cGMP produced in response to ANP and CNP regulates proliferation and differentiation of osteoblastic cells.

The effects of natriuretic peptides on the proliferation and differentiation of osteoblast-like cells from rat calvariae were examined. Natriuretic peptides are physiological agonists that activate receptor guanylate cyclases, namely, natriuretic peptide receptor (NPR)-A and NPR-B. Exposure of cells to atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) resulted in large increases in the rate of intracellular production of guanosine 3',5'-cyclic monophosphate (cGMP). Moreover, CNP-like immunoreactivity was detected in the conditioned medium from osteoblast-like cells, while ANP was undetectable. In cells exposed to natriuretic peptides, a dose-dependent reduction in the rate of DNA synthesis was observed. Natriuretic peptides also stimulated the activity of alkaline phosphatase (ALPase) and the expression of mRNA for ALPase and osteocalcin and the mineralization of nodules by the cultured cells. These results could be reproduced by treating cells with 8-bromo-cGMP. Endothelin-1, whose physiological functions are the opposite of those of natriuretic peptides, decreased the ALPase activity and the mineralization of nodules. In the present study, natriuretic peptides were demonstrated to promote bone formation via the action of cGMP in a signal-transduction pathway mediated by specific receptors in osteoblast-like cells.