Bisphosphonates and connexin 43: a critical review of evidence

Local application of zoledronate inhibits early bone resorption and promotes bone formation

Nonunion resulting from early bone resorption is common after bone transplantation surgery. In these patients, instability or osteoporosis causes hyperactive catabolism relative to anabolism, leading to graft resorption instead of fusion. Systemic zoledronate administration inhibits osteoclastogenesis and is widely used to prevent osteoporosis; however, evidence on local zoledronate application is controversial due to osteoblast cytotoxicity, uncontrolled dosing regimens, and local release methods. We investigated the effects of zolendronate on osteoclastogenesis and osteogenesis and explored the corresponding signaling pathways. In vitro cytotoxicity and differentiation of MC3T3E1 cells, rat bone marrow stromal cells (BMSCs) and preosteoclasts (RAW264.7 cells) were evaluated with different zolendronate concentrations. In vivo bone regeneration ability was tested by transplanting different concentrations of zolendronate with β-tricalcium phosphate (TCP) bone substitute into rat femoral critical-sized bone defects. In vitro, zolendronate concentrations below 2.5 × 10-7 M did not compromise viability in the three cell lines and did not promote osteogenic differentiation in MC3T3E1 cells and BMSCs. In RAW264.7 cells, zoledronate inhibited extracellular regulated protein kinases and c-Jun n-terminal kinase signaling, downregulating c-Fos and NFATc1 expression, with reduced expression of fusion-related dendritic cell‑specific transmembrane protein and osteoclast-specific Ctsk and tartrate-resistant acid phosphatase (. In vivo, histological staining revealed increased osteoid formation and neovascularization and reduced fibrotic tissue with 500 μM and 2000 μM zolendronate. More osteoclasts were found in the normal saline group after 6 weeks, and sequential osteoclast formation occurred after zoledronate treatment, indicating inhibition of bone resorption during early callus formation without inhibition of late-stage bone remodeling. In vivo, soaking β-TCP artificial bone with 500 μM or 2000 μM zoledronate is a promising approach for bone regeneration, with potential applications in bone transplantation.

Cx43 deficiency confers EMT-mediated tamoxifen resistance to breast cancer via c-Src/PI3K/Akt pathway

Tamoxifen (TAM) resistance has indicated a significant challenge during endocrine therapy for hormone-sensitive breast cancer. Thus, it is significant to elucidate the molecular events endowing TAM resistance to endocrine therapy. In this study, we found that epithelial-mesenchymal transition (EMT) was an important event to confer TAM resistance, and attenuating EMT by elevating connexin (Cx) 43 expression could reverse TAM resistance. Specifically, Cx43 overexpression improved TAM sensitivity, while Cx43 depletion facilitated TAM insensitivity by modulating EMT in T47D TAM-resistant and -sensitive cells, and transplanted xenografts. Importantly, we found a novel reciprocal regulation between Cx43 and c-Src/PI3K/Akt pathway contributing to EMT and TAM resistance in breast cancer. Moreover, we identified that Cx43 deficiency was significantly correlated with poor relapse-free survival in patients undergoing TAM treatment. Therefore, Cx43 represents a prognostic marker and an attractive target for breast cancer treatments. Therapeutic strategies designed to increase or maintain Cx43 function may be beneficial to overcome TAM resistance.

Lineage-associated connexin 43 expression in bisphosphonate-exposed rat bones

Expression of signaling proteins in bone cells depends on their embryological mesoderm-derived (e.g. tibia) or cranial neural crest (CNC)-derived (e.g. jaw) origin. Connexin 43 (Cx43) is a gap junction protein that plays an essential role in the mode of action of bisphosphonates (BP). This study aimed to investigate Cx43 expression and the influence of BP application on mesoderm- and CNC-derived bone. Using a rat model, molar extraction and tibia osteotomy with (Group 4) or without (Group 3) previous BP application was performed. Untreated (Group 1) and animals selectively treated with BPs (Group 2) served as controls. Cx43 expression was immunohistochemically determined 12 and 16 weeks postoperatively via a labeling index. Cx43 expression in CNC-derived bone was significantly higher compared with mesodermal bone. BP application decreased Cx43 expression; however, detected expression levels were still higher in jawbone (Group 2 tibia vs jaw: 5.83 ± 5.06 vs 23.52 ± 6.42; p = 0.007). During bone healing after surgical intervention (Group 3) there were no expression differences between tibia and jawbone. BP treatment prior to surgery resulted in significantly lower Cx43 expression in CNC-derived compared with tibia bone (Group 4 tibia vs jaw: 56.84 ± 15.57 vs 16.40 ± 5.66; p < 0.01). Increased Cx43 expression in jaw compared with tibia bone is in line with their embryological origins. A significant Cx43 suppression in jawbone after BP application and surgery might contribute to the selectively altered osseous turnover and development of MRONJ in CNC-derived bone.

