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Calcitonin Induces 25-Hydroxyvitamin D₃ 1-Hydroxylase mRNA Expression via Protein Kinase C Pathway in LLC-PK₁ Cells

Department of Internal Medicine, School of Medicine, Keio University, Tokyo, Japan.

Correspondence to Dr. Matsuhiko Hayashi, Department of Internal Medicine, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku, Tokyo 160-8582, Japan. Phone: +81 3 3353 1211; Fax: +81 3 3359 2745; E-mail: matuhiko{at} mc.med.keio.ac.jp

Abstract. The biosynthesis of 1, 25-dihydroxyvitamin D₃ from 25-hydroxyvitamin D₃ is catalyzed by 25-hydroxyvitamin D₃ 1-hydroxylase (CYP27B1) in renal proximal tubules. It was recently demonstrated that LLC-PK₁ cells express CYP27B1 mRNA, which is regulated by intracellular cAMP but not vitamin D₃. To clarify the effect of calcitonin on vitamin D₃ metabolism in vitro, LLC-PK₁ cells were incubated with hormonal factors, and expression of CYP27B1 mRNA was measured by quantitative reverse transcription-PCR. Calcitonin at 100 nmol/L significantly increased CYP27B1 mRNA expression by 24 h (271 ± 21% of control). Incubation with calcitonin over a range of 1 µmol/L to 1 pmol/L resulted in a concentration-dependent increase in CYP27B1 mRNA levels. It is known that the calcitonin receptor has dual intracellular signaling pathways, via protein kinases A and C. Both 500 µmol/L 8-bromo-cAMP, a protein kinase A activator, and 100 nmol/L phorbol 12-myristate 13-acetate, a protein kinase C activator, increased CYP27B1 mRNA levels at 24 h (207 ± 54 and 246 ± 58% of control, respectively). However, calcitonin-induced CYP27B1 mRNA expression was only inhibited by the protein kinase C inhibitors staurosporine and calphostin C. The protein kinase A inhibitors Rp-cAMPS at 10 and 100 µmol/L and H-89 at 10 µmol/L had no effect on the action of calcitonin, in spite of cAMP-activation by calcitonin. The present data suggest that calcitonin upregulates CYP27B1 mRNA expression via the protein kinase C pathway in LLC-PK₁ cells.

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