Estrogen via the Estrogen Receptor Blocks cAMP‐Mediated Parathyroid Hormone (PTH)‐Stimulated Osteoclast Formation

Abstract Several lines of evidence indicate that estrogen inhibits parathyroid hormone (PTH)‐induced bone resorption in vivo and in vitro. However, its precise mechanism remains unknown. The present study was performed to investigate whether osteoclast precursor cells possess the receptors for PTH/PTH‐related protein (PTHrP) and/or estrogen and to clarify the mechanism by which estrogen affects PTH‐induced osteoclast‐like cell (Ocl) formation. The polymerase chain reaction (PCR) product corresponding in size to the mouse PTH/PTHrP receptor cDNA was detected in mouse hemopoietic blast cells supported by granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) as well as in osteoblastic MC3T3‐E1 cells. The nucleotide sequence of the PTH/PTHrP receptor PCR product of hemopoietic blast cells was found to be 95.4% identical to that of PTH/PTHrP receptor cDNA of rat osteoblastic ROS cells. The PCR product corresponding in size to the mouse estrogen receptor cDNA was detected in mouse hemopoietic blast cells supported by GM‐CSF as well as in MC3T3‐E1 cells. The nucleotide sequence of the estrogen receptor PCR product of hemopoietic blast cells was completely identical to that of mouse estrogen receptor cDNA. 17β‐estradiol (17β‐E 2 ) but not 17α‐E 2 dose dependently antagonized Ocl formation stimulated by human (h) PTH(1–34) at a minimal effective concentration of 10 −10 M in the hemopoietic blast cell culture. 17β‐E 2 also significantly inhibited Ocl formation stimulated by 10 −8 M hPTHrP(1–34), while it did not affect 1,25‐dihydroxyvitamin D 3 –induced Ocl formation. However, 10 −8 M 17β‐E 2 significantly inhibited Ocl formation stimulated by dibutyryladenosine cAMP (10 −4 M) and Sp‐cAMPS (10 −4 M), an activator of cAMP‐dependent protein kinase (PKA) as well as forskolin (10 −5 M). In contrast, 17β‐E 2 did not affect Ocl formation by either phorbol myristate acetate (10 −7 M), an activator of protein kinase C (PKC), or A23187 (10 −7 M), a calcium ionophore. The pretreatment with 17β‐E 2 significantly inhibited Ocl formation induced by the combined treatment with PTH and PKC inhibitors (H7 or staurosporine), while it did not affect Ocl formation stimulated by the combined treatment with PTH and Rp‐cAMPS, a PKA inhibitor. The present data indicate that estrogen inhibits PTH‐stimulated Ocl formation by directly acting on hemopoietic blast cells, possibly through blocking a PKA pathway but not a calcium/PKC pathway.