Protein kinase D mediates the synergistic effects of BMP-7 and IGF-I on osteoblastic cell differentiation

IGF signaling pathway in bone and cartilage development, homeostasis, and disease

The skeleton plays a fundamental role in the maintenance of organ function and daily activities. The insulin-like growth factor (IGF) family is a group of polypeptide substances with a pronounced role in osteoblast differentiation, bone development, and metabolism. Disturbance of the IGFs and the IGF signaling pathway is inextricably linked with assorted developmental defects, growth irregularities, and jeopardized skeletal structure. Recent findings have illustrated the significance of the action of the IGF signaling pathway via growth factors and receptors and its interactions with dissimilar signaling pathways (Wnt/β-catenin, BMP, TGF-β, and Hh/PTH signaling pathways) in promoting the growth, survival, and differentiation of osteoblasts. IGF signaling also exhibits profound influences on cartilage and bone development and skeletal homeostasis via versatile cell-cell interactions in an autocrine, paracrine, and endocrine manner systemically and locally. Our review summarizes the role and regulatory function as well as a potentially integrated gene network of the IGF signaling pathway with other signaling pathways in bone and cartilage development and skeletal homeostasis, which in turn provides an enlightening insight into visualizing bright molecular targets to be eligible for designing effective drugs to handle bone diseases and maladies, such as osteoporosis, osteoarthritis, and dwarfism.

The role of E3 ubiquitin ligases in bone homeostasis and related diseases

The ubiquitin-proteasome system (UPS) dedicates to degrade intracellular proteins to modulate demic homeostasis and functions of organisms. These enzymatic cascades mark and modifies target proteins diversly through covalently binding ubiquitin molecules. In the UPS, E3 ubiquitin ligases are the crucial constituents by the advantage of recognizing and presenting proteins to proteasomes for proteolysis. As the major regulators of protein homeostasis, E3 ligases are indispensable to proper cell manners in diverse systems, and they are well described in physiological bone growth and bone metabolism. Pathologically, classic bone-related diseases such as metabolic bone diseases, arthritis, bone neoplasms and bone metastasis of the tumor, etc., were also depicted in a UPS-dependent manner. Therefore, skeletal system is versatilely regulated by UPS and it is worthy to summarize the underlying mechanism. Furthermore, based on the current status of treatment, normal or pathological osteogenesis and tumorigenesis elaborated in this review highlight the clinical significance of UPS research. As a strategy possibly remedies the limitations of UPS treatment, emerging PROTAC was described comprehensively to illustrate its potential in clinical application. Altogether, the purpose of this review aims to provide more evidence for exploiting novel therapeutic strategies based on UPS for bone associated diseases.

Protein kinase D3 conditional knockout impairs osteoclast formation and increases trabecular bone volume in male mice

Studies using kinase inhibitors have shown that the protein kinase D (PRKD) family of serine/threonine kinases are required for formation and function of osteoclasts in culture. However, the involvement of individual protein kinase D genes and their in vivo significance to skeletal dynamics remains unclear. In the current study we present data indicating that protein kinase D3 is the primary form of PRKD expressed in osteoclasts. We hypothesized that loss of PRKD3 would impair osteoclast formation, thereby decreasing bone resorption and increasing bone mass. Conditional knockout (cKO) of Prkd3 using a murine Cre/Lox system driven by cFms-Cre revealed that its loss in osteoclast-lineage cells reduced osteoclast differentiation and resorptive function in culture. Examination of the Prkd3 cKO mice showed that bone parameters were unaffected in the femur at 4 weeks of age, but consistent with our hypothesis, Prkd3 conditional knockout resulted in 18 % increased trabecular bone mass in male mice at 12 weeks and a similar increase at 6 months. These effects were not observed in female mice. As a further test of our hypothesis, we asked if Prkd3 cKO could protect against bone loss in a ligature-induced periodontal disease model but did not see any reduction in bone destruction in this system. Together, our data indicate that PRKD3 promotes osteoclastogenesis both in vitro and in vivo.

