Osteogenic protein-1 prevents renal fibrogenesis associated with ureteral obstruction

Bone morphogenic protein-7 inhibits progression of chronic renal fibrosis associated with two genetic mouse models

Tubulointerstitial fibrosis is a hallmark feature of chronic renal injury. Specific therapies to control the progression of renal fibrosis toward end-stage renal failure are limited. Previous studies have demonstrated that expression of endogenous bone morphogenic protein-7 (BMP-7) is reduced in the kidneys of several inducible mouse models of acute and chronic renal disease and that administration of exogenous recombinant human BMP-7 (rhBMP-7) has a beneficial effect on kidney function. Here we report that treatment with rhBMP-7 leads to improved renal function, histology, and survival in mice deficient in the alpha3-chain of type IV collagen and MRL/MpJlpr/lpr lupus mice, two genetic models for chronic renal injury and fibrosis. Such therapeutic benefit is also associated with a significant decrease in the expression of profibrotic molecules, such as type I collagen and fibronectin, in renal fibroblasts. Additionally, rhBMP-7 induces expression of active matrix metalloproteinase-2, which is potentially important for removal of fibrotic matrix. Collectively, these studies provide further evidence for rhBMP-7 as an important bone-associated protein with protective function against renal pathology.

BMP-7 counteracts TGF-beta1-induced epithelial-to-mesenchymal transition and reverses chronic renal injury

Bone morphogenic protein (BMP)-7 is a 35-kDa homodimeric protein and a member of the transforming growth factor (TGF)-beta superfamily. BMP-7 expression is highest in the kidney, and its genetic deletion in mice leads to severe impairment of eye, skeletal and kidney development. Here we report that BMP-7 reverses TGF-beta1-induced epithelial-to-mesenchymal transition (EMT) by reinduction of E-cadherin, a key epithelial cell adhesion molecule. Additionally, we provide molecular evidence for Smad-dependent reversal of TGF-beta1-induced EMT by BMP-7 in renal tubular epithelial cells and mammary ductal epithelial cells. In the kidney, EMT-induced accumulation of myofibroblasts and subsequent tubular atrophy are considered key determinants of renal fibrosis during chronic renal injury. We therefore tested the potential of BMP-7 to reverse TGF-beta1-induced de novo EMT in a mouse model of chronic renal injury. Our results show that systemic administration of recombinant human BMP-7 leads to repair of severely damaged renal tubular epithelial cells, in association with reversal of chronic renal injury. Collectively, these results provide evidence of cross talk between BMP-7 and TGF-beta1 in the regulation of EMT in health and disease.

BMP-7 is an efficacious treatment of vascular calcification in a murine model of atherosclerosis and chronic renal failure

Chronic renal failure is complicated by high cardiovascular mortality. One key contributor to this mortality is vascular calcification, for which no therapy currently exists. Bone morphogenetic protein 7 is an essential renal morphogen that maintains renal tubular differentiation in the adult and is downregulated in renal failure. Several studies have demonstrated its efficacy in treating various renal diseases in rodents, and it was hypothesized that it would also be an effective treatment of vascular calcification in this setting. Uremia was imposed on LDL receptor null mice (a model of atherosclerosis), which were then treated with bone morphogenetic protein 7 for 15 wk. Uremic animals had increased vascular calcification by histology and chemical analysis. Calcification in treated animals was similar to or less than non-uremic control animals. Cells exhibiting an osteoblast-like phenotype in the vessel wall may be important in the etiology of vascular calcification. Expression of osteocalcin was assessed as a marker of osteoblastic function, and it is shown that it is increased in untreated uremic animals but downregulated to levels similar to non-uremic control animals with treatment. The data are compatible with bone morphogenetic protein 7 deficiency as a pathophysiologic factor in chronic renal failure, and they demonstrate its efficacy as a potential treatment of vascular calcification.

