Second messenger signaling in the regulation of collagenase production by osteogenic sarcoma cells

Recent work indicates that PTH can stimulate osteoblastic cells to secrete neutral collagenase, an enzyme thought to be linked to bone matrix turnover. Since recent studies suggest that the calcium/protein kinase-C (PKC) message system is involved in signal transduction stimulated by PTH, we examined the role of these putative second messengers of PTH in the regulation of collagenase production by the osteoblastic tumor cell line UMR 106-01. Immunohistochemical staining of cells exposed to PTH (10^-7 M) revealed that about 20% of the entire population was positive for collagenase, compared to less than 3% staining positively in control untreated cells. Incubation with the cAMP analog 8-bromo-cAMP (8BrcAMP) increased the number of collagenase-staining cells in a dose-dependent manner (ED₀₅ = 2.5 x 10^-4 M), but to a lower level than PTH, with the maximal effect producing about 15% positive cells. The calcium ionophore ionomycin (10^-7 M) was ineffective, whereas phorbol 12-myristate 13-acetate (PMA), a PKC activator, increased collagenase-specific staining to about 5%, but only at high concentrations (10^-5 M). Incubation of UMR 106-01 cells with ionomycin and PMA did not change the effect of the latter. When the three agents were used in combination, an additive effect was observed, which fully reproduced that of PTH. Similarly, the amount of collagenase released into the medium by cells stimulated with maximal concentrations of 8BrcAMP (10^-3 M) was only 80% of that induced by maximal doses of PTH (10^-7 M). PMA (10^-5 M) was slightly stimulatory, and ionomycin was ineffective alone, but they were synergistic with submaximal doses of 8BrcAMP (10^-4 M). In agreement with the immunohistochemical results, the full hormonal effect was reproduced when the three second messenger analogs were used in combination. In conclusion, signal transduction from PTH receptor to collagenase production is mediated mainly by cAMP; the Ca²⁺/PKC system appears to have a contributory role necessary for the full expression of hormonal response. These results support the hypothesis of a dual pathway of target cell activation by PTH.