The Role of Bone Morphogenetic Protein 7 (BMP-7) in Inflammation in Heart Diseases

Bone morphogenetic protein-7 is (BMP-7) is a potent anti-inflammatory growth factor belonging to the Transforming Growth Factor Beta (TGF-β) superfamily. It plays an important role in various biological processes, including embryogenesis, hematopoiesis, neurogenesis and skeletal morphogenesis. BMP-7 stimulates the target cells by binding to specific membrane-bound receptor BMPR 2 and transduces signals through mothers against decapentaplegic (Smads) and mitogen activated protein kinase (MAPK) pathways. To date, rhBMP-7 has been used clinically to induce the differentiation of mesenchymal stem cells bordering the bone fracture site into chondrocytes, osteoclasts, the formation of new bone via calcium deposition and to stimulate the repair of bone fracture. However, its use in cardiovascular diseases, such as atherosclerosis, myocardial infarction, and diabetic cardiomyopathy is currently being explored. More importantly, these cardiovascular diseases are associated with inflammation and infiltrated monocytes where BMP-7 has been demonstrated to be a key player in the differentiation of pro-inflammatory monocytes, or M1 macrophages, into anti-inflammatory M2 macrophages, which reduces developed cardiac dysfunction. Therefore, this review focuses on the molecular mechanisms of BMP-7 treatment in cardiovascular disease and its role as an anti-fibrotic, anti-apoptotic and anti-inflammatory growth factor, which emphasizes its potential therapeutic significance in heart diseases.

Deletion of protein kinase D1 in osteoprogenitor cells results in decreased osteogenesis in vitro and reduced bone mineral density in vivo

Protein kinase D1 (PRKD1) is thought to play a role in a number of cellular functions, including proliferation and differentiation. We hypothesized that PRKD1 in bone marrow-derived mesenchymal stem cells (BMMSC) could modulate osteogenesis. In BMMSCs from floxed PRKD1 mice, PRKD1 ablation with adenovirus-mediated Cre-recombinase expression inhibited BMMSC differentiation in vitro. In 3- and 6-month-old conditional knockout mice (cKO), in which PRKD1 was ablated in osteoprogenitor cells by osterix promoter-driven Cre-recombinase, bone mineral density (BMD) was significantly reduced compared with floxed control littermates. Microcomputed tomography analysis also demonstrated a decrease in trabecular thickness and bone volume fraction in cKO mice at these ages. Dynamic bone histomorphometry suggested a mineralization defect in the cKO mice. However, by 9 months of age, the bone appeared to compensate for the lack of PRKD1, and BMD was not different. Taken together, these results suggest a potentially important role for PRKD1 in bone formation.

A Conditional Knockout Mouse Model Reveals a Critical Role of PKD1 in Osteoblast Differentiation and Bone Development

The protein kinase D family of serine/threonine kinases, particularly PKD1, has been implicated in the regulation of a complex array of fundamental biological processes. However, its function and mechanism underlying PKD1-mediated the bone development and osteoblast differentiation are not fully understood. Here we demonstrate that loss of PKD1 function led to impaired bone development and osteoblast differentiation through STAT3 and p38 MAPK signaling using in vitro and in vivo bone-specific conditional PKD1-knockout (PKD1-KO) mice models. These mice developed markedly craniofacial dysplasia, scapula dysplasia, long bone length shortage and body weight decrease compared with wild-type littermates. Moreover, deletion of PKD1 in vivo reduced trabecular development and activity of osteoblast development, confirmed by Micro-CT and histological staining as well as expression of osteoblastic marker (OPN, Runx2 and OSX). Mechanistically, loss of PKD1 mediated the downregulation of osteoblast markers and impaired osteoblast differentiation through STAT3 and p38 MAPK signaling pathways. Taken together, these results demonstrated that PKD1 contributes to the osteoblast differentiation and bone development via elevation of osteoblast markers through activation of STAT3 and p38 MAPK signaling pathways.

