therapeutic potential of PDGF-D inhibition, Col4a3-/- mice were treated with a PDGF-D neutralizing antibody. Surprisingly, PDGF-D antibody treatment had no effect on renal function, glomerulosclerosis, fibrosis, or other indices of kidney injury compared with control treatment with unspecific IgG. To characterize the role of PDGF-D in disease development, Col4a3-/- mice with a constitutive genetic deletion of Pdgfd were generated and analyzed. No difference in pathologic features or kidney function was observed in Col4a3-/-Pdgfd-/- mice compared with Col4a3-/-Pdgfd+/+ littermates, confirming the antibody treatment data. Mechanistically, lack of proteolytic PDGF-D activation in Col4a3-/- mice might explain the lack of effects in vivo. In conclusion, despite its established role in kidney fibrosis, PDGF-D, without further activation, does not mediate the development and progression of Alport syndrome in mice.

Novel therapeutic perspectives for crescentic glomerulonephritis through targeting parietal epithelial cell activation and proliferation

INTRODUCTION

Kidney injury is clinically classified as crescentic glomerulonephritis (CrGN) when ≥50% of the glomeruli in a biopsy sample contain crescentic lesions. However, current strategies, such as systemic immunosuppressive therapy and plasmapheresis for CrGN, are partially effective, and these drugs have considerable systemic side effects. Hence, targeted therapy to prevent glomerular crescent formation and expansion remains an unmet clinical need.

AREAS COVERED

Hyperproliferative parietal epithelial cells (PECs) are the main constituent cells of the glomerular crescent with cell-tracing evidence. Crescents obstruct the flow of primary urine, pressure the capillaries, and degenerate the affected nephrons. We reviewed the markers of PEC activation and proliferation, potential therapeutic effects of thrombin and thrombin receptor inhibitors, and how podocytes cross-talk with PECs. These experiments may help identify potential early specific targets for the prevention and treatment of glomerular crescentic injury.

EXPERT OPINION

Inhibiting PEC activation and proliferation in CrGN can alleviate glomerular crescent progression, which has been supported by preclinical studies with evidence of genetic deletion. Clarifying the outcome of PEC transformation to the podocyte phenotype and suppressing thrombin, thrombin receptors, and PEC hyperproliferation in early therapeutic strategies will be the research goals in the next ten years.

Expression and function of PDGF-C in development and stem cells

Platelet-derived growth factor C (PDGF-C) is a relatively new member of the PDGF family, discovered nearly 20 years after the finding of platelet-derived growth factor A (PDGF-A) and platelet-derived growth factor B (PDGF-B). PDGF-C is generally expressed in most organs and cell types. Studies from the past 20 years have demonstrated critical roles of PDGF-C in numerous biological, physiological and pathological processes, such as development, angiogenesis, tumour growth, tissue remodelling, wound healing, atherosclerosis, fibrosis, stem/progenitor cell regulation and metabolism. Understanding PDGF-C expression and activities thus will be of great importance to various research disciplines. In this review, however, we mainly discuss the expression and functions of PDGF-C and its receptors in development and stem cells.

PDGF in organ fibrosis

Fibrosis is part of a tissue repair response to injury, defined as increased deposition of extracellular matrix. In some instances, fibrosis is beneficial; however, in the majority of diseases fibrosis is detrimental. Virtually all chronic progressive diseases are associated with fibrosis, representing a huge number of patients worldwide. Fibrosis occurs in all organs and tissues, becomes irreversible with time and further drives loss of tissue function. Various cells types initiate and perpetuate pathological fibrosis by paracrine activation of the principal cellular executors of fibrosis, i.e. stromal mesenchymal cells like fibroblasts, pericytes and myofibroblasts. Multiple pathways are involved in fibrosis, platelet-derived growth factor (PDGF)-signaling being one of the central mediators. Stromal mesenchymal cells express both PDGF receptors (PDGFR) α and β, activation of which drives proliferation, migration and production of extracellular matrix, i.e. the principal processes of fibrosis. Here, we review the role of PDGF signaling in organ fibrosis, with particular focus on the more recently described ligands PDGF-C and -D. We discuss the potential challenges, opportunities and open questions in using PDGF as a potential target for anti-fibrotic therapies.

