C-type natriuretic peptide/guanylate cyclase B system in rat osteogenic ROB-C26 cells and its down-regulation by dexamethazone

There is recent evidence that natriuretic peptides are important regulators of bone and cartilage, although they were originally identified as the car diac hormones causing natriuresis and hypotension. Three members of natriur etic peptide family are known: atrial natriuretic peptide (ANP), brain natr iuretic peptide (BNP), and C-type natriuretic peptide (CNP). The biological ly active receptors for these peptides are particulate guanylate cyclases; the two known types are GC-A and GC-B. ANP and BNP have high affinities for GC-A, and CNP is the preferred Ligand for GC-B. In this paper we report th e results of our study of the expression and possible role(s) of natriureti c peptides in the ROB-C26 cell, which is an osteogenic cell Line with multi ple potentials for differentiating into myoblast, osteoblast, and adipocyte . ROB-C26 cells produced cGMP in response to natriuretic peptides at both t heir basal state and after enhanced differentiation into osteoblast which w as induced by bone morphogenetic protein [(BMP)-2]. CNP was far more patent than ANP in cGMP production. In contrast, enhanced differentiation into ad ipocyte by dexamethasone resulted in the marked decrease in their responsiv eness to natriuretic peptides. Although the messages for GC-A and GC-B were demonstrated by Northern blot analysis at both the basal stage and after B MP treatment, they were down-regulated after dexamethasone treatment. The p resence of CNP was shown by RT-PCR and immunohistochemistry in ROB-C26 cell s. C3H10T1/2, which is another and more primitive mesenchymal cell line, al so produced cGMP in response to CNP, and less potently to ANP. Culturing-RO B-C26 cells with CNP or X-bromo cGMP decreased [H-3]thymidine uptake and sl ightly increased the message for alkaline phosphatase, which is a marker fo r osteoblast differentiation. These results suggest that the CNP/GC-B syste m is preferentially expressed in the cells of osteogenic lineage and their expression is down-regulated with differentiation into adipocyte lineage. T he CNP/GC-B system is likely to be an autocrine/paracrine regulator of oste oblast growth and differentiation.