Estrogen inhibition of PTH-stimulated osteoclast formation and attachment in vitro: involvement of both PKA and PKC.

Estrogens modulate the catabolic effects of PTH on bone in vivo and in vitro. PTH-stimulated cAMP accumulation in osteoblasts is thought to be linked to increased osteoclastic activity, but the precise mechanism is still unknown. In cocultures of clonal marrow stromal cells (MS1) and normal mouse spleen cells, both 1,25-dihydroxyvitamin D3 and rat PTH (rPTH)-(1-34) can induce the formation of tartrate-resistant acid phosphatase- and calcitonin receptor-positive multinucleated osteoclast-like cells, which can attach to dentine slices and produce resorption pits. In this system, osteoclastogenesis stimulated by PTH, but not by 1,25-dihydroxyvitamin D3, was suppressed by 17beta-E2 (10(-10)-10(-8) M), whereas 17alpha-E2 (10(-8) M) had no effect. Exposure to 10(-8) M 17beta-E2, but not 17alpha-E2, also significantly decreased the PTH-induced attachment of osteoclast-like cells to dentine slices. 17beta-E2 inhibited osteoclast-like cell formation induced by 8-bromo-cAMP (10(-4) M), 12-O-tetradecanoylphorbol 13-acetate (10(-8) M), or rat PTH-(1-34) (10(-7) M) in combination with either rp-adenosine-3',5'-cyclic monophosphorothioate (10(-4) M) or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (10(-5) M). 17beta-E2 suppressed the partial stimulation of tartrate-resistant acid phosphatase-positive multinucleated osteoclast-like cell formation induced by [Arg(2)]human (h) PTH-(1-34) (10(-7) M) or hPTH-(3-34) (10(-7) M), but not that caused by 10(-7) M hPTH-(53-84). We conclude that estrogens suppress PTH-stimulated osteoclast-like cell formation by blocking both the cAMP-dependent PKA pathway and the PLC-coupled calcium/PKC pathway. In addition to inhibiting formation of osteoclasts and promoting their apoptosis, estrogen may regulate bone resorption by blocking attachment of osteoclasts to bone.