New insights on glomerular hyperfiltration: a Japanese autopsy study

It has been suggested that low nephron number contributes to glomerular hypertension and hyperperfusion injury in progressive chronic kidney disease (CKD). The incidence of CKD in Japan is among the highest in the world, but the reasons remain unclear. We estimated total nephron (glomerular) number (NglomTOTAL) as well as numbers of nonsclerosed (NglomNSG) and globally sclerosed glomeruli (NglomGSG), and the mean volume of nonsclerosed glomeruli (VglomNSG) in Japanese normotensive, hypertensive, and CKD subjects and investigated associations between these parameters and estimated glomerular filtration rate (eGFR). Autopsy kidneys from age-matched Japanese men (9 normotensives, 9 hypertensives, 9 CKD) had nephron number and VglomNSG estimated using disector/fractionator stereology. Subject eGFR, single-nephron eGFR (SNeGFR), and the ratio SNeGFR/VglomNSG were calculated. NglomNSG in Japanese with hypertension (392,108 ± 87,605; P < 0.001) and CKD (268,043 ± 106,968; P < 0.001) was less than in normotensives (640,399 ± 160,016). eGFR was directly correlated with NglomNSG (r = 0.70, P < 0.001) and inversely correlated with VglomNSG (r = -0.53, P < 0.01). SNeGFR was higher in hypertensives than normotensives (P = 0.03), but was similar in normotensives and CKD, while the ratio SNeGFR/VglomNSG was similar in normotensives and hypertensives but markedly reduced in CKD. Nephron number in Japanese with hypertension or CKD was low. This results in a higher SNeGFR in hypertensives compared with normotensive and CKD subjects, but lowered SNeGFR/VglomNSG in CKD subjects, suggesting that changes in GFR are accommodated by glomerular hypertrophy rather than glomerular hypertension. These findings suggest glomerular hypertrophy is a dominant factor in maintenance of GFR under conditions of low nephron number.

The phenotypes of podocytes and parietal epithelial cells may overlap in diabetic nephropathy

Reversal of diabetic nephropathy (DN) has been achieved in humans and mice, but only rarely and under special circumstances. Since progression of DN is related to podocyte loss, reversal of DN requires restoration of podocytes. Here we identified and quantified potential glomerular progenitor cells that could be a source for restored podocytes. DN was identified in 31 human renal biopsy cases and separated into morphologically early or advanced lesions. Markers of podocytes (WT-1, p57), parietal epithelial cells (claudin-1) and cell proliferation (Ki-67) were identified by immunohistochemistry. Podocyte density was progressively reduced with DN. Cells marking as podocytes (p57) were present infrequently on Bowman's capsule in controls, but significantly increased in histologically early DN. Ki-67 expressing cells were identified on the glomerular tuft and Bowman's capsule in DN, but rarely in controls. Cells marking as PECs were present on the glomerular tuft, particularly in morphologically advanced DN. These findings show evidence of phenotypic plasticity in podocyte and PEC populations and are consistent with studies in the BTBR ob/ob murine model in which reversibility of DN occurs with podocytes potentially regenerating from PEC precursors. Thus, our findings support, but do not prove, that podocytes may regenerate from PEC progenitors in human DN. If so, progression of DN may represent a modifiable net balance between podocyte loss and regeneration.

Characterization of binding mode of action of a blocking anti-platelet-derived growth factor (PDGF)-B monoclonal antibody, MOR8457, reveals conformational flexibility and avidity needed for PDGF-BB to bind PDGF receptor-β

Platelet derived growth factor-BB (PDGF-BB) is an important mitogen and cell survival factor during development. PDGF-BB binds PDGF receptor-β (PDGFRβ) to trigger receptor dimerization and tyrosine kinase activation. We present the pharmacological and biophysical characterization of a blocking PDGF-BB monoclonal antibody, MOR8457, and contrast this to PDGFRβ. MOR8457 binds to PDGF-BB with high affinity and selectivity, and prevents PDGF-BB induced cell proliferation competitively and with high potency. The structural characterization of the MOR8457-PDGF-BB complex indicates that MOR8457 binds with a 2:1 stoichiometry, but that binding of a single MOR8457 moiety is sufficient to prevent binding to PDGFRβ. Comparison of the MOR8457-PDGF-BB structure with that of the PDGFRβ-PDGF-BB complex suggested the potential reason for this was a substantial bending and twisting of PDGF-BB in the MOR8457 structure, relative to the structures of PDGF-BB alone, bound to a PDGF-BB aptamer or PDGFRβ, which makes it nonpermissive for PDGFRβ binding. These biochemical and structural data offer insights into the permissive structure of PDGF-BB needed for agonism as well as strategies for developing specific PDGF ligand antagonists.

