Kruppel-like factor 4 regulates matrix metalloproteinase and aggrecanase gene expression in chondrocytes

Deletion of Trps1 regulatory elements recapitulates postnatal hip joint abnormalities and growth retardation of Trichorhinophalangeal syndrome in mice

Trichorhinophalangeal syndrome (TRPS) is a genetic disorder caused by point mutations or deletions in the gene-encoding transcription factor TRPS1. TRPS patients display a range of skeletal dysplasias, including reduced jaw size, short stature, and a cone-shaped digit epiphysis. Certain TRPS patients experience early onset coxarthrosis that leads to a devastating drop in their daily activities. The etiologies of congenital skeletal abnormalities of TRPS were revealed through the analysis of Trps1 mutant mouse strains. However, early postnatal lethality in Trps1 knockout mice has hampered the study of postnatal TRPS pathology. Here, through epigenomic analysis we identified two previously uncharacterized candidate gene regulatory regions in the first intron of Trps1. We deleted these regions, either individually or simultaneously, and examined their effects on skeletal morphogenesis. Animals that were deleted individually for either region displayed only modest phenotypes. In contrast, the Trps1Δint/Δint mouse strain with simultaneous deletion of both genomic regions exhibit postnatal growth retardation. This strain displayed delayed secondary ossification center formation in the long bones and misshaped hip joint development that resulted in acetabular dysplasia. Reducing one allele of the Trps1 gene in Trps1Δint mice resulted in medial patellar dislocation that has been observed in some patients with TRPS. Our novel Trps1 hypomorphic strain recapitulates many postnatal pathologies observed in human TRPS patients, thus positioning this strain as a useful animal model to study postnatal TRPS pathogenesis. Our observations also suggest that Trps1 gene expression is regulated through several regulatory elements, thus guaranteeing robust expression maintenance in skeletal cells.

1α,25-Dihydroxyvitamin D3 Regulates microRNA Packaging in Extracellular Matrix Vesicles and Their Release in the Matrix

Growth plate chondrocytes are regulated by numerous factors and hormones as they mature during endochondral bone formation, including transforming growth factor beta-1 (TGFb1), bone morphogenetic protein 2 (BMP2), insulin-like growth factor-1 (IFG1), parathyroid hormone and parathyroid hormone related peptide (PTH, PTHrP), and Indian hedgehog (IHH). Chondrocytes in the growth plate's growth zone (GC) produce and export matrix vesicles (MVs) under the regulation of 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃]. 1α,25(OH)₂D₃ regulates MV enzyme composition genomically and 1α,25(OH)₂D₃ secreted by the cells acts on the MV membrane nongenomically, destabilizing it and releasing MV enzymes. This study examined the regulatory role 1α,25(OH)₂D₃ has over production and packaging of microRNA (miRNA) into MVs by GC cells and the release of miRNA by direct action on MVs. Costochondral cartilage GC cells were treated with 1α,25(OH)₂D₃ and the miRNA in the cells and MVs sequenced. We also treated MVs with 1α,25(OH)₂D₃ and determined if the miRNA was released. To assess whether MVs can act directly with chondrocytes and if this is regulated by 1α,25(OH)₂D₃, we stained MVs with a membrane dye and treated GC cells with them. 1α,25(OH)₂D₃ regulated production and packaging of a unique population of miRNA into MVs compared to the vehicle control population. 1α,25(OH)₂D₃ treatment of MVs did not release miRNA. Stained MVs were endocytosed by GC cells and this was increased with 1α,25(OH)₂D₃ treatment. This study adds new regulatory roles for 1α,25(OH)₂D₃ with respect to packaging and transport of MV miRNAs.

Pregnane X receptor (PXR) represses osteoblast differentiation through repression of the Hedgehog signaling pathway

The pregnane X receptor (PXR, NR1I2) belongs to the nuclear receptor family and functions as a xenobiotic and endobiotic sensor by binding to various molecules through its relatively flexible ligand-binding domain. In addition to these well-known canonical roles, we previously reported that PXR represses osteoblast differentiation. However, the mechanisms underlying the PXR-mediated repression of osteoblast differentiation remains unknown. In this study, we analyzed the changes in global gene expression profiles induced by PXR in calvarial osteoblasts cultured in standard fetal bovine serum (in which PXR induces repression of differentiation), and in those cultured in charcoal-stripped fetal bovine serum (in which PXR does not induce repression of differentiation). The comparison revealed that PXR attenuated the Hedgehog-mediated signaling in culture conditions that induced PXR-mediated repression of differentiation. Real-time PCR analysis showed that PXR repressed the Hedgehog signaling-induced genes such as Gli1 and Hhip, and conversely induced the Hedgehog signaling-repressed genes such as Cdon, Boc, and Gas1. Activation of Smo-mediated signaling in osteoblasts following treatment with a Smo agonist (SAG) significantly restored Gli-mediated transcriptional activity and osteoblast differentiation. Our results demonstrate the osteoblast-autonomous effects of PXR and identify a novel regulation of Hedgehog signaling by nuclear receptors.

