Osteoblast-osteoclast coordination is critical in the maintenance of skeletal integrity. The modulation of osteoclastogenesis by immature cells of the osteoblastic lineage is mediated through receptor activator of NF kappa B (RANK), its ligand RANKL, and osteoprotegerin (OPG), a natural decoy receptor for RANKL. Here, the expression of OPG and RANKL in primary mouse osteoblastic cultures was investigated to determine whether the osteoclastogenic stimulus depended on the stage of osteoblastic differentiation and the presence of the calciotrophic hormone 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)). OPG mRNA expression was increased in osteoblastic cultures after the onset of mineralisation relative to less mature cultures, but did not alter in response to 1,25-(OH)(2)D(3) treatment. In contrast, basal RANK L mRNA expression did not change during differentiation but was significantly enhanced by 1,25-(OH)(2)D(3) treatment at all times. The stimulatory effects of 1,25-(OH)(2)D(3) on RANKL were lessened in more mature cultures, however. The RANKL/OPG ratio, an index of osteoclastogenic stimulus, was therefore increased by 1,25-(OH)(2)D(3) treatment at all stages of osteoblastic differentiation, but to a lesser degree in cultures after the onset of mineralisation. Thus the 1,25-(OH)(2)D(3)-driven increase in osteoclastogenic potential of immature osteoblasts appears to be mediated by increased RANKL mRNA expression, with mature osteoblasts having relatively decreased osteoclastogenic activity due to increased OPG mRNA expression. These findings suggest a possible mechanism for the recently proposed negative regulatory role of mature osteoblasts on osteoclastogenesis and indicate that the relative proportions of immature and mature osteoblasts in the local microenvironment may control the degree of resorption at each specific bone site.

Activins are growth and differentiation factors which have been shown to have proliferative and antiproliferative actions in many tissues. In addition, they have been implicated in tumourigenesis in reproductive tissues. Although activin and inhibin are present in rat ventral prostate, inhibin beta, but not alpha, subunit proteins have been detected in the human prostate epithelial tumour cell lines LNCaP, DU145 and PC3. With this absence of capacity to produce inhibins, the aims of this study were to determine the effect of activin A and B and follistatin on DNA synthesis by these human prostate tumour cell lines. The results demonstrate a differential response to exogenously added activin A and B on DNA synthesis in vitro by the tumour cell lines. The inhibitory effects were observed on LNCaP cells in the absence or presence of stimulation with 1 nM 5 alpha-dihydrotestosterone and on the androgen-independent DU145 cells, but not the PC3 cells. Activin A caused a dose-dependent inhibition of DNA synthesis and proliferation by LNCaP and androgen-independent DU145 cells which was maximal at 8 ng/ml. The effect of exogenously added activin A was completely reversed by follistatin, but not by inhibin A. The addition of human recombinant FS 288 alone (400 ng/ml) did not have any effect on DNA synthesis, whereas inhibin A alone (400 ng/ml) caused a significant inhibition of DNA synthesis. The capacity of all three cell lines to produce activins and follistatins was demonstrated by the expression of the mRNAs and confirmed by the localisation of immunoreactivity for these ligands to the cytoplasm of the tumour cells. The growth inhibitory response to activins A and B by LNCaP and DU145 cells, and the ability of follistatin to block these effects, suggest that the autocrine interactions between activins and follistatins have a role in the regulation of LNCaP and DU145 prostate tumour cell growth.