The matricellular protein Cyr61 is a key mediator of platelet-derived growth factor-induced cell migration

Platelet-derived growth factor (PDGF), a potent chemoattractant, induces cell migration via the MAPK and PI3K/Akt pathways. However, the downstream mediators are still elusive. In particular, the role of extracellular mediators is largely unknown. In this study, we identified the matricellular protein Cyr61, which is de novo synthesized in response to PDGF stimulation, as the key downstream mediator of the ERK and JNK pathways, independent of the p38 MAPK and AKT pathways, and, thereby, it mediates PDGF-induced smooth muscle cell migration but not proliferation. Our results revealed that, when Cyr61 was newly synthesized by PDGF, it was promptly translocated to the extracellular matrix and physically interacted with the plasma membrane integrins α6β1 and αvβ3. We further demonstrate that Cyr61 and integrins are integral components of the PDGF signaling pathway via an "outside-in" signaling route to activate intracellular focal adhesion kinase (FAK), leading to cell migration. Therefore, this study provides the first evidence that the PDGF-induced endogenous extracellular matrix component Cyr61 is a key mediator in modulating cell migration by connecting intracellular PDGF-ERK and JNK signals with integrin/FAK signaling. Therefore, extracellular Cyr61 convergence with growth factor signaling and integrin/FAK signaling is a new concept of growth factor-induced cell migration. The discovered signaling pathway may represent an important therapeutic target in growth factor-mediated cell migration/invasion-related vascular diseases and tumorigenesis.

Platelet-derived growth factors (PDGFs) in glomerular and tubulointerstitial fibrosis

Renal fibrosis is the hallmark of chronic kidney disease progression and is characterized by an exaggerated wound-healing process with the production of renal scar tissue. It comprises both the glomerular and the tubulointerstitial compartments. Among the factors that contribute to kidney fibrosis, the members of the platelet-derived growth factor (PDGF) family are among the best characterized ones. They appear to be the key factors in driving renal fibrosis, independent of the underlying kidney disease. The PDGF family consists of four isoforms (PDGF-A, -B, -C, and -D) and two receptor chains (PDGFR-α and -β), which are constitutively or inducibly expressed in most renal cells. These components have an irreplaceable role in kidney development by recruitment of mesenchymal cells to the glomerular and tubulointerstitial compartments. They further regulate multiple pathophysiologic processes including cell proliferation, cell migration, expression and accumulation of extracellular matrix, production and secretion of pro- and anti-inflammatory mediators, vascular permeability, and hemodynamics. This review provides a brief update on the role of different PDGF isoforms in the development of glomerulosclerosis and tubulointerstitial fibrosis, newly identified endogeneous PDGF antagonists, and resulting potential therapies.

Signal transduction in podocytes--spotlight on receptor tyrosine kinases

The mammalian kidney filtration barrier is a complex multicellular, multicomponent structure that maintains homeostasis by regulating electrolytes, acid-base balance, and blood pressure (via maintenance of salt and water balance). To perform these multiple functions, podocytes--an important component of the filtration apparatus--must process a series of intercellular signals. Integrating these signals with diverse cellular responses enables a coordinated response to various conditions. Although mature podocytes are terminally differentiated and cannot proliferate, they are able to respond to growth factors. It is possible that the initial response of podocytes to growth factors is beneficial and protective, and might include the induction of hypertrophic cell growth. However, extended and/or uncontrolled growth factor signalling might be maladaptive and could result in the induction of apoptosis and podocyte loss. Growth factors signal via the activation of receptor tyrosine kinases (RTKs) on their target cells and around a quarter of the 58 RTK family members that are encoded in the human genome have been identified in podocytes. Pharmacological inhibitors of many RTKs exist and are currently used in experimental and clinical cancer therapy. The identification of pathological RTK-mediated signal transduction pathways in podocytes could provide a starting point for the development of novel therapies for glomerular disorders.

Reversibility of structural and functional damage in a model of advanced diabetic nephropathy

The reversibility of diabetic nephropathy remains controversial. Here, we tested whether replacing leptin could reverse the advanced diabetic nephropathy modeled by the leptin-deficient BTBR ob/ob mouse. Leptin replacement, but not inhibition of the renin-angiotensin-aldosterone system (RAAS), resulted in near-complete reversal of both structural (mesangial matrix expansion, mesangiolysis, basement membrane thickening, podocyte loss) and functional (proteinuria, accumulation of reactive oxygen species) measures of advanced diabetic nephropathy. Immunohistochemical labeling with the podocyte markers Wilms tumor 1 and p57 identified parietal epithelial cells as a possible source of regenerating podocytes. Thus, the leptin-deficient BTBR ob/ob mouse provides a model of advanced but reversible diabetic nephropathy for further study. These results also suggest that restoration of lost podocytes is possible but is not induced by RAAS inhibition, possibly explaining the limited efficacy of RAAS inhibitors in promoting repair of diabetic nephropathy.

