No abstract

Periodontal ligament fibroblasts (PLF) sense and respond to mechanical stimuli and participate in alveolar bone resorption during orthodontic treatments. This study examined how PLF influence osteoclastogenesis from bone marrow-derived macrophages (BMM) after application of tension or compression force. We also investigated whether lymphocytes could be a primary stimulator of osteoclastic activation during alveolar bone remodeling. We found that mechanical forces inhibited osteoclastic differentiation from BMM in co-cultures with PLF, with PLF producing predominantly osteoprotegerin (OPG) rather than receptor activator of nuclear factor-kappaB (NF-κB) ligand (RANKL). In particular, PLF increased the expression of tumor necrosis factor (TNF)-α in response to compression. Additional experiments showed the presence of CD4- and B220-positive cells with a subsequent increase in tartrate-resistant acid phosphatase (TRAP)-positive cells and RANKL expression only at the compression side of the force-subjected periodontal tissues. Exogenous TNF-α increased the number of TRAP-positive cells and pit formation in the co-cultures of BMM with Jurkat, but not with BJAB cells and this effect was almost completely inhibited by antibodies to TNF-α or TNF receptor. Collectively, the current findings suggest that PLF secrete relatively higher levels of TNF-α at the compression side than at the tension side and this imbalance leads to RANKL expression by activating CD4+ T cells, thereby facilitating bone resorption during orthodontic tooth movement.

Type I collagen (COL I) is the predominant collagen in the extracellular matrix of periodontal ligament (PDL), and its expression in PDL fibroblasts (PLF) is sensitive to mechanical force. However, the mechanism by which PLF induces COL I to respond to mechanical force is unclear. This study examined the nature of human PLF in mediating COL I expression in response to centrifugal force. Signal transduction pathways in the early stages of mechanotransduction involved in the force-driven regulation of COL I expression were also investigated. Centrifugal force up-regulated COL I without cytotoxicity and activated extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 kinase. ERK and JNK inhibitor blocked the expression of COL I but p38 kinase inhibitor had no effect. Centrifugal force activated activator protein-1 (AP-1) through dimerization between c-Fos and c-Jun transcription factors. ERK and JNK inhibitors also inhibited AP-1-DNA binding, c-Fos nuclear translocation, and c-Jun phosphorylation that were increased in the force-exposed PLF. Further, transfecting the cells with c-Jun antisense oligonucleotides almost completely abolished the force-induced increase of c-Jun phosphorylation and COL I induction. Our findings suggest that mechanical signals are transmitted into the nucleus by ERK/JNK signaling pathways and then stimulate COL I expression through AP-1 activation in force-exposed human PLF.

Purinergic receptors of the P2Y family are G protein-coupled surface receptors that respond to extracellular nucleotides and can mediate responses to local cell damage. P2Y-dependent signaling contributes to thrombotic and/or inflammatory consequences of tissue injury by altering platelet and endothelial activation and immune cell phagocytosis. Here, we have demonstrated that P2Y 14 modifies cell senescence and cell death in response to tissue stress, thereby enabling preservation of hematopoietic stem/progenitor cell function. In mice, P2Y 14 deficiency had no demonstrable effect under homeostatic conditions; however, radiation stress, aging, sequential exposure to chemotherapy, and serial bone marrow transplantation increased senescence in animals lacking P2Y 14 . Enhanced senescence coincided with increased ROS, elevated p16 INK4a expression, and hypophosphorylated Rb and was inhibited by treatment with a ROS scavenger or inhibition of p38/MAPK and JNK. Treatment of WT cells with pertussis toxin recapitulated the P2Y 14 phenotype, suggesting that P2Y 14 mediates antisenescence effects through Gi/o protein-dependent pathways. Primitive hematopoietic cells lacking P2Y 14 were compromised in their ability to restore hematopoiesis in irradiated mice. Together, these data indicate that P2Y 14 on stem/progenitor cells of the hematopoietic system inhibits cell senescence by monitoring and responding to the extracellular manifestations of tissue stress and suggest that P2Y 14 -mediated responses prevent the premature decline of regenerative capacity after injury.

Hypoxia alters the biological functions of skeletal muscle cells to proliferate and differentiate into myotubes. However, the cellular responses of myoblasts to hypoxia differ according to the levels of oxygen and the types of cells studied. This study examined the effect of hypoxia (1% oxygen) on bovine satellite cells. Hypoxia significantly increased the proliferation of satellite cells cultured in a growth medium. In addition, the levels of PCNA, cyclin D1, cyclin-dependent kinase-1 (CDK1) and CDK2 expression were increased. Hypoxia facilitated the formation of myotubes as well as the stimulation of MyoD, myogenin, and myosin heavy chain (MHC) expression in differentiating medium (DM) cultures. In particular, satellite cells cultured under hypoxic/DM conditions showed increased p21 expression but not p27. The transfection of satellite cells with antisense MyoD oligonucleotides resulted in a decrease in the MHC, myogenin, MRF4 RNA and protein levels with the concomitant decrease in fused cells to levels similar to those observed under normoxia/DM conditions. This indicates that MyoD up-regulation is closely associated with hypoxia-stimulated myogenic differentiation. In conclusion, hypoxia stimulates the proliferation of satellite cells and promotes their myogenic differentiation with MyoD playing an important role.