BMP7 antagonizes TGF-beta -dependent fibrogenesis in mesangial cells

Exogenous administration of recombinant human bone morphogenetic protein (BMP)-7 was recently shown to ameliorate renal glomerular and interstitial fibrosis in rodents with experimental renal diseases. We tested the hypothesis that BMP7 functions by antagonizing profibrogenic events that are induced by transforming growth factor (TGF)-beta in cultured mesangial cells. Incubation of murine mesangial cells with TGF-beta (50-200 pM) increased cell-associated collagen type IV and fibronectin, soluble collagen type IV, thrombospondin, and connective tissue growth factor (CTGF). Coincubation with recombinant human BMP7 (200 pM) reduced the increase of these ECM proteins and CTGF. The changes in collagen type IV and fibronectin proteins occurred without concomitant changes in collagen type alpha(1)IV and fibronectin mRNA levels, suggesting that TGF-beta and BMP7 act primarily by affecting ECM protein degradation. Indeed, TGF-beta decreases the levels and activity of matrix metalloprotease (MMP)-2, the major metalloprotease that is secreted by mesangial cells. Moreover, BMP7 inhibits TGF-beta-induced activation of MMP2. Because TGF-beta reduces the activity of MMPs through increasing plasminogen activator inhibitor (PAI)-1, we tested whether BMP7 interferes with this TGF-beta effect. BMP7 reduces, by about two-thirds, the activation of a PAI-1 promoter/luciferase reporter in cells stably transfected with this construct. The findings from these studies indicate that BMP7 reduces TGF-beta-induced ECM protein accumulation in cultured mesangial cells primarily by maintaining levels and activity of MMP2 partially through prevention of TGF-beta-dependent upregulation of PAI-1.

Progression of chronic renal disease

Risk factors for progression of kidney disease include hypertension, proteinuria, male sex, obesity, diabetes mellitus, hyperlipidemia, smoking, high-protein diets, phosphate retention, and metabolic acidosis. Angiotensin II production upregulates the expression of transforming growth factor-beta1, tumor necrosis factor-alpha, nuclear factor-kappaB, and several adhesion molecules and chemoattractants. In addition to angiotensin, other vasoactive compounds, such as thromboxane A(2), endothelin, and prostaglandins, are upregulated. Treatment with one of several growth factors may ameliorate the progression of kidney disease: insulin-like growth factor-1, hepatocyte growth factor, and bone morphogenetic protein-7.

Role of renin-angiotensin system and nuclear factor-kappaB in the obstructed kidney of rats with unilateral ureteral obstruction

The present study was conducted to elucidate the role of oxidative stress and nuclear factor-kappaB (NF-kappaB) in the beneficial effects of angiotensin receptor blockade on obstructive nephropathy. Unilateral ureteral occlusion in rats elicited tubulo-interstitial fibrosis with concomitant macrophage infiltration and increased expression of monocyte chemoattractant protein-1. These changes were accompanied by an induction of renal cortical lipid peroxidation and activation of NF-kappaB. Both an AT(1) antagonist, candesartan, and a NF-kappaB inhibitor, pyrrolidine dithiocarbamate, markedly attenuated these changes and to a similar extent. These results suggest that the beneficial effects of angiotensin blockade are mediated by the inhibition of oxidative stress and subsequent NF-kappaB activation in obstructive nephropathy.

Obstructive nephropathy and renal fibrosis

Interstitial fibrosis has a major role in the progression of renal diseases. Several animal models are available for the study of renal fibrosis. The models of aminonucleoside-induced nephrotic syndrome, cyclosporin nephrotoxicity, and passive Heyman nephritis are characterized by molecular and cellular events similar to those that occur in obstructive nephropathy. Additionally, inhibition of angiotensin-converting enzyme exerts salutary effects on the progression of renal fibrosis in obstructive nephropathy. Unilateral ureteral obstruction (UUO) has emerged as an important model for the study of the mechanisms of renal fibrosis and also for the evaluation of the impact of potential therapeutic approaches to ameliorate renal disease. Many quantifiable pathophysiological events occur over the span of 1 wk of UUO, making this an attractive model for study. This paper reviews some of the ongoing studies that utilized a rodent model of UUO. Some of the findings of the animal model have been compared with observations made in patients with obstructive nephropathy. Most of the evidence suggests that the rodent model of UUO is reflective of human renal disease processes.