Parietal epithelial cells and podocytes in glomerular diseases

In recent years, it has become apparent that parietal epithelial cells (PECs) play an important role within the renal glomerulus, in particular in diseased conditions. In this review, we examine current knowledge about the role of PECs and their interactions with podocytes in development and under physiological conditions. A particular focus is on the crucial role of PECs and podocytes in two major glomerular disease entities. In rapidly progressive glomerulonephritis, PECs and podocytes proliferate and obstruct the tubular outlet, resulting in loss of the affected nephron. In focal and segmental glomerulosclerosis, PECs become activated and invade a segment of the glomerular tuft via an adhesion. From this entry site, activated PECs displace podocytes and deposit matrix. Thus, activated PECs are involved in inflammatory as well as degenerative glomerular diseases, which both can lead to irreversible loss of renal function.

Growth arrest-specific protein 1 is a novel endogenous inhibitor of glomerular cell activation and proliferation

Growth arrest-specific protein-1 (GAS1) is a GPI-anchored protein which is highly expressed in embryonic mouse fibroblasts and inhibits their proliferation. Glomerular mesangial cells release soluble GAS1 protein into the supernatant in vitro. Growth arrest led to GAS1 overexpression and increased release. Secretion involved disintegrin and metalloproteinase 10 and 17 as signified by inhibition experiments. Recombinant soluble GAS1 protein inhibited the proliferation of mesangial cells. Conversely, the induction of mesangial cell proliferation by PDGF-BB or -DD led to downregulation of GAS1 mRNA. Specific ligands of the PDGF α-receptor, PDGF-AA and -CC, had no effect. The GAS1 protein was localized in podocytes in kidneys from healthy rats. During the time course of mesangioproliferative glomerulonephritis in anti-Thy1.1-treated rats, glomerular GAS1 expression decreased prior to the onset of mesangial cell proliferation and increased at later stages during glomerular recovery. Finally, a plasmid expressing soluble GAS1 fused to an Fc fragment was systemically overexpressed in rats with mesangioproliferative glomerulonephritis. This ameliorated renal damage was indicated by decreased albuminuria and serum creatinine. Gas1/Fc-transfected rats also exhibited a reduction of the glomerular mesangial cell activation and proliferation. Thus, GAS1 is a novel endogenous inhibitor of glomerular mesangial cell proliferation and may be a novel therapeutic target in mesangioproliferative glomerular diseases.

The PDGF family in renal fibrosis

The platelet-derived growth factor (PDGF) family plays an important role in embryonic development, malignancy, wound healing, atherosclerosis, and fibrosis in multiple organs. It belongs to the best-characterized growth factor systems in normal and diseased kidneys, and there is accumulating evidence that members of the PDGF family are key players in the development of renal fibrosis independent of the underlying kidney disease. All components of the PDGF system, consisting of four isoforms (PDGF-A, -B, -C, -D) and two receptor chains (PDGFR-α and -β), are constitutively or inducibly expressed in most renal cells. They regulate multiple pathophysiologic events, ranging from cell proliferation and migration, extracellular matrix accumulation and production of pro- and anti-inflammatory mediators, to tissue permeability and hemodynamics. This review focuses on advances in defining the roles of different PDGF isoforms in the development of glomerulosclerosis and tubulointerstitial fibrosis. The recent identification of endogenous PDGF inhibitors offers additional novel therapeutic strategies.

The platelet-derived growth factor system in renal disease: an emerging role of endogenous inhibitors

The platelet-derived growth factor (PDGF) family consists of four isoforms which are secreted as homodimers (PDGF-AA, PDGF-BB, PDGF-CC and PDGF-DD) or heterodimers (PDGF-AB), and two receptor chains (PDGFR-α and -β). All members of the PDGF system are constitutively or inducibly expressed in renal cells and are involved in the regulation of cell proliferation and migration, the accumulation of extracellular matrix proteins and the secretion of pro- and anti-inflammatory mediators. Particular roles have been identified in mediating mesangioproliferative changes, renal interstitial fibrosis and glomerular angiogenesis. Different endogenous inhibitors of PDGF-induced biological responses exist which affect the activation/deactivation of PDGF isoforms, the activity of the PDGFRs, or which block downstream signaling pathways of the autophosphorylated PDGFRs. The novel endogenous inhibitor nephroblastoma overexpressed gene (NOV, CCN3) reduces PDGF-induced cell proliferation and is downregulated by PDGF isoforms itself. Among all identified inhibitors only few "true" PDGF antagonists have been identified. A better understanding of these inhibitors may aid in the design of novel therapeutic approaches to PDGF-mediated diseases.

