EXPRESSION OF ANNEXIN-I, ANNEXIN-II, ANNEXIN-V, AND ANNEXIN-VI BY RATOSTEOBLASTS IN PRIMARY CULTURE - STIMULATION OF ANNEXIN-I EXPRESSION BY DEXAMETHASONE

To determine whether rat osteoblasts synthesize proteins of the annexi n family and to evaluate the extent to which glucocorticoids modulate the expression of annexins by these cells, osteoblasts were grown in p rimary cultures in the absence or presence of dexamethasone, and the e xpression of annexins was evaluated by immunoblotting using polyclonal antibodies against human annexins. Four different annexins (I, II, V, and VI) were found to be expressed by rat osteoblasts. The expression of annexin I, but not the other annexins studied, was increased in os teoblasts cultured in the presence of dexamethasone (173 +/- 33% incre ase comparing untreated cells and cells treated for 10 days with 5 x 1 0(-7) M dexamethasone). Increased expression of annexin I was observed after the third day' of exposure to dexamethasone and rose thereafter until day 10; annexin I expression increased with dexamethasone conce ntrations above 10(-10) M throughout the range of concentrations studi ed. The increase in annexin I protein was associated with an increase in annexin I mRNA and was completely blocked by the concomitant additi on of the glucocorticoid receptor antagonist RU 38486. The increase in annexin I content following dexamethasone treatment was associated wi th an increase in alkaline phosphatase activity and PTH-induced cAMP s timulation, whereas phospholipase A2 activity in the culture medium wa s reduced to undetectable levels. The finding that four annexins are e xpressed in rat osteoblasts in primary culture raises the possibility that these proteins could play an important role in bone formation by virtue of their ability to bind calcium and phospholipids, serve as Ca 2+ channels, interact with cytoskeletal elements, and/or regulate phos pholipase A2 activity. In addition, the dexamethasone-induced increase in annexin I may represent a mechanism by which glucocorticoids modif y osteoblast function.