Systemic Activation of Activin A Signaling Causes Chronic Kidney Disease-Mineral Bone Disorder.

The high cardiovascular mortality associated with chronic kidney disease (CKD) is caused in part by the CKD-mineral bone disorder (CKD-MBD) syndrome. The CKD-MBD consists of skeletal, vascular and cardiac pathology caused by metabolic derangements produced by kidney disease. The prevalence of osteopenia/osteoporosis resulting from the skeletal component of the CKD-MBD, renal osteodystrophy (ROD), in patients with CKD exceeds that of the general population and is a major public health concern. That CKD is associated with compromised bone health is widely accepted, yet the mechanisms underlying impaired bone metabolism in CKD are not fully understood. Therefore, clarification of the molecular mechanisms by which CKD produces ROD is of crucial significance. We have shown that activin A, a member of the transforming growth factor (TGF)-ß super family, is an important positive regulator of receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis with Smad-mediated signaling being crucial for inducing osteoclast development and function. Recently, we have demonstrated systemic activation of activin receptors and activin A levels in CKD mouse models, such as diabetic CKD and Alport (AL) syndrome. In these CKD mouse models, bone remodeling caused by increased osteoclast numbers and activated osteoclastic bone resorption was observed and treatment with an activin receptor ligand trap repaired CKD-induced-osteoclastic bone resorption and stimulated individual osteoblastic bone formation, irrespective of parathyroid hormone (PTH) elevation. These findings have opened a new field for exploring mechanisms of activin A-enhanced osteoclast formation and function in CKD. Activin A appears to be a strong candidate for CKD-induced high-turnover ROD. Therefore, the treatment with the decoy receptor for activin A might be a good candidate for treatment for CKD-induced osteopenia or osteoporosis, indicating that the new findings from in these studies will lead to the identification of novel therapeutic targets for CKD-related and osteopenia and osteoporosis in general. In this review, we describe the impact of CKD-induced Smad signaling in osteoclasts, osteoblasts and vascular cells in CKD.

The activin receptor is stimulated in the skeleton, vasculature, heart, and kidney during chronic kidney disease.

We examined activin receptor type IIA (ActRIIA) activation in chronic kidney disease (CKD) by signal analysis and inhibition in mice with Alport syndrome using the ActRIIA ligand trap RAP-011 initiated in 75-day-old Alport mice. At 200 days of age, there was severe CKD and associated Mineral and Bone Disorder (CKD-MBD), consisting of osteodystrophy, vascular calcification, cardiac hypertrophy, hyperphosphatemia, hyperparathyroidism, elevated FGF23, and reduced klotho. The CKD-induced bone resorption and osteoblast dysfunction was reversed, and bone formation was increased by RAP-011. ActRIIA inhibition prevented the formation of calcium apatite deposits in the aortic adventitia and tunica media and significantly decreased the mean aortic calcium concentration from 0.59 in untreated to 0.36 mg/g in treated Alport mice. Aortic ActRIIA stimulation in untreated mice increased p-Smad2 levels and the transcription of sm22α and αSMA. ActRIIA inhibition reversed aortic expression of the osteoblast transition markers Runx2 and osterix. Heart weight was significantly increased by 26% in untreated mice but remained normal during RAP-011 treatment. In 150-day-old mice, GFR was significantly reduced by 55%, but only by 30% in the RAP-011-treated group. In 200-day-old mice, the mean BUN was 100 mg/dl in untreated mice compared to 60 mg/dl in the treated group. In the kidneys of 200-day-old mice, ActRIIA and p-Smad2 were induced and MCP-1, fibronectin, and interstitial fibrosis were stimulated; all were attenuated by RAP-011 treatment. Hence, the activation of ActRIIA signaling during early CKD contributes to the CKD-MBD components of osteodystrophy and cardiovascular disease and to renal fibrosis. Thus, the inhibition of ActRIIA signaling is efficacious in improving and delaying CKD-MBD in this model of Alport syndrome.

CKD-induced wingless/integration1 inhibitors and phosphorus cause the CKD-mineral and bone disorder.