PDGF: the nuts and bolts of signalling toolbox

PDGF is a growth factor and is extensively involved in multi-dimensional cellular dynamics. It switches on a plethora of molecules other than its classical pathway. It is engaged in various transitions of development; however, if the unleashed potentials lead astray, it brings forth tumourigenesis. Conventionally, it has been assumed that the components of this signalling pathway show fidelity and act with a high degree of autonomy. However, as illustrated by the PDGF signal transduction, reinterpretation of recent data suggests that machinery is often shared between multiple pathways, and other components crosstalk to each other through multiple mechanisms. It is important to note that metastatic cascade is an intricate process that we have only begun to understand in recent years. Many of the early steps of this PDGF cascade are not readily targetable in the clinic. In this review, we will unravel the paradoxes with reference to mitrons and cellular plasticity and discuss how disruption of signalling cascade triggers cellular proliferation phase transition and metastasis. We will also focus on the therapeutic interventions to counteract resultant molecular disorders.

Renal fibrosis

Renal fibrosis, characterized by tubulointerstitial fibrosis and glomerulosclerosis, is the final manifestation of chronic kidney disease. Renal fibrosis is characterized by an excessive accumulation and deposition of extracellular matrix components. This pathologic result usually originates from both underlying complicated cellular activities such as epithelial-to-mesenchymal transition, fibroblast activation, monocyte/macrophage infiltration, and cellular apoptosis and the activation of signaling molecules such as transforming growth factor beta and angiotensin II. However, because the pathogenesis of renal fibrosis is extremely complicated and our knowledge regarding this condition is still limited, further studies are needed.

The enigmatic parietal epithelial cell is finally getting noticed: a review

Although the normal glomerulus comprises four resident cell types, least is known about the parietal epithelial cells (PECs). This comprehensive review addresses the cellular origin of PECs, discusses the normal structure and protein makeup of PECs, describes PEC function, and defines the responses to injury in disease and how these events lead to clinical events. The data show that PECs have unique properties and that new functions are being recognized such as their role in differentiating into podocytes during disease.

Histone deacetylase-1 is enriched at the platelet-derived growth factor-D promoter in response to interleukin-1beta and forms a cytokine-inducible gene-silencing complex with NF-kappab p65 and interferon regulatory factor-1

Understanding the mechanisms governing cytokine control of growth factor expression in smooth muscle cells would provide invaluable insight into the molecular regulation of vascular phenotypes and create future opportunities for therapeutic intervention. Here, we report that the proinflammatory cytokine interleukin (IL)-1beta suppresses platelet-derived growth factor (PDGF)-D promoter activity and mRNA and protein expression in smooth muscle cells. NF-kappaB p65, induced by IL-1beta, interacts with a novel element in the PDGF-D promoter and inhibits PDGF-D transcription. Interferon regulatory factor-1 (IRF-1) is also induced by IL-1beta and binds to a different element upstream in the promoter. Immunoprecipitation and chromatin immunoprecipitation experiments showed that IL-1beta stimulates p65 interaction with IRF-1 and the accumulation of both factors at the PDGF-D promoter. Mutation of the IRF-1 and p65 DNA-binding elements relieved the promoter from IL-1beta-mediated repression. PDGF-D repression by IL-1beta involves histone deacetylation and interaction of HDAC-1 with IRF-1 and p65. HDAC-1 small interfering RNA ablates complex formation with IRF-1 and p65 and abrogates IRF-1 and p65 occupancy of the PDGF-D promoter. Thus, HDAC-1 is enriched at the PDGF-D promoter in cells exposed to IL-1beta and forms a cytokine-inducible gene-silencing complex with p65 and IRF-1.

Identification of platelet-derived growth factor D in human chronic allograft nephropathy

Chronic allograft nephropathy (CAN), a descriptive term denoting chronic scarring injury of the renal parenchyma and vasculature in allograft kidneys arising from various etiologies including chronic rejection, is the most common cause of late allograft failure, but mediators of this progressive injury largely remain unknown. We hypothesized that platelet-derived growth factor D (PDGF-D) and its specific receptor PDGF-Rbeta may be an important mediator in the pathogenesis of CAN and, hence, sought to identify its expression in this setting. Allograft nephrectomies demonstrating CAN, obtained from patients with irreversible transplant kidney failure (n = 15), were compared with renal tissues without prominent histopathological abnormalities (n = 18) and a series of renal allograft biopsies demonstrating acute vascular rejection (AVR) (n = 12). Antibodies to PDGF-D and PDGF-Rbeta were used for immunohistochemistry. Double and triple immunohistochemistry was used to identify cell types expressing PDGF-D. PDGF-D was widely expressed in most neointimas in arteries exhibiting the chronic arteriopathy of CAN and only weakly expressed in a small proportion of sclerotic arteries in the other 2 groups. Double and triple immunolabeling demonstrated that the neointimal cells expressing PDGF-D were alpha-smooth muscle actin-expressing cells, but not infiltrating macrophages or endothelial cells. PDGF-Rbeta expression evaluated in serial sections was localized to the same sites where neointimal PDGF-D was expressed. PDGF-Rbeta was expressed in interstitial cells more abundantly in the CAN group compared with the normal and AVR groups, without demonstrable colocalization of PDGF-D. PDGF-D is present in the neointima of the arteriopathy of CAN, where it can engage PDGF-Rbeta to promote mesenchymal cell migration, proliferation, and neointima formation. PDGF-D may engage the PDGF-Rbeta to promote interstitial injury in chronic allograft injury, but its sources within the interstitium were unidentified.

