Functional Receptors for Vasoactive Intestinal Peptide on Human Osteosarcoma Cells*

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Abstract
Vasoactive intestinal peptide (VIP) stimulates bone resorption in organ culture via a cAMP-dependent mechanism. Functional receptors for VIP on a clonal line of human osteosarcoma cells, SaOs-2 are described. SaOs-2 cells respond to VIP with an increase in cAMP. The effect was rapid (2 min) and dose dependent from 0.15-15 nM VIP, with half-maximal stimulation at 1.4 nM. SaOs-2 cells produce prostaglandin [PG]E2 and respond to exogenous PGE2 with increases in cAMP .apprx. 1/3 as great as those induced by VIP. However, the VIP-stimulated increases in cAMP occurred without detectable increases in PGE2 production, and increases in cAMP were unaffected by the cyclooxygenase inhibitor indomethacin. SaOs-2 cells pretreated with VIP for 24 h were significantly less responsive to a 2nd acute challenge with VIP, but retained their ability to respond to PGE2. Similarly, pretreatment with PGE2 induced homologous desensitization to PGE2, but had no effect on the VIP-stimulated increase in cAMP. These patterns of response paralleled those previoulsy described in whole bone in organ culture. Binding studies with [125I]VIP demonstrated specific, saturable, high affinity receptors for VIP on SaOs-2 cells. Scatchard analysis of [125I]VIP binding at 37.degree. C resulted in a curvilinear plot. Analysis based on the assumption of 2 independent binding sites gave Kd values of 0.44 and 17 nM for high and low affinity binding sites, respectively. The numbers of high and low affinity sites per cell were 8500 and 57,000, respectively. Binding of [125I]VIP was partially inhibited by 2 related peptides, secretion and PHI-27, but not by parathyroid hormone, calcitonin or a variety of unrelated peptides. The action of VIP on human SaOs-2 cells is similar to that observed in intact mouse calvaria; these cells provide a good model for the study of the initial steps of VIP action in bone.