In chronic kidney disease, vascular calcification, renal osteodystrophy, and phosphate contribute substantially to cardiovascular risk and are components of CKD-mineral and bone disorder (CKD-MBD). The cause of this syndrome is unknown. Additionally, no therapy addresses cardiovascular risk in CKD. In its inception, CKD-MBD is characterized by osteodystrophy, vascular calcification, and stimulation of osteocyte secretion. We tested the hypothesis that increased production of circulating factors by diseased kidneys causes the CKD-MBD in diabetic mice subjected to renal injury to induce stage 2 CKD (CKD-2 mice). Compared with non-CKD diabetic controls, CKD-2 mice showed increased renal production of Wnt inhibitor family members and higher levels of circulating Dickkopf-1 (Dkk1), sclerostin, and secreted klotho. Neutralization of Dkk1 in CKD-2 mice by administration of a monoclonal antibody after renal injury stimulated bone formation rates, corrected the osteodystrophy, and prevented CKD-stimulated vascular calcification. Mechanistically, neutralization of Dkk1 suppressed aortic expression of the osteoblastic transcription factor Runx2, increased expression of vascular smooth muscle protein 22-α, and restored aortic expression of klotho. Neutralization of Dkk1 did not affect the elevated plasma levels of osteocytic fibroblast growth factor 23 but decreased the elevated levels of sclerostin. Phosphate binder therapy restored plasma fibroblast growth factor 23 levels but had no effect on vascular calcification or osteodystrophy. The combination of the Dkk1 antibody and phosphate binder therapy completely treated the CKD-MBD. These results show that circulating Wnt inhibitors are involved in the pathogenesis of CKD-MBD and that the combination of Dkk1 neutralization and phosphate binding may have therapeutic potential for this disorder.

Expression of DGCR8-dependent microRNAs is indispensable for osteoclastic development and bone-resorbing activity.

Recently, microRNAs (miRs) have been implicated in bone formation and homeostasis. We previously reported that Dicer generated miRs have pivotal roles in differentiation and activity of osteoclasts. However, recent studies have demonstrated that Dicer is implicated in production of endogenous small interfering RNAs, non-canonical miRs, and other small RNAs in mammals. Hence, a challenging question is the extent to which expression of canonical miRs is obligatory for osteoclastic control of bone metabolism. DiGeorge syndrome critical region gene 8 (DGCR8) is exclusively related to expression of miRs by a canonical processing pathway together with the nuclear RNase III enzyme Drosha. Osteoclast-specific deletion of DGCR8 led to impaired osteoclastic development and bone resorption so that bone development was significantly retarded. In culture, the expression levels of osteoclastic phenotype-related genes and proteins were remarkably inhibited during osteoclastogenesis in DGCR8-deficiency. Thus, we have identified that DGCR8-dependent miRs are indispensable for osteoclastic control of bone metabolism.

Down-regulation of miR-21 biogenesis by estrogen action contributes to osteoclastic apoptosis.

Estrogen inhibits osteoclastogenesis and induces osteoclastic apoptosis; however, the molecular mechanisms remain controversial. Recently, a group has demonstrated that osteoclasts are a direct target for estrogen because estrogen stimulates transcription of the Fas Ligand (FasL) gene in osteoclasts, which in turn causes cell death through an autocrine mechanism. In contrast, other groups have shown that the cells are an indirect target for estrogen because estrogen fails to stimulate the transcription of that in osteoclasts. Thus, two quite different molecular mechanisms have been suggested to explain the effects of estrogen in osteoclastic apoptosis. Here we show that the proapoptotic effect of estrogen during osteoclastogenesis is regulated by a posttranscriptional increase in FasL production by down-regulated microRNA-21 (miR-21) biogenesis. Previously, we reported that miR-21 is highly expressed in osteoclastogenesis. We found that estrogen down-regulates miR-21 biogenesis so that FasL, the targets of miR-21, protein levels are posttranscriptionally increased that induce osteoclastic apoptosis. Moreover, the gain-of-function of miR-21 rescued the apoptosis. In addition, we failed to detect estrogen-enhanced FasL levels at mRNA levels. Thus, osteoclastic survival is controlled by autocrine actions of FasL regulated by estrogen and miR-21 plays a central role during estrogen-controlled osteoclastogenesis.

A microRNA expression signature of osteoclastogenesis.

