Strontium Promotes Transforming Growth Factors β1 and β2 Expression in Rat Chondrocytes Cultured In Vitro

Calcitriol-Loaded Multifunctional Nanospheres with Superlubricity for Advanced Osteoarthritis Treatment

Osteoarthritis (OA) is characterized by the lubrication dysfunction of a cartilage sliding interface caused by chronic joint inflammation, and effective nonsurgical therapy for advanced OA remains lacking. Addressing chronic joint inflammation, lubrication dysfunction, and cartilage-tissue degradation simultaneously may hopefully tackle this challenge. Herein, we developed superlubricative zein@alginate/strontium@calcitriol (ZASC) nanospheres to treat advanced OA. ZASC was confirmed to significantly improve joint lubrication through traditional tribological tests and our proposed tribological experiment to mimic the intra-articular condition based on the human medial tibiofemoral joint tissues. This finding was attributed to the hydration lubrication formed around the alginate-strontium spheres that enabled ball-bearing lubrication and the filling of cartilage defects. Moreover, ZASCs that released calcitriol in a sustained manner showed proliferative, anti-inflammatory, and anti-apoptosis effects in vitro. Further experiments demonstrated that ZASC exerted chondroprotective effects by inhibiting the breakdown of the extracellular matrix in patient-derived OA cartilage explants. In vivo results demonstrated that ZASC can effectively maintain a normal gait to improve joint function, inhibit abnormal bone remodeling and cartilage degradation in early OA and can effectively reverse the advanced OA progression. Therefore, ZASC is a potentially nonsurgical therapeutic strategy for advanced OA treatments.

Transcriptomic and Proteomic Analyses Reveal New Insights into Regulatory Mechanisms of Strontium in Bovine Chondrocytes

Strontium (Sr) is a trace element found mainly in bone, and it performs a dual action by promoting bone formation and inhibiting bone resorption. Sr has been used to evaluate the gastrointestinal calcium (Ca) absorption capacity of dairy cows due to the similar physicochemical properties of the two elements. However, the possible effects of Sr on dairy cows remain unclear. This study aimed to explore the potential regulatory mechanism of Sr in bovine chondrocytes by performing transcriptomic and proteomic analyses. A total of 111 genes (52 up-regulated and 59 down-regulated) were identified as significantly altered (1.2-fold change and p < 0.05) between control and Sr-treated groups. Moreover, LC-MS-based proteomic analysis detected 286 changed proteins (159 up-regulated and 127 down-regulated) between the control and Sr-treated groups (1.2-fold change and p < 0.05). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations of a combination analysis of the transcriptomic and proteomic data revealed that the genes were predominantly involved in chondrocyte proliferation and differentiation, fat metabolism, the inflammation process, and immune responses. Overall, our data reveal a potential regulatory mechanism of strontium in bovine chondrocytes, thus providing further insights into the functions and application of Sr in ruminants.

Strontium Regulates the Proliferation and Differentiation of Isolated Primary Bovine Chondrocytes via the TGFβ/SMAD Pathway

The present study evaluated the effects of strontium (Sr) on proliferation and differentiation of chondrocytes isolated from dairy cows, and whether Sr exerts its effects via transforming growth factor β (TGFβ) signaling. The chondrocytes were isolated from patellar cartilage from newborn Holstein bull calves (n = 3, 1 day old, 38.0 ± 2.8 kg, fasting) within 15 min after euthanasia, and treated with different concentrations of Sr (0, 0.1, 1, and 10 μg/ml, as SrCl₂·6H₂O). After pretreatment with or without activin receptor-like kinase 5 (ALK5) inhibitor (10 μM SB-505124) for 4 h, chondrocytes were incubated with Sr for another 4 h. Overall effects of Sr were evaluated relative to NaCl as the control. In contrast, the 1 μg/ml Sr-treated group served as the control to determine effects of preincubating with SB-505124. Western blot and qRT-PCR were used for measuring expression of proliferation-, differentiation-, and TGFβ1-responsive factors. Data were analyzed using one-way ANOVA in GraphPad Prism 7.0. Incubation with all doses of Sr increased TGFβ1/ALK5-induced SMAD3 phosphorylation, and at 10 μg/ml it inhibited ALK1-induced SMAD1/5/9 phosphorylation. Expression of mRNA and protein of the proliferation-responsive factors type Ⅱ Collagen α1 (COL2A1) and aggrecan (ACAN) was induced by Sr at 1 μg/ml. In contrast, Sr at 10 μg/ml inhibited the expression of differentiation-responsive factors type Ⅹ Collagen α1 (COL10A1) and secreted phosphoprotein 1 (SPP1), and at 1 μg/ml it had the same effect on alkaline phosphatase (ALPL) mRNA and protein levels. Cells were stained with PI/RNase Staining buffer to assess cell cycle activity using flow-cytometry. Incubation with Sr at 1 and 10 μg/ml induced an increase in the number of cells in the S-phase, leading to an increase in the proliferation index. Incubation with SB-505124 inhibited phosphorylation of SMAD3. Abundance of ACAN and COL2A1 mRNA and protein was lower when cells were pre-incubated with SB-505124. Overall, data indicated that Sr promotes proliferation and inhibits differentiation of primary chondrocytes by directing TGFβ1 signaling towards SMAD3 phosphorylation rather than SMAD1/5/9 phosphorylation. Whether these effects occur in vivo remains to be determined and could impact future application of Sr as an experimental tool in livestock.

Strontium stimulates alkaline phosphatase and bone morphogenetic protein-4 expression in rat chondrocytes cultured in vitro

The trace element strontium has a significant impact on cartilage metabolism. However, the direct effects of strontium on alkaline phosphatase (ALP), a marker of bone growth, and bone morphogenetic protein-4 (BMP-4), which plays a key role in the regulation of bone and cartilage development, are not entirely clear. In order to understand the mechanisms involved in these processes, the chondrocytes were isolated from Wistar rat articular cartilage by enzymatic digestion and cultured under standard conditions. They were then treated with strontium at 0.5, 1.0, 2.0, 5.0, 20.0 and 100.0 μg/mL for 72 h. The mRNA abundance and protein expression levels of ALP and BMP-4 were measured using real-time polymerase chain reaction (real-time PCR) and Western blot analysis. The results showed that the levels of expression of ALP and BMP-4 in chondrocytes increased as the concentration of strontium increased relative to the control group, and the difference became significant at 1.0 μg/mL strontium (P<0.05). These results indicated that strontium could be involved in cartilage development via regulating ALP and BMP-4 expression.