The Role of HDACs and HDACi in Cartilage and Osteoarthritis

Epigenetics plays an important role in the pathogenesis and treatment of osteoarthritis (OA). In recent decades, HDAC family members have been associated with OA. This paper aims to describe the different role of HDACs in the pathogenesis of OA through interaction with microRNAs and the regulation of relevant signaling pathways. We found that HDACs are involved in cartilage and chondrocyte development but also play a crucial role in OA. However, the distinct HDAC mechanism in the pathogenesis and treatment of OA require further investigation. Furthermore, HDAC inhibitors (HDACi) can protect cartilage from disease, which may represent a potential therapeutic approach against OA.

Kruppel-Like Factor 4 represses osteoblast differentiation via ciliary Hedgehog signaling

Primary cilia are appendages observed in most types of cells, and serve as cellular antennae for sensing environmental signals. Evidence is accumulating that correct ciliogenesis and ciliary functions are indispensable for normal skeletal development by regulating signaling pathways important for bone development. However, whether ciliogenesis is regulated by bone-related factors in osteoblasts is largely unknown. Here we show that Kruppel-Like Factor 4 (KLF4), which is known to repress osteoblast differentiation, supports the formation and maintenance of cilia in cultured osteoblasts; however, the length of the cilia observed in KLF4-induced cells were significantly shorter compared to the control cells. Basal Hedgehog signaling was repressed by KLF4. Significantly, activating Hedgehog signaling using a Smoothened agonist significantly rescued osteoblast mineralization and osteoblastic gene expressions. Global gene expression analysis showed that KLF4 induced number of genes including the nuclear receptor, Pregnane X receptor (PXR), and PXR repressed calvarial osteoblast mineralization and repressed Gli1 expression similar as the effect observed by inducing KLF4. Our results implicate that KLF4 plays important roles for maintaining osteoblasts in an immature state by repressing basal activation of the Hedgehog signaling.

2 dyn/cm2 shear force upregulates kruppel-like factor 4 expression in human chondrocytes to inhibit the interleukin-1β-activated nuclear factor-κB

The shear force effect on human chondrocytes is time and magnitude dependent. Recently, kruppel-like factor (KLF) 4 has been identified as a pleiotropic protein and its activity in cells is dependent on different stimuli and/or cell types. The role of KLF4 in chondrocytes is still unclear and there has been no report determining whether shear force regulates KLF4 levels in chondrocytes. Hence, this study was carried out to investigate the role of KLF4 in human chondrocytes under shear force stimulation and the underlying mechanism. Human primary and SW1353 chondrocytes were used in this study. The shear forces at 2, 5, or 15 dyn/cm² intensity were applied to both types of human chondrocytes. The specific small interfering RNAs, activators, and inhibitors were used to study the detailed mechanism of shear force. The presented results showed that 2, but not 5 and 15, dyn/cm² shear force increases KLF4 expression in human primary and SW1353 chondrocytes. Extracellular signal-regulated kinase 5 induced peroxisome proliferator-activated receptor γ transcription activity to increase KLF4 transcription. Moreover, the KLF4 induction in human chondrocytes in response to 2 dyn/cm² shear force could attenuate interleukin (IL)-1β-stimulated nuclear factor-κB activation. These results elucidate the role of KLF4 in antagonizing the effect of IL-1β in human chondrocytes under 2 dyn/cm² shear force stimulation and provide a possible mechanism to demonstrate the protection of moderate forces or exercises in cartilage.

Kruppel-like factor 4 (KLF-4) plays a crucial role in simvastatin (SVT)-induced differentiation of rabbit articular chondrocytes

Simvastatin is a cholesterol-lowing reagent that is derived synthetically from the fermentation of Aspergillus terreus. Recently, SVT has been shown to possess a protective effect of chondrocytes. Kruppel-like factor 4 (KLF-4) is a zinc finger transcription factor that plays crucial roles during the development and maintenance of multiple organs. However, the roles of KLF-4 in chondrocytes have not been well unknown. Here, we investigated whether KLF-4 regulates SVT-caused differentiated phenotype of chondrocytes. A KLF-4 cDNA or KLF-4 siRNA was transfected into SVT-treated chondrocytes. Western blot analysis, RT-PCR and immunofluorescence staining analyzed expression of type II collagen and SOX-9, marker proteins of differentiation. The results showed overexpression of KLF-4 accelerates SVT-induced type II collagen expression, as determined by western blot analysis and causes sulfated-proteoglycan synthesis, as detected by Alcian blue staining. RT-PCR revealed that ectopic expression of KLF-4 induces SVT-caused SOX-9, a transcription factor of type II collagen, expression. Transfection of KLF-4 siRNA reversed SVT-caused type II collagen and SOX-9 expression and inhibited SVT-induced sulfated proteoglycan production. This study indicates that KLF-4 plays critical role in SVT-caused chondrocytes differentiation.