The role of adaptive bone formation in the etiology of stress fracture

Stress fractures are common injuries with load-bearing activities. Stress fractures have been reported in the scientific literature for over a century; however, the etiology continues to be investigated with important distinctions made between the contributions of the tissue-level processes of bone remodeling and modeling. In response to novel repetitive loading, increased bone remodeling may serve to replace fatigue-damaged bone while at the same time creating temporary porosity. Much attention has been given to the role of remodeling in the etiology of stress fracture; however, the role of bone modeling has received less attention. Modest increases in modeling, via bone formation on the periosteal surface of long bones in response to mechanical loading, greatly increases the fatigue resistance of bone. Thus, enhancing this adaptive bone formation is a promising target for stress fracture prevention, and a focus on adaptive bone formation may reveal novel risk factors for stress fracture.

Histone demethylase Jmjd3 regulates osteoblast apoptosis through targeting anti-apoptotic protein Bcl-2 and pro-apoptotic protein Bim

Posttranslational modifications including histone methylation regulate gene transcription through directly affecting the structure of chromatin. Trimethylation of histone H3K27 (H3K27me3) contributes to gene silencing and the histone demethylase Jumonji domain-containing 3 (Jmjd3) specifically removes the methylation of H3K27me3, followed by the activation of gene expression. In the present study, we explored the roles of Jmjd3 in regulating osteoblast apoptosis. Knockdown of Jmjd3 promoted osteoblast apoptosis induced by serum deprivation with decreased mitochondrial membrane potential and increased levels of caspase-3 activation, PARP cleavage, and DNA fragmentation. B cell lymphoma-2 (Bcl-2), an anti-apoptotic protein, was down-regulated by knockdown of Jmjd3 through retaining H3K27me3 on its promoter region. Knockdown of Jmjd3 increased the pro-apoptotic activity of Bim through inhibiting ERK-dependent phosphorylation of Bim. Protein kinase D1 (PKD1), which stimulates ERK phosphorylation, decreased in the Jmjd3-knockdown cells and introduction of PKD1 relieved osteoblast apoptosis in the Jmjd3-knockdown cells through increasing ERK-regulated Bim phosphorylation. These results suggest that Jmjd3 regulates osteoblast apoptosis through targeting Bcl-2 expression and Bim phosphorylation.

IGF-1 Receptor Insufficiency Leads to Age-Dependent Attenuation of Osteoblast Differentiation

In the current study, we determined the effects of IGF-1 receptor haploinsufficiency on osteoblast differentiation and bone formation throughout the lifespan. Bone mineral density was significantly decreased in femurs of male and female Igf1r(+/-) mice compared with wild-type mice. mRNA expression of osteoblast differentiation markers was significantly decreased in femurs and calvariae from Igf1r(+/-) mice compared with cells from wild-type mice. Bone morphogenetic protein-7-induced ectopic bone in Igf1r(+/-) mice was significantly smaller with fewer osteoblasts but more lipid droplets and had reduced expression of osteoblast differentiation markers compared with wild-type mice. In bone marrow cells from middle-aged and old wild-type and Igf1r(+/-) male mice, palmitate inhibited osteoblast markers expression. In cells from young wild-type male mice, palmitate did not inhibit marker expression, but in cells from young male Igf1r(+/-) mice, palmitate inhibited bone sialoprotein and osterix but not osteocalcin or type I collagen (TIC). In female wild-type mice, palmitate inhibited osteoblast markers expression in cells from young, middle-aged, and old mice except TIC in cells from middle-aged mice. Palmitate inhibited bone sialoprotein expression in cells from middle-aged and old female Igf1r(+/-) mice and osteocalcin, osterix, and TIC expression in young and middle-aged female Igf1r(+/-) mice but stimulated expression in cells from old female Igf1r(+/-) mice. We conclude that IGF-1 receptor haploinsufficiency results in a prolipid accrual phenotype in bone in association with inhibition of growth factor-induced osteoblast differentiation, a situation which may phenocopy age-related decreases in bone formation.