Inhibitory smads and tgf-Beta signaling in glomerular cells

Smad6 and Smad7 are inhibitory SMADs with putative functional roles at the intersection of major intracellular signaling networks, including TGF-beta, receptor tyrosine kinase (RTK), JAK/STAT, and NF-kappaB pathways. This study reports differential functional roles and regulation of Smad6 and Smad7 in TGF-beta signaling in renal cells, in murine models of renal disease and in human glomerular diseases. Smad7 is upregulated in podocytes in all examined glomerular diseases (focal segmental glomerulosclerosis [FSGS], minimal-change disease [MCD], membranous nephropathy [MNP], lupus nephritis [LN], and diabetic nephropathy [DN]) with a statistically significant upregulation in "classical" podocyte-diseases such as FSGS and MCD. TGF-beta induces Smad7 synthesis in cultured podocytes and Smad6 synthesis in cultured mesangial cells. Although Smad7 expression inhibited both Smad2- and Smad3-mediated TGF-beta signaling in podocytes, it inhibited only Smad3 but not Smad2 signaling in mesangial cells. In contrast, Smad6 had no effect on TGF-beta/Smad signaling in podocytes and enhanced Smad3 signaling in mesangial cells. These data suggest that Smad7 is activated in injured podocytes in vitro and in human glomerular disease and participates in negative control of TGF-beta/Smad signaling in addition to its pro-apoptotic activity, whereas Smad6 has no role in TGF-beta response and injury in podocytes. In contrast, Smad6 is upregulated in the mesangium in human glomerular diseases and may be involved in functions independent of TGF-beta/Smad signaling. These data indicate an important role for Smad6 and Smad7 in glomerular cells in vivo that could be important for the cell homeostasis in physiologic and pathologic conditions.

Overview of bone morphogenetic proteins

STUDY DESIGN

A literature review was conducted.

OBJECTIVES

To review the discovery of the bone morphogenetic proteins and describe the bone morphogenetic protein products that will or may be available for clinical use.

SUMMARY OF BACKGROUND DATA

Bone morphogenetic proteins comprise the osteoinductive component of several tissue engineering products in late-stage development as replacements for autogenous bone graft, and for bone augmentation and repair.

METHODS

The literature on bone morphogenetic proteins was reviewed.

RESULTS

Bone morphogenetic proteins were discovered originally on the basis of their presence in osteoinductive extracts of bone matrix. Molecular cloning of bone morphogenetic proteins demonstrated that they are a family of related differentiation factors, each capable of inducing the formation of new bone tissue when implanted. Two of the molecules in clinical use, recombinant human bone morphogenetic protein-2 and recombinant human bone morphogenetic protein-7 (OP-1) are produced in a biotechnology process using recombinant deoxyribonucleic acid technology that offers unlimited supply and substantial control over purity and reproducible activity. A third material, bovine bone morphogenetic protein extract, is extracted from bone, and contains a mixture of bone morphogenetic protein molecules. Each of these molecules, although osteoinductive in vivo, has different physiologic roles and biologic activities in vivo and in vitro. Successful development of a product for use in spinal fusion involves selecting the osteoinductive molecule, the amount of the bone morphogenetic protein required, and the method of delivery, as well as conducting subsequent preclinical and clinical studies to evaluate its efficacy and safety.

CONCLUSIONS

On the basis of the data provided in this issue of Spine, some of these bone morphogenetic protein-based products provide for revolutionary therapies in orthopedic practice.

Activation of bone morphogenetic protein/Smad signaling in bronchial epithelial cells during airway inflammation

Bone morphogenetic proteins (BMPs) are pleiotropic secreted proteins, structurally related to transforming growth factor (TGF)-beta and activins. BMPs play pivotal roles in the regulation of embryonic lung development and branching of airways and have recently been considered to influence inflammatory processes in adults due to their chemotactic activity on fibroblasts, myocytes, and inflammatory cells. In this study, we have investigated the possible involvement of BMPs in a model of experimental allergic-airway inflammation in situ using antibodies that detect activated Smad proteins, and have monitored the modulation of BMP ligands during the inflammatory response. Inflamed bronchial epithelial cells and a few scattered alveolar cells expressed levels of phosphorylated Smad1 (pSmad1/5), indicative of active BMP/Smad signaling. This was in contrast to healthy epithelium, which was devoid of immunoreactivity. A mechanistic explanation for increased pSmad1/5 staining during inflammation was provided by the upregulated expression of all the BMP type I receptors, i.e., activin receptor-like kinase (ALK)2, ALK3, and ALK6, in the inflamed bronchial epithelial cells. Furthermore, the mRNA and protein profiles for BMP ligands were significantly altered during airway inflammation with induction of BMP2, BMP4, and BMP6, and downregulation of BMP5 and BMP7. Collectively, our data demonstrate for the first time active BMP/Smad signaling during airway inflammation in bronchial epithelial cells and thus raise the possibility that BMPs could play a determining role in respiratory pathophysiology.