Structural and functional specificities of PDGF-C and PDGF-D, the novel members of the platelet-derived growth factors family

The platelet-derived growth factor (PDGF) family was for more than 25 years assumed to consist of only PDGF-A and -B. The discovery of the novel family members PDGF-C and PDGF-D triggered a search for novel activities and complementary fine tuning between the members of this family of growth factors. Since the expansion of the PDGF family, more than 60 publications on the novel PDGF-C and PDGF-D have been presented, highlighting similarities and differences to the classical PDGFs. In this paper we review the published data on the PDGF family covering structural (gene and protein) similarities and differences among all four family members, with special focus on PDGF-C and PDGF-D expression and functions. Little information on the protein structures of PDGF-C and -D is currently available, but the PDGF-C protein may be structurally more similar to VEGF-A than to PDGF-B. PDGF-C contributes to normal development of the heart, ear, central nervous system (CNS), and kidney, while PDGF-D is active in the development of the kidney, eye and brain. In adults, PDGF-C is active in the kidney and the central nervous system. PDGF-D also plays a role in the lung and in periodontal mineralization. PDGF-C is expressed in Ewing family sarcoma and PDGF-D is linked to lung, prostate and ovarian cancers. Both PDGF-C and -D play a role in progressive renal disease, glioblastoma/medulloblastoma and fibrosis in several organs.

Inducible platelet-derived growth factor D-chain expression by angiotensin II and hydrogen peroxide involves transcriptional regulation by Ets-1 and Sp1

Platelet-derived growth factor D-chain (PDGF-D) is the newest member of the PDGF family of mitogens and chemoattractants expressed in a wide variety of cell types, including vascular smooth muscle cells (SMCs). The molecular mechanisms regulating PDGF-D transcription are not known. Primer extension analysis mapped a single transcriptional start site to the ccAGCGC motif with several potential Ets motifs located upstream. Ets-1, but not Ets-1 bearing only the DNA-binding domain, activates the PDGF-D promoter and mRNA expression in SMCs. Ets site D3 ((-470)GGAT(-467)) is singly required for basal and Ets-1-inducible PDGF-D promoter-dependent expression. D3 supports the interaction of endogenous and recombinant Ets-1 and Sp1. Sp1, like Ets-1, induces PDGF-D transcription and mRNA expression, which is blocked by mutant Ets-1. H2O2 stimulates Ets-1, but not Sp1, and activates D3-dependent PDGF-D transcription. Ets-1 and Sp1 siRNA block peroxide-inducible PDGF-D expression. Angiotensin II (ATII) induction of PDGF-D and Ets-1 was blocked by prior incubation of the cells with PEG-catalase, but not BSA, indicating that ATII-inducible Ets-1 and PDGF-D expression is mediated via H2O2. Thus, 2 separate trans-acting factors regulate PDGF-D transcription, alone and in response to oxidative stress.

Blocking platelet-derived growth factor-D/platelet-derived growth factor receptor beta signaling inhibits human renal cell carcinoma progression in an orthotopic mouse model

Renal cell carcinoma is a highly malignant and often fatal disease of the kidney. It is difficult to treat, often because metastases are common at the time of presentation. Platelet-derived growth factor-D (PDGF-D) is a newly discovered member of the PDGF family; its function in tumor progression is largely unknown. Here, we examined the expression level of PDGF-D in human renal cell carcinoma by immunohistochemical staining using tissue arrays. We showed that human renal cell carcinoma expresses high levels of PDGF-D protein. The human renal cell carcinoma cell line SN12-C was stably transfected with pdgf-d cDNA. Overexpression of PDGF-D in SN12-C cells promoted tumor growth, angiogenesis, and metastasis of human renal cell carcinoma in an orthotopic severe combined immunodeficient (SCID) mouse model. PDGF-D overproduction in SN12-C cells increased the proliferation and migration of mural cells in vitro and improved perivascular cell coverage in vivo. Overexpression of PDGF-D led to increased expression of angiopoietin-1 and matrix metalloproteinase-9 in tumor tissues. ShRNAi and Gleevec were used to block PDGF-D expression and PDGF receptor beta (PDGFRbeta) signaling. Inhibition of PDGF-D expression by short hairpin RNA interference (shRNAi) and blockage of PDGFRbeta signaling by Gleevec inhibited the growth and lung metastasis of SN12-C cells grown orthotopically in SCID mice. Thus, PDGF-D is a potential candidate for controlling the progression of metastatic renal cell carcinoma. This opens up an avenue of investigation into novel therapeutic strategies for the treatment of renal cell carcinoma, including the use of recently developed tyrosine kinase inhibitors, such as Gleevec, which inhibit PDGF activity through inhibition of its receptor tyrosine kinase.