IGFBP-2 directly stimulates osteoblast differentiation

Insulin-like growth factor binding protein 2 (IGFBP-2) is important for acquisition of normal bone mass in mice; however, the mechanism by which IGFBP-2 functions is not defined. These studies investigated the role of IGFBP-2 in stimulating osteoblast differentiation. MC-3T3 preosteoblasts expressed IGFBP-2, and IGFBP-2 knockdown resulted in a substantial delay in osteoblast differentiation, reduced osteocalcin expression and Alizarin red staining. These findings were replicated in primary calvarial osteoblasts obtained from IGFBP-2(-/-) mice, and addition of IGFBP-2 rescued the differentiation program. In contrast, overexpression of IGFBP-2 accelerated the time course of differentiation as well as increasing the total number of differentiating cells. By day 6, IGFBP-2-overexpressing cells expressed twice as much osteocalcin as control cultures and this difference persisted. To determine the mechanism by which IGFBP-2 functions, the interaction between IGFBP-2 and receptor tyrosine phosphatase β (RPTPβ) was examined. Disruption of this interaction inhibited the ability of IGFBP-2 to stimulate AKT activation and osteoblast differentiation. Knockdown of RPTPβ enhanced osteoblast differentiation, whereas overexpression of RPTPβ was inhibitory. Adding back IGFBP-2 to RPTPβ-overexpressing cells was able to rescue cell differentiation via enhancement of AKT activation. To determine the region of IGFBP-2 that mediated this effect, an IGFBP-2 mutant that contained substitutions of key amino acids in the heparin-binding domain-1 (HBD-1) was prepared. This mutant had a major reduction in its ability to stimulate differentiation of calvarial osteoblasts from IGFBP-2(-/-) mice. Addition of a synthetic peptide that contained the HBD-1 sequence to calvarial osteoblasts from IGFBP-2(-/-) mice rescued differentiation and osteocalcin expression. In summary, the results clearly demonstrate that IGFBP-2 stimulates osteoblast differentiation and that this effect is mediated through its heparin-binding domain-1 interacting with RPTPβ. The results suggest that stimulation of differentiation is an important mechanism by which IGFBP-2 regulates the acquisition of normal bone mass in mice.

Palmitate attenuates osteoblast differentiation of fetal rat calvarial cells

Aging is associated with the accumulation of ectopic lipid resulting in the inhibition of normal organ function, a phenomenon known as lipotoxicity. Within the bone marrow microenvironment, elevation in fatty acid levels may produce an increase in osteoclast activity and a decrease in osteoblast number and function, thus contributing to age-related osteoporosis. However, little is known about lipotoxic mechanisms in intramembraneous bone. Previously we reported that the long chain saturated fatty acid palmitate inhibited the expression of the osteogenic markers RUNX2 and osteocalcin in fetal rat calvarial cell (FRC) cultures. Moreover, the acetyl CoA carboxylase inhibitor TOFA blocked the inhibitory effect of palmitate on expression of these two markers. In the current study we have extended these observations to show that palmitate inhibits spontaneous mineralized bone formation in FRC cultures in association with reduced mRNA expression of RUNX2, alkaline phosphatase, osteocalcin, and bone sialoprotein and reduced alkaline phosphatase activity. The effects of palmitate on osteogenic marker expression were inhibited by TOFA. Palmitate also inhibited the mRNA expression of fatty acid synthase and PPARγ in FRC cultures, and as with osteogenic markers, this effect was inhibited by TOFA. Palmitate had no effect on FRC cell proliferation or apoptosis, but inhibited BMP-7-induced alkaline phosphatase activity. We conclude that palmitate accumulation may lead to lipotoxic effects on osteoblast differentiation and mineralization and that increases in fatty acid oxidation may help to prevent these lipotoxic effects.

