Humoral factors of ascites sarcoma 180 stimulate osteoblastic UMR 106-01 cell proliferation and bone resorption via signal transduction pathways, which are clearly different from those of parathyroid hormone.

Ascites sarcoma 180 (S180A) is a transplantable tumor that induces hypercalcemia in tumor-bearing mice and stimulates bone resorption in cultured neonatal mouse calvaria without parathyroid hormone (PTH)-like activity. The serum-free conditioned media of S180A cell cultures (S180A-CM) stimulated [3H]thymidine incorporation (178.3% of the control) and inhibited alkaline phosphatase activity (39.0% of the control) in the osteoblastic osteosarcoma cell line UMR 106-01, contrary to PTH. To investigate signal transduction by S180A-CM, we determined the levels of intracellular free calcium ([Ca2+]i), inositol 1,4,5-triphosphate (IP3), 1,2-diacylglycerol (DAG), phosphatidylcholine (PC) and protein kinase (PK) C activity in UMR 106-01 cells. PTH and PTH-related protein (PTHrP), both potent bone-resorbing factors (BRFs), caused an increase in [Ca2+]i and stimulated IP3 production, whereas S180A-CM had little or no effect on these parameters. On the other hand, S180A-CM stimulated DAG production, accompanied by PC breakdown, and the translocation of PKC activity from the cytosol to the membrane fraction. Sphingosine, a specific PKC inhibitor, inhibited bone-resorbing activity (BRA) in S180A-CM more effectively than PTH or PTHrP-stimulated resorption. H-7, an inhibitor of both cAMP-dependent PKA and PKC, completely inhibited BRA in S180A-CM. These results suggest that BRFs of S180A-CM stimulate osteoblastic cell proliferation and bone resorption via two signal transduction pathways, which are different from those of PTH: 1) activation of PKC by DAG resulting from PC hydrolysis and 2) activation of PKA subsequent to prostaglandin E2 production by bone.