Transforming growth factor-beta induces renal epithelial jagged-1 expression in fibrotic disease

For elucidation of the mechanisms by which growth factors and cytokines affect renal epithelial cells, gene array analysis of renal cells cultured in the presence of transforming growth factor-beta1 (TGF-beta1) was performed. Many genes that were not previously considered to be involved in renal cell biologic processes were affected, one of which was jagged-1. The jagged ligand/notch receptor family controls the formation of boundaries between groups of cells and regulates cell fates. On the basis of the array analysis, jagged-1 expression was further evaluated in cultured cells and in C57BL/6 mice with a model of unilateral ureteral obstruction (UUO). Recombinant human TGF-beta1 increased jagged-1 mRNA levels at concentrations between 10(-11) and 10(-10) M. There was a commensurate increase in jagged-1 protein levels, as assessed by Western blotting. The expression of jagged-1 mRNA and protein was observed to be significantly increased in the kidneys of C57BL/6 mice with obstructed ureters, compared with the contralateral kidneys, at 7 and 14 d of UUO. Immunohistochemical analyses demonstrated jagged-1 expression in distal tubules of kidneys from normal mice or contralateral kidneys from mice with UUO. Jagged-1 protein expression was increased in tubules not yet in apparent atrophy in the kidneys with an obstructed ureter. Jagged-1 expression was significantly increased in the kidneys of normal mice treated with TGF-beta1 and was decreased in the kidneys of mice with UUO treated with a TGF-beta receptor II-Fc chimera. These results suggest that jagged-1 is expressed in normal kidneys and that this expression is upregulated during renal disease, in a TGF-beta-dependent manner.

New approaches to delay the progression of chronic renal failure

BACKGROUND

The molecular cloning of many bone morphogenetic proteins (BMP)-encoding genes and their identification as transforming growth factor-beta (TGF-beta) relatives enhanced the interest in these molecules and allowed expression and functional studies to be performed.

METHODS

Rats with ureteral obstruction were distributed into four groups. Group 1 received vehicle, group 2 received enalapril 12.5 mg/kg body wt/day, group 3 received 50 or 300 microg/kg body wt BMP-7, and group 4 received both enalapril and the high dose of BMP-7. We also studied the effects of BMP-7 administration in a model streptozocin-induced diabetes.

RESULTS

Treatment with BMP-7 in rats with ureteral obstruction of 3 days duration and subsequent release indicated that this compound decreases interstitial volume and accelerates the return of renal function. After 16 weeks of diabetes, the rats were treated with BMP-7. The administration of BMP-7 partially reversed renal hypertrophy, restored GFR to normal, and decreased proteinuria.

CONCLUSIONS

These studies indicate that administration of BMP-7 maintains and restores renal function and structure in animals with ureteral obstruction and diabetic nephropathy.

Treatment of a murine model of high-turnover renal osteodystrophy by exogenous BMP-7

BACKGROUND

The secondary hyperparathyroidism of chronic kidney disease (CKD) produces a high turnover osteodystrophy that is associated with peritrabecular fibrosis. The nature of the cells involved in the development of peritrabecular fibrosis may represent osteoprogenitors expressing a fibroblastic phenotype that are retarded from progressing through osteoblast differentiation.