MicroRNAs (miRs) are small noncoding RNAs that principally function in the spatiotemporal regulation of protein translation in animal cells. Although emerging evidence suggests that some miRs play important roles in osteoblastogenesis and skeletal homeostasis, much less is known in osteoclastogenesis. Here, we show that receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclastogenesis is mediated by miR-21. MiR-21 was identified as an miR expression signature of RANKL-induced osteoclastogenesis that down-regulates programmed cell death 4 (PDCD4) protein levels. Diminished PDCD4 removes a repression from c-Fos, a critical transcription factor for osteoclastogenesis and osteoclast-specific downstream target genes. In addition, RANKL-induced c-Fos up-regulates miR-21 gene expression. Bone marrow-derived monocyte/macrophage precursors deficient of DiGeorge syndrome critical region gene 8, an RNA binding protein associated with miR biogenesis, and Dicer, an endoribonuclease in the RNaseIII family associated with miR biogenesis, possessed significantly decreased miR-21 levels and increased PDCD4 protein levels so that RANKL-induced osteoclastogenesis was impaired in those cells. However, forced expression of miR-21 rescued osteoclast development because of down-regulation of PDCD4 protein expression levels. Thus, our studies provide a new molecular mechanism, including a positive feedback loop of c-Fos/miR-21/PDCD4, regulating osteoclastogenesis.

Impaired micro-RNA pathways diminish osteoclast differentiation and function.

Micro-RNAs (miRNAs) are important in regulating cell fate determination because many of their target mRNA transcripts are engaged in cell proliferation, differentiation, and apoptosis. DGCR8, Dicer, and Ago2 are essential factors for miRNA homeostasis. Here we show that these three factors have critical roles in osteoclast differentiation and function. Gene silencing of DGCR8, Dicer, or Ago2 by small interfering RNA revealed global inhibition of osteoclast transcription factor expression and function, decreased osteoclastogenesis, and decreased bone resorption in vitro. In vivo, CD11b(+)-cre/Dicer-null mice had mild osteopetrosis caused by decreased osteoclast number and bone resorption. These results suggest that miRNAs play important roles in differentiation and function of osteoclasts in vitro and in vivo. We found a novel mechanism mediating these results in which PU.1, miRNA-223, NFI-A, and the macrophage colony-stimulating factor receptor (M-CSFR) are closely linked through a positive feedback loop. PU.1 stimulates miRNA-223 expression, and this up-regulation is implicated in stimulating differentiation and function of osteoclasts through negative regulation of NFI-A levels. Down-regulation of NFI-A levels is important for expression of the M-CSFR, which is critical for osteoclast differentiation and function. NFI-A overexpression decreased osteoclast formation and function with down-regulation of M-CSFR levels. Forced expression of the M-CSFR in M-CSF-dependent bone marrow macrophages from Dicer-deficient mice rescued osteoclast differentiation with up-regulation of PU.1 levels. Our studies provide new molecular mechanisms controlling osteoclast differentiation and function by the miRNA system and specifically by miRNA-223, which regulates NFI-A and the M-CSFR levels.

The mechanism of phosphorus as a cardiovascular risk factor in CKD.

Hyperphosphatemia and vascular calcification have emerged as cardiovascular risk factors among those with chronic kidney disease. This study examined the mechanism by which phosphorous stimulates vascular calcification, as well as how controlling hyperphosphatemia affects established calcification. In primary cultures of vascular smooth muscle cells derived from atherosclerotic human aortas, activation of osteoblastic events, including increased expression of bone morphogenetic protein 2 (BMP-2) and the transcription factor RUNX2, which normally play roles in skeletal morphogenesis, was observed. These changes, however, did not lead to matrix mineralization until the phosphorus concentration of the media was increased; phosphorus stimulated expression of osterix, a second critical osteoblast transcription factor. Knockdown of osterix with small interference RNA (siRNA) or antagonism of BMP-2 with noggin prevented matrix mineralization in vitro. Similarly, vascular BMP-2 and RUNX2 were upregulated in atherosclerotic mice, but significant mineralization occurred only after the induction of renal dysfunction, which led to hyperphosphatemia and increased aortic expression of osterix. Administration of oral phosphate binders or intraperitoneal BMP-7 decreased expression of osterix and aortic mineralization. It is concluded that, in chronic kidney disease, hyperphosphatemia stimulates an osteoblastic transcriptional program in the vasculature, which is mediated by osterix activation in cells of the vascular tunica media and neointima.

