Platelet-Activating Factor Receptor Signals in Rat Osteoblasts during Spaceflight
The platelet-activating factor (PAF) is a lipid mediator. The G-protein-coupled receptor of PAF (PAF-R) is activated by inflammatory and stressful conditions in numerous cell types. PAF/PAF-R is involved in apoptotic and antiapoptotic processes. We examined microgravity effects on the expression of PAF-R and second messengers in rat osteoblasts. The PAF-R signals are transmitted via arachidonic acid, phospholipase C (PLC), protein kinase C (PKC), and mitogen-activated protein kinase. Rat osteoblasts were cultured for 4 and 5 days aboard a space shuttle and solubilized on board. PAF-R gene expression in flight cultures increased to 26-fold higher than in ground controls. Gene expression of the G-protein ? subunit G?q in flight cultures increased to 3-fold and higher than in ground controls. It is known that Gaq stimulates the effecter PLC?, activating PKC. The mRNA levels of PKC? and PKC? in flight cultures were increased to 25-fold higher than in ground controls. The PKC? mRNA level in flight cultures was increased to 3-fold higher than in ground controls on the 4th day. Gene expression of catalytic and regulatory subunits of protein kinase A was suppressed in flight cultures. PKC? and PKC? are novel PKCs that can be target substrates of caspases. The PAF-R gene may act as a mechano-sensitive gene that is involved in the apoptotic and antiapoptotic processes of osteoblasts under microgravity.
platelet-activating factor receptor osteoblast spaceflight g-protein protein kinase c apoptosis