METHODS

To test the hypothesis that osteoblast differentiation is retarded in secondary hyperparathyroidism due to CKD producing bone marrow fibrosis, we administered bone morphogenetic protein 7 (BMP-7), a physiologic regulator of osteoblast regulation, to C57BL6 mice that had CKD produced by electrocautery of one kidney followed by contralateral nephrectomy two weeks later. Following the second surgical procedure, a subgroup of mice received daily intraperitoneal injections of BMP-7 (10 microg/kg). Three to six weeks later, the animals were sacrificed, blood was obtained for measurements of blood urea nitrogen (BUN) and parathyroid hormone (PTH) levels, and the femora and tibiae were processed for histomorphometric analysis.

RESULTS

The animals had significant renal insufficiency with BUN values of 77.79 +/- 22.68 mg/dL, and the level of renal impairment between the CKD untreated mice and the CKD mice treated with BMP-7 was the same in the two groups. PTH levels averaged 81.13 +/- 51.36 and 75.4 +/- 43.61 pg/mL in the CKD and BMP-7 treated groups, respectively. The animals with CKD developed significant peritrabecular fibrosis. In addition, there was an increase in osteoblast surface and osteoid accumulation as well as increased activation frequency and increased osteoclast surface consistent with high turnover renal osteodystrophy. Treatment with BMP-7 eliminated peritrabecular fibrosis, increased osteoblast number, osteoblast surface, mineralizing surface and single labeled surface. There was also a significant decrease in the eroded surface induced by treatment with BMP-7.

CONCLUSIONS

These findings indicate that BMP-7 treatment in the setting of high turnover renal osteodystrophy prevents the development of peritrabecular fibrosis, affects the osteoblast phenotype and mineralizing surfaces, and decreases bone resorption. This is compatible with a role of osteoblast differentiation in the pathophysiology of osteitis fibrosa.

BMP-7 regulates chemokine, cytokine, and hemodynamic gene expression in proximal tubule cells

BACKGROUND

Proximal tubule epithelial cells (PTEC) play a central role in the response of the kidney to insult by virtue of their production of chemokines and cytokines that signal an inflammatory response. Bone morphogenic protein-7 (BMP-7/OP-1), a member of the transforming growth factor-beta (TGF-beta) superfamily, has previously been demonstrated to reduce macrophage infiltration and tissue damage in animal models of acute and chronic renal failure. The present study was designed to define the molecular mechanism of BMP-7 action in human PTEC.

METHODS

Expression of BMP-7 in the adult mouse kidney was determined indirectly through X-gal staining of heterozygous BMP-7/lacZ mice in combination with cell-type specific markers. Primary human PTEC were cultured in the presence of the pro-inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha), with and without BMP-7. RNA isolated from these two populations was then used to identify differentially regulated genes via gene-array analysis. Modulation of potential target genes was subsequently confirmed through ELISA and/or quantitative PCR.

RESULTS

Expression from the BMP-7/lacZ transgene was detected in the collecting duct, thick ascending limb, distal convoluted tubule, and podocytes within glomeruli. No expression was detected within PTEC; however, these cells were found to express mRNA for BMP receptors including, ActR-I, BMPR-IA, ActR-II, ActR-IIB, and BMPR-II. BMP-7 significantly reduced TNF-alpha stimulated increases in mRNA for the pro-inflammatory genes, interleukin-6 (IL-6) and interleukin-1beta (IL-1beta), and the chemoattractants monocyte chemotactic protein-1 (MCP-1) and interleukin-8 (IL-8) in primary human PTEC. In addition, BMP-7 also reduced the expression of mRNA for endothelin-2 (ET-2), a vasoconstrictor, and increased the expression of mRNA for heme oxygenase-1 (HO-1), a vasodilator, although the latter was not statistically significant. In experiments designed to examine MCP-1 and IL-6 protein levels in response to additional TGF-beta superfamily members, TGF-beta1 was unable to mimic the effects of BMP-7 in reducing IL-6 production. However, the closely related BMP-6 exhibited similar properties to those of BMP-7. Each of the factors reduced MCP-1 expression.

CONCLUSIONS

BMP-7 represses the basal and TNF-alpha-stimulated expression of the pro-inflammatory cytokines IL-6 and IL-1beta, the chemokines MCP-1 and IL-8, and the vasoconstrictor ET-2 in PTEC. This data are consistent with the in vivo observations that BMP-7 administration in a model of chronic and acute renal failure results in a reduction in the infiltration of macrophages in the renal interstitium. Taken together, these observations suggest that BMP-7 may be a novel therapeutic agent for kidney disorders involving inflammation and ischemic damage of PTEC.

