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C-Terminal Parathyroid Hormone-Related Protein (PTHrP) (107–139) Stimulates Intracellular Ca²⁺ through a Receptor Different from the Type 1 PTH/PTHrP Receptor in Osteoblastic Osteosarcoma UMR 106 Cells¹

Bone and Mineral Metabolism Laboratory, Research Unit, Fundación Jiménez Díaz, 28040 Madrid, Spain

Address all correspondence and requests for reprints to: P. Esbrit, Ph.D., Bone and Mineral Metabolism Laboratory, Research Unit, Fundación Jiménez Díaz, Avda. Reyes Católicos 2, 28040 Madrid, Spain. E-mail: pesbrit{at}fjd.es

Studies were undertaken to determine whether PTH-related protein (PTHrP) (107–139) mobilizes [Ca²⁺]_i in osteoblastic osteosarcoma UMR 106 cells. PTHrP (107–139), in a manner similar to PTHrP (107–111), induced a rapid [Ca²⁺]_i response in these cells that was dose dependent (EC₅₀ of 0.1 pM) and more efficient than that of PTHrP (1–36) (EC₅₀ of 1 nM). This effect of PTHrP (107–139) was abrogated by micromolar doses of verapamil or nifedipine. However, it was unaffected by 10 µM U73122 (a phospholipase C inhibitor), 100 µg/ml heparin (an inositol 1,4,5-trisphosphate receptor inhibitor), or 400 ng/ml pertussis toxin (a G_i inhibitor), which inhibited the [Ca²⁺]_i response to PTHrP (1–36), or by either 25 nM bisindolylmaleimide I (BIM), a protein kinase (PK) C inhibitor, or 1 µM phorbol-12-myristate-13-acetate preincubation (22 h). PTHrP (107–139) and PTHrP (1–36), at 100 nM, desensitized the [Ca²⁺]_i response to a second challenge with the same peptide, but not with the other peptide in these cells. PTHrP (7–34), a type 1 PTH/PTHrP receptor (PTH1R) antagonist, decreased the effect of PTHrP (1–36) on [Ca²⁺]_i. In contrast, PTHrP (107–111), but neither PTHrP (109–138) nor PTHrP (7–34), abolished this effect of PTHrP (107–139). Both PTHrP (107–139) and PTHrP (1–36), added together at submaximal doses, induced a higher [Ca²⁺]_i response. Moreover, PTHrP (107–139) increased the efficacy of PTHrP (1–36) on [Ca²⁺]_i, but decreased its induced increase in PKA activity in these cells. Verapamil or nifedipine (at 50 µM) or 25 nM BIM, but not 25 µM adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer, a PKA inhibitor, abolished the PTHrP (107–139)-induced increase in interleukin 6 messenger RNA (assessed by RT, followed by PCR) in UMR 106 cells. This peptide also increased c-fos messenger RNA in these cells; an effect inhibited by BIM, but unaffected by either verapamil or EGTA. These findings support the existence of high-affinity receptors for PTHrP (107–139), associated with an induced Ca²⁺ influx, different from the PTH1R in UMR 106 cells. The present results suggest that PTHrP could affect bone turnover by interacting with the PTH1R and other yet unknown receptors in bone cells through complex mechanisms.

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