Association between bisphosphonate therapy and outcomes from rehabilitation in older people

BACKGROUND

Bisphosphonate therapy may have actions beyond bone, including effects on cardiovascular, immune and muscle function. We tested whether bisphosphonate treatment is associated with improved outcomes in older people undergoing inpatient rehabilitation.

METHODS

Analysis of prospectively collected, linked routine clinical datasets. Participants were divided into never users of bisphosphonates, use prior to rehabilitation only, use after rehabilitation only, and current users (use before and after rehabilitation). We calculated change in 20-point Barthel scores during rehabilitation, adjusting for comorbid disease and laboratory data using multivariable regression analysis. Cox regression analyses were performed to analyse the association between bisphosphonate use and time to death or hospitalisation.

RESULTS

2797 patients were included in the analysis. Current bisphosphonate users showed greater improvement in Barthel score during rehabilitation than non-users (5.0 points [95%CI 4.3-5.7] vs 3.8 [95%CI 3.6-3.9]), but no difference compared to those receiving bisphosphonates only after discharge (5.1 [95%CI 4.6-5.5]). Previous bisphosphonate use was significantly associated with time to death (adjusted hazard ratio 1.41 [95%CI 1.15-1.73]) but less strongly with time to combined endpoint of hospitalisation or death (adjusted hazard ratio 1.18 [95%CI 0.98-1.48]). Use after discharge from rehabilitation was associated with reduced risk of death (adjusted hazard ratio 0.64 [95%CI 0.55-0.73]; hazard ratio per year of bisphosphonate prescription 0.98 [95%CI 0.97-0.99]).

CONCLUSION

Bisphosphonate use is unlikely to be causally associated with improved physical function in older people, but continuing use may be associated with lower risk of death.

The anti-metastatic micro-environment of the bone: Importance of osteocyte Cx43 hemichannels

Bone metastases of tumor cells are a common and life-threatening feature of a variety of late-stage cancers, including breast cancers. However, until now, much less has been known about the intrinsic anti-metastatic properties of the bones and how these could be exploited to prevent or treat bone metastases. Very recently, native Cx43 hemichannels present in osteocytes have been identified as important anti-metastatic signaling complexes by establishing high local extracellular ATP levels. Moreover, bisphosphonate drugs, applied as adjuvant therapies in the treatment of breast cancer patients and bone diseases, are known to display anti-metastatic properties. Now, it became clear that these compounds exert their effects through osteocyte Cx43 hemichannels, thereby triggering their opening and promoting ATP release in the extracellular micro-environment. Hence, endogenous osteocyte Cx43 hemichannels emerge as important and promising therapeutic targets for the prevention of bone metastases and/or clinical treatment of bone-metastasized breast cancers.

ZO-1 expression shows prognostic value in chronic B cell leukemia

Connexin-mediated gap junctions are vital for tumor cell function. Intracellular pathways of connexin signaling use Zonula Occludens protein-1 (ZO-1) as an intermediate. This report describes the ZO-1 and connexin 43 (Cx43) expression pattern in lymphocytes from chronic B-cell leukemia (B-CLL) patients. The ZO-1 and Cx43 expression in the B cells of 113 B-CLL patients was identified. Western blot and flow cytometry were used to determine protein expression. Results indicated that ZO-1 and Cx43 expression was reduced and correlated negatively with CD38 and Zap-70 expression. Inhibition of intercellular communication with anti-Cx43 antibodies, 1-octanol, or carbenoxolone resulted in induced cell apoptosis. These data suggest that ZO-1, along with CD38 and Zap-70, plays a role in cell cycle regulation in B-CLL and may be used as a prognostic marker in B-CLL monitoring.