The role of the insulin‑like growth factor (IGF) axis in osteogenic and odontogenic differentiation

The insulin-like growth factor (IGF) axis is a multicomponent molecular network which has important biological functions in the development and maintenance of differentiated tissue function(s). One of the most important functions of the IGF axis is the control of skeletal tissue metabolism by the finely tuned regulation of the process of osteogenesis. To achieve this, the IGF axis controls the activity of several cell types—osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts to achieve the co-ordinated development of appropriate hard tissue structure and associated matrix deposition. In addition, there is an increasing awareness that the IGF axis also plays a role in the process of odontogenesis (tooth formation). In this review, we highlight some of the key findings in both of these areas. A further understanding of the role of the IGF axis in hard tissue biology may contribute to tissue regeneration strategies in cases of skeletal tissue trauma.

Protein kinase D1 is essential for bone acquisition during pubertal growth

Bone formation and maintenance represents the summation of the balance of local and endocrine hormonal stimuli within a complex organ. Protein kinase D (PKD) is a member of the Ca(2+)/calmodulin-dependent kinase superfamily of serine/threonine kinases and has been described as the crossroads for the bone morphogenetic protein (BMP)-IGF-I signaling axis, which plays a major role in bone formation. The current study exploits the PKD1-deficient mouse model to examine the role of PKD in vivo in the skeleton. Dual-energy x-ray absorptiometry scan analysis of male and female pubescent mice demonstrated significantly decreased bone mineral density in the whole body and femoral bone compartments of PKD1 (+/-) mice, compared with their wild-type littermates. The body weight, nasal-anal length, and percentage body fat of the mice were not significantly different from their wild-type littermates. Cultured bone marrow stromal cells from PKD1 (+/-) mice demonstrated lower alkaline phosphatase activity in early differentiating osteoblasts and decreased mineralized nodule formation in mature osteoblasts. Quantitative RT-PCR analysis of osteoblast differentiation markers and osteoclast markers exhibited lower levels of expression in PKD1 (+/-) male mice than wild type. In female mice, however, only markers of osteoblast differentiation were reduced. PKD1 (+/-) mice also demonstrated a profound reduction in mRNA expression levels of BMP type II receptor and IGF-I receptor and in BMP-7 responsiveness in vitro. Together these data suggest that in mice, PKD1 action contributes to the regulation of osteoblastogenesis by altering gene expression with gender-specific effects on osteoclastogenesis, subsequently affecting skeletal matrix acquisition during puberty.

Physiological functions of protein kinase D in vivo

The cellular functions of the serine/threonine protein kinase D (PKD) have been extensively studied within the last decade and distinct roles such as fission of vesicles at the Golgi compartment, coordination of cell migration and invasion, and regulation of gene transcription have been correlated with this kinase family. Here, we highlight the current state of in vivo studies on PKD function with a focus on animal models and discuss the molecular basis of the observed phenotypic characteristics associated with this kinase family.

Low-intensity pulsed ultrasound regulates proliferation and differentiation of osteoblasts through osteocytes

Low-intensity pulsed ultrasound (LIPUS) has been used as a safe and effective modality to enhance fracture healing. As the most abundant cells in bone, osteocytes orchestrate biological activities of effector cells via direct cell-to-cell contacts and by soluble factors. In this study, we have used the osteocytic MLO-Y4 cells to study the effects of conditioned medium from LIPUS-stimulated MLO-Y4 cells on proliferation and differentiation of osteoblastic MC3T3-E1 cells. Conditioned media from LIPUS-stimulated MLO-Y4 cells (LIPUS-Osteocyte-CM) were collected and added on MC3T3-E1 cell cultures. MC3T3-E1 cells cultured in LIPUS-Osteocyte-CM demonstrated a significant inhibition of proliferation and an increased alkaline phosphatase activity. The results of PGE(2) and NO assay showed that LIPUS could enhance PGE(2) and NO secretion from MLO-Y4 cells at all time points within 24h after LIPUS stimulation. We conclude that LIPUS regulates proliferation and differentiation of osteoblasts through osteocytes in vitro. Increased secretion of PGE(2) from osteocytes may play a role in this effect.