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Involvement of protein kinase C in the stimulation of sodium-dependent phosphate transport by parathyroid hormone in osteoblast-like cells

Arao M, Yamaguchi T, Sugimoto T, Fukase M, Chihara K. Involvement of protein kinase C in sodiumdependent phosphate transport by parathyroid hormone in osteoblast-like cells. Eur J Endocrinol 1994;131:646–51. ISSN 0804–4643

The rat osteosarcoma cell line UMR-106 has an osteoblast-like phenotype and possesses parathyroid hormone (PTH)-responsive dual signal transduction systems [adenosine 3',5'-cyclic monophosphatedependent protein kinase (PKA) and calcium-protein kinase C (Ca-PKC)]. These cells transport inorganic phosphate (P_i) by a Na⁺-dependent carrier under stimulation by PTH. The present study aimed to clarify PTH-responsive signal transduction mechanisms in the regulation of Na⁺-dependent P_i transport by PTH in UMR-106 cells. Exposure of these cells to 10^–7 mol/l PTH induced a significant increase in P_i uptake within 30 min of incubation and it became maximal after 2 h. Parathyroid hormone (10^–9 –10^–7 mol/l) stimulated P_i uptake dose dependently. Activation of PKC by 12-O-tetradecanoyl phorbol- 13-acetate (TPA) also increased P_i uptake in time- and dose-dependent manners similar to PTH In contrast, neither PKA activation by 10 mol/l forskolin or by 10^–4 mol/l dibutyryladenosine 3',5'-cyclic monophosphate nor calcium ionophore treatment with 10^–7 mol/l A23187 [GenBank] or with 10^–7 mol/l ionomycin during 3-h incubations affect P_i uptake, except its increase by 10^–4 mol/l forskolin at a 3-h incubation. These agents had no influence on P_i uptake even in combined treatments with TPA. The PTH-induced increase in P_i uptake was abolished almost completely by pretreating cells with PKC inhibitors, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine dihydrochloride (H-7) (50 µmol/l) or staurosporin (10 and 50 nmol/l), and by down-regulating PKC with a prolonged TPA treatment. These results indicate that the messenger system mediated by PKC, rather than by PKA or by cytosolic calcium, plays a crucial role in the regulation of Na⁺-dependent P_i transport by PTH within a few hours of exposure of the hormone in the osteoblast-like cells.

Toru Yamaguchi, Third Division, Department of Medicine, Kobe University School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650, Japan

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