Loss of tubular bone morphogenetic protein-7 in diabetic nephropathy

Bone morphogenetic protein-7 (BMP7), a member of the transforming growth factor-beta (TGF-beta) superfamily of cytokines, is highly expressed in renal tubules and generally promotes maintenance of epithelial phenotype. It was examined whether, during the evolution of experimental diabetic nephropathy, the renal expression of BMP7 and BMP7 receptors declines, and the hypothesis that loss of BMP7 activity is profibrogenic in proximal tubular cells was tested. Moreover, in vitro studies in cultured proximal tubular cells were performed to examine putative mechanisms that cause these changes. At 15 wk of streptozotocin-induced diabetes, renal expression of BMP7 is declined by about half, and it decreased further by 30 wk to <10% of timed controls. Renal expression of the high-affinity BMP type II receptor and the type I receptor Alk2 (activin receptor-like kinase-2) decreased. Alk3 tended to decrease, but Alk6 remained unchanged. During the evolution of diabetic nephropathy, the secreted BMP antagonist gremlin increased substantially. In cultured tubular cells, TGF-beta reduced BMP7 and Alk3 expression and increased gremlin but did not interrupt BMP7-induced activation of smad5 or Erk1 and -2. In contrast, BMP7 did not alter TGF-beta expression. Neutralization of endogenous BMP7 in cultured proximal tubular cells raised the expression of fibronectin and tended to increase collagen alpha(1) III mRNA levels. In conclusion, in experimental diabetic nephropathy, renal tubular BMP7 and some of its receptors decreased and gremlin, a secreted BMP antagonist, increased. Some, but not all, of these changes are explained by increased TGF-beta. The loss of BMP7 activity per se is profibrogenic in tubular cells.

Renal fibrosis: an update

Tubulointerstitial fibrosis invariably accompanies the course of chronic renal failure towards end-stage renal disease. Tubular epithelial cells, the predominant cell type in the tubulointerstitium, are increasingly being recognized for playing a dominant role as mediators of renal fibrogenesis. Tubular epithelial cells become activated either by the glomerular ultrafiltrate from their apical side or by mononuclear cells from their basolateral side. They initiate the scarring process by secreting chemokines, which in return attract mononuclear cells as well as growth factors that stimulate interstitial fibroblasts. In later phases of renal fibrogenesis, cellular changes of tubular epithelial cells contribute to the chronic impairment of renal function. Whereas tubular epithelial cells react by proliferation or hypertrophy to initial stimuli, they may undergo apoptosis or transdifferentiate into fibroblasts, and thus contribute to tubular atrophy in later stages of progressive renal disease. Resident interstitial fibroblasts are also important in renal fibrogenesis, and recent research has demonstrated that these cells are much more heterogeneous than expected. Cytokines such as fibroblast growth factor type 2 and epithelial growth factor have been shown to be pro-fibrogenic, whereas hepatocyte growth factor and bone morphogenic protein type 7 may inhibit fibrogenesis. Despite recent progress, further research is mandatory for a better understanding and the development of novel therapeutic approaches.

Progression of chronic renal disease

Progressive chronic renal disease is characterized histologically by the accumulation of extracellular matrix proteins within the renal interstitium and progressive tubular atrophy. A number of kidney diseases and their progression to end-stage renal failure are driven by the intercrine, autocrine, paracrine, and endocrine effects of angiotensin II. Growth factors appear to have an important role in the development and progression of diabetic nephropathy. Substantial evidence indicates that induction of transforming growth factor beta (TGF-beta) is mediated by high levels of glucose. TGF-beta is pivotal for the hypertrophy of mesangial and tubular cells. Other factors such as hypertension, protein glycation products, and other mediators may further amplify the synthesis of TGF-beta and/or the expression of its receptors in the diabetic state. Other cytokines, such as tumor necrosis factor alpha also contribute to the progression of chronic renal disease. The overall picture derived from several studies of chronic renal disease suggests that treatment of the progression of the renal fibrosis may require the use of several strategies to prevent the advent of end-stage renal disease.