Akt1/Akt2 and mammalian target of rapamycin/Bim play critical roles in osteoclast differentiation and survival, respectively, whereas Akt is dispensable for cell survival in isolated osteoclast precursors.

Akt, also known as protein kinase B, is a serine/threonine protein kinase with antiapoptotic activities; also, it is a downstream target of phosphatidylinositol 3-kinase. Here we show that Akt1/Akt2 play a critical role in osteoclast differentiation but not cell survival and that mammalian target of rapamycin (mTOR) and Bim, a pro-apoptotic Bcl-2 family member, are required for cell survival in isolated osteoclast precursors. To investigate the function of Akt1, Akt2, mTOR, and Bim, we employed a retroviral system for delivery of small interfering RNA into cells. Loss of Akt1 and/or Akt2 protein inhibited osteoclast differentiation due to down-regulation of IkappaB-kinase (IKK) alpha/beta activity, phosphorylation of IkappaB-alpha, nuclear translocation of nuclear factor-kappaB (NFkappaB) p50, and NFkappaB p50 DNA-binding activity. Surprisingly, deletion of Akt1 and/or Akt2 protein did not stimulate cleaved caspase-3 activity and failed to promote apoptosis. Conversely, loss of mTOR protein induced apoptosis due to up-regulation of cleaved caspase-3 activity. In addition, we found that mTOR is downstream of phosphatidylinositol 3-kinase (but not Akt) and that macrophage colony-stimulating factor regulates Bim expression through mTOR activation for cell survival. These results demonstrate that Akt1/Akt2 are key elements in osteoclast differentiation and that the macrophage colony-stimulating factor stimulation of mTOR leading to Bim inhibition is essential for cell survival in isolated osteoclast precursors.

Effect of the phosphodiesterase 4 inhibitor, rolipram, on retinoic acid-increased alkaline phosphatase activity in the mouse fibroblastic C3H10T1/2 cell line.

We have evaluated effects of a phosphodiesterase (PDE) 4 inhibitor on retinoic acid-increased alkaline phosphatase activity in the mouse fibroblastic C3H10T1/2 clone 8 (10T1/2) cell line. 10T1/2 cells were cultured in minimum essential medium (MEM) and 10% fetal bovine serum with or without 1 microM retinoic acid and/or the PDE 4 inhibitor, rolipram, and harvested at specific intervals before measurement of alkaline phosphatase activity, cAMP production in response to parathyroid hormone, osteocalcin synthesis and expression, and phosphodiesterase activity. Retinoic acid-increased alkaline phosphatase activity, and slightly enhanced cAMP production in response to parathyroid hormone. However, it did not affect osteocalcin synthesis and expression. In the presence of retinoic acid, PDE 4 activity was not changed. A PDE 4 inhibitor, rolipram, and cAMP analog, 8-bromo-cAMP dramatically increased retinoic acid's ability to induce alkaline phosphatase activity. This is the first report that PDE 4 may be involved in regulation of retinoic acid-increased alkaline phosphatase activity.

PTEN regulates RANKL- and osteopontin-stimulated signal transduction during osteoclast differentiation and cell motility.

PTEN (also known as MMAC-1 or TEP-1) is a frequently mutated tumor suppressor gene in human cancer. PTEN functions have been identified in the regulation of cell survival, growth, adhesion, migration, and invasiveness. Here, we characterize the diverse signaling networks modulated by PTEN in osteoclast precursors stimulated by RANKL and osteopontin (OPN). RANKL dose-dependently stimulated transient activation of Akt before activation of PTEN, consistent with a role for PTEN in decreasing Akt activity. PTEN overexpression blocked RANKL-activated Akt stimulated survival and osteopontin-stimulated cell migration while a dominant-negative PTEN increased the actions of RANKL and OPN. PTEN overexpression suppressed RANKL-mediated osteoclast differentiation and OPN-stimulated cell migration. The PTEN dominant-negative constitutively induced osteoclast differentiation and cell migration. Our data demonstrate multiple roles for PTEN in RANKL-induced osteoclast differentiation and OPN-stimulated cell migration in RAW 264.7 osteoclast precursors.