PDGF and cardiovascular disease

Platelet-derived growth factor (PDGF) was identified in a search for serum factors that stimulate smooth muscle cell (SMC) proliferation. During the development of lesions of atherosclerosis that can ultimately lead to vessel occlusion, SMC are stimulated by inflammatory factors to migrate from their normal location in the media. They accumulate within the forming lesion where they contribute to lesion expansion by proliferation and deposition of extracellular matrix. Different genetic manipulations in vascular cells combined with various inhibitory strategies have provided strong evidence for PDGF playing a prominent role in the migration of SMC into the neointima following acute injury and in atherosclerosis. Other activities of PDGF identified in vivo suggest additional functions for PDGF in the pathogenesis of cardiovascular disease.

From hyperglycemia to diabetic kidney disease: the role of metabolic, hemodynamic, intracellular factors and growth factors/cytokines

At present, diabetic kidney disease affects about 15-25% of type 1 and 30-40% of type 2 diabetic patients. Several decades of extensive research has elucidated various pathways to be implicated in the development of diabetic kidney disease. This review focuses on the metabolic factors beyond blood glucose that are involved in the pathogenesis of diabetic kidney disease, i.e., advanced glycation end-products and the aldose reductase system. Furthermore, the contribution of hemodynamic factors, the renin-angiotensin system, the endothelin system, and the nitric oxide system, as well as the prominent role of the intracellular signaling molecule protein kinase C are discussed. Finally, the respective roles of TGF-beta, GH and IGFs, vascular endothelial growth factor, and platelet-derived growth factor are covered. The complex interplay between these different pathways will be highlighted. A brief introduction to each system and description of its expression in the normal kidney is followed by in vitro, experimental, and clinical evidence addressing the role of the system in diabetic kidney disease. Finally, well-known and potential therapeutic strategies targeting each system are discussed, ending with an overall conclusion.

Stem-cell therapy for renal diseases

Significant attention is currently directed to the biological and therapeutic capabilities of stem cells for developing novel treatments for acute and chronic kidney diseases. To date, viable sources of stem cells for renal therapies include adult bone marrow and embryonic tissues, including the metanephric mesenchyme and mesonephros. Native adult kidney stem cells have yet to be identified. Systemically introduced stem cells can engraft in sites of renal disease and injury to show donor phenotypes. Stem cells can differentiate into cells similar to glomeruli, mesangium, and tubules in the kidneys. The impact of stem-cell engraftment and differentiation on renal function presently is unknown. Identification of renal diseases treatable with stem-cell therapies is expected to evolve as stem-cell technologies advance. Methods of modifying stem cells to improve homing, differentiation, and integration into host tissues need further characterization. Ethical and legal controversies about embryonic research and cloning are shaping the regulation and funding of stem-cell research for kidney diseases. Scientific and clinical understanding of stem cells and their potential for renal treatments are in the early stage of development. This field offers great promise, and there are significant opportunities for future investigation in clinical, biological, and ethical aspects of stem-cell therapy for kidney diseases.

Obstructive Uropathy in Mice and Humans: Potential Role for PDGF-D in the Progression of Tubulointerstitial Injury

ABSTRACT. Tubulointerstitial fibrosis is a major characteristic of progressive renal diseases. Platelet-derived growth factor (PDGF) is a family of growth regulatory molecules consisting of PDGF-A and -B, along with the newly discovered PDGF-C and -D. They signal through cell membrane receptors, PDGF receptor α (PDGF-Rα) and receptor β (PDGF-Rβ). Involvement of PDGF-B and PDGF-Rβ in the initiation and progression of renal fibrosis has been well documented. The authors studied the localization of PDGF ligands and receptors by immunohistochemistry, with emphasis on the role of PDGF-D in murine renal fibrosis induced by unilateral ureteral obstruction (UUO). In mice with UUO,de novoexpression of PDGF-D was detected in interstitial cells at day 4, which increased to maximal expression at day 14. Increased expression of PDGF-B by interstitial cells and in some tubules was observed after day 4. The diseased mice did not show augmentation of PDGF-A or PDGF-C proteins in the areas of fibrosis. PDGF-Rα and -Rβ protein expression was increased in interstitial cells after day 4 and reached maximal expression at day 14. Human renal nephrectomies (n= 10) of chronic obstructive nephropathy demonstrated similarde novoexpression of PDGF-D in interstitial cells, correlating with expression of PDGF-Rβ and PDGF-B, as it did in the murine model. These observations suggest that PDGF-D plays an important role in the pathogenesis of tubulointerstitial injury through binding of PDGF-Rβ in both human obstructive nephropathy and the corresponding murine model of UUO. E-mail: calp@u.washington.edu

