The connexin 43 (Cx43) hemichannel (HC) in the mechanosensory osteocytes is a major portal for the release of factors responsible for the anabolic effects of mechanical loading on bone formation and remodeling. However, little is known about how the Cx43 molecule responds to mechanical stimulation leading to the opening of the HC. Here, we demonstrate that integrin α5β1 interacts directly with Cx43 and that this interaction is required for mechanical stimulation-induced opening of the Cx43 HC. Direct mechanical perturbation via magnetic beads or conformational activation of integrin α5β1 leads to the opening of the Cx43 HC, and this role of the integrin is independent of its association with an extracellular fibronectin substrate. PI3K signaling is responsible for the shear stress-induced conformational activation of integrin α5β1 leading to the opening of the HC. These results identify an unconventional function of integrin that acts as a mechanical tether to induce opening of the HC and provide a mechanism connecting the effect of mechanical forces directly to anabolic function of the bone.

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Gap junctional intercellular communication (GJIC) mediated by connexins, in particular connexin 43 (Cx43), plays important roles in regulating signal transmission among different bone cells and thereby regulates development, differentiation, modeling and remodeling of the bone. GJIC regulates osteoblast formation, differentiation, survival and apoptosis. Osteoclast formation and resorptive ability are also reported to be modulated by GJIC. Furthermore, osteocytes utilize GJIC to coordinate bone remodeling in response to anabolic factors and mechanical loading. Apart from gap junctions, connexins also form hemichannels, which are localized on the cell surface and function independently of the gap junction channels. Both these channels mediate the transfer of molecules smaller than 1.2 kDa including small ions, metabolites, ATP, prostaglandin and IP3. The biological importance of the communication mediated by connexin-forming channels in bone development is revealed by the low bone mass and osteoblast dysfunction in the Cx43-null mice and the skeletal malformations observed in occulodentodigital dysplasia (ODDD) caused by mutations in the Cx43 gene. The current review summarizes the role of gap junctions and hemichannels in regulating signaling, function and development of bone cells.

Gap junction intercellular communication in osteocytes plays an important role in bone remodeling in response to mechanical loading; however, the responsible molecular mechanisms remain largely unknown. Here, we show that phosphoinositide-3 kinase (PI3K)/Akt signaling activated by fluid flow shear stress and prostaglandin E 2 (PGE 2 ) had a stimulatory effect on both connexin 43 (Cx43) mRNA and protein expression. PGE 2 inactivated glycogen synthase kinase 3 (GSK-3) and promoted nuclear localization and accumulation of ␤-catenin. Knockdown of ␤-catenin expression resulted in a reduction in Cx43 protein. Furthermore, the chromatin immunoprecipitation (ChIP) assay demonstrated an association of ␤-catenin with the Cx43 promoter, suggesting that ␤-catenin could regulate Cx43 expression at the level of gene transcription. We have previously reported that PGE 2 activates cyclic AMP (cAMP)-protein kinase A (PKA) signaling and increases Cx43 and gap junctions. Interestingly, the activation of PI3K/Akt appeared to be independent of the activation of PKA, whereas both PI3K/Akt and PKA signaling inactivated GSK-3 and increased ␤-catenin translocation. Together, these results suggest that shear stress, through PGE 2 release, activates both PI3K/Akt and cAMP-PKA signaling, which converge through the inactivation of GSK-3, leading to the increase in nuclear accumulation of ␤-catenin. ␤-Catenin binds to the Cx43 promoter, stimulating Cx43 expression and functional gap junctions between osteocytes.

Purpose: While effective targeted therapies exist for estrogen receptor-positive and HER2-positive breast cancer, no such effective therapies exist for triple-negative breast cancer (TNBC); thus, it is clear that additional targets for radiosensitization and treatment are critically needed.Experimental Design: Expression microarrays, qRT-PCR, and Western blotting were used to assess MELK RNA and protein expression levels. Clonogenic survival assays were used to quantitate the radiosensitivity of cell lines at baseline and after MELK inhibition. The effect of MELK knockdown on DNA damage repair kinetics was determined using gH2AX staining. The in vivo effect of MELK knockdown on radiosensitivity was performed using mouse xenograft models. Kaplan-Meier analysis was used to estimate local control and survival information, and a Cox proportional hazards model was constructed to identify potential factors impacting local recurrence-free survival.