A fully human monoclonal antibody (CR002) identifies PDGF-D as a novel mediator of mesangioproliferative glomerulonephritis

PDGF-B is of central importance in mesangioproliferative diseases. PDGF-D, a new PDGF isoform, like PDGF-B, signals through the PDGF betabeta-receptor. The present study first determined that PDGF-D is mitogenic for rat mesangial cells and is not inhibited by a PDGF-B antagonist. Low levels of PDGF-D mRNA were detected in normal rat glomeruli. After induction of mesangioproliferative nephritis in rats by anti-Thy 1.1 mAb, glomerular PDGF-D mRNA and protein expression increased significantly from days 4 to 9 in comparison with nonnephritic rats. Peak expression of PDGF-D mRNA occurred 2 d later than peak PDGF-B mRNA expression. In addition, PDGF-D serum levels increased significantly in the nephritic animals on day 7. For investigating the functional role of PDGF-D, neutralizing fully human mAb were generated using the XenoMouse technology. Rats with anti-Thy 1.1-induced nephritis were treated on days 3 and 5 with different amounts of a fully human PDGF-DD-specific neutralizing mAb (CR002), equal amounts of irrelevant control mAb, or PBS by intraperitoneal injection. Specific antagonism of PDGF-D led to a dose-dependent (up to 67%) reduction of glomerular cell proliferation. As judged by double immunostaining for 5-bromo-2'-deoxyuridine and alpha-smooth muscle actin, glomerular mesangial cell proliferation was reduced by up to 57%. Reduction of glomerular cell proliferation in the rats that received CR002 was not associated with reduced glomerular expression of PDGF-B mRNA. PDGF-D antagonism also led to reduced glomerular infiltration of monocytes/macrophages (day 5) and reduced accumulation of fibronectin (day 8). In contrast, no effect was noted in normal rats that received an injection of CR002. These data show that PDGF-D is overexpressed in mesangioproliferative states and can act as an auto-, para-, or even endocrine glomerular cell mitogen, indicating that antagonism of PDGF-D may represent a novel therapeutic approach to mesangioproliferative glomerulonephritides.

Over-expression of platelet-derived growth factor in human diabetic nephropathy

BACKGROUND

The pathogenetic mechanisms responsible for progressive renal impairment of diabetic nephropathy are still poorly understood, despite its growing incidence. Increasing evidence suggests that growth factors may contribute to the initiation and progressive fibrosis of diabetic nephropathy. In this study, the gene expression and protein distribution of platelet-derived growth factor-A and -B (PDGF-A and PDGF-B) in human diabetic nephropathy were examined.

METHODS

PDGF-A and PDGF-B mRNA levels in surplus renal biopsy tissue from seven patients with overt diabetic nephropathy and six nephrectomy samples were examined using quantitative reverse transcription-polymerase chain reaction (RT-PCR). In addition, each sample was also examined immunohistochemically to quantify and localize peptide expression of each PDGF isoform.

RESULTS

Gene expression of PDGF-A and PDGF-B mRNA were increased 22- and 6-fold, respectively, in biopsies from patients with diabetic nephropathy compared with control tissue. Immunostaining also demonstrated increased peptide expression of both PDGF-A and PDGF-B in diabetic nephropathy, with each isoform showing a specific pattern of tissue distribution.

CONCLUSIONS

The findings of increased gene and protein expression of PDGF in renal biopsies from patients with diabetic nephropathy imply a potential role for this prosclerotic growth factor in the development of the progressive fibrosis that characterizes human diabetic kidney disease.

Novel insights into renal fibrosis

PURPOSE OF REVIEW

Renal fibrosis characterizes a common endpoint of diverse renal diseases which leads to functional impairment ultimately resulting in terminal renal failure.

RECENT FINDINGS

Recent advances in this field led to the discovery of several novel mediators as well as novel aspects of known mediators. Studies on the origin and role of specific renal cell types involved in renal fibrosis identified bone marrow derived mesangial progenitors and offered substantial evidence for the concept of epithelial to mesenchymal transition. Much progress has also been made in better understanding of the interactions between different mediators and between mediators and renal target cells. Compounds designed on the basis of this current knowledge have proven to be potent inhibitors of the development of renal fibrosis or might even induce resolution of renal fibrosis.

SUMMARY

The number and diversity of recent studies in this field offer hope for new treatment regimes in our clinical efforts towards prevention and regression of progressive fibrosing renal diseases.