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Originally published In Press as doi:10.1074/jbc.M108576200 on March 7, 2002

J. Biol. Chem., Vol. 277, Issue 20, 17687-17695, May 17, 2002
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Biosynthesis and Secretion of Parathyroid Hormone Are Sensitive to Proteasome Inhibitors in Dispersed Bovine Parathyroid Cells*

Amos M. SakweDagger , Åke Engström, Mårten Larsson, and Lars Rask

From the Department of Medical Biochemistry and Microbiology, Uppsala Biomedical Center, Uppsala University, SE-751 23 Uppsala, Sweden

Preproparathyroid hormone (prepro-PTH) is one of the proteins abundantly synthesized by parathyroid chief cells; yet under normal growth conditions, little or no prepro-PTH can be detected in these cells. Although this may be attributed to effective cotranslational translocation and proteolytic processing, proteasome-mediated degradation of PTH precursors may be important in the regulation of the levels of these precursors and hence PTH secretion. The effects of N-acetyl-Leu-Leu-norleucinal, N-acetyl-Leu-Leu-methional, carbobenzoxy-Leu-Leu-leucinal (MG132), benzyloxycarbonyl-Ile-Glu(t-butyl)-Ala-leucinal (proteasome inhibitor I), and lactacystin on the biosynthesis and secretion of PTH were examined in dispersed bovine parathyroid cells. We demonstrate that treatment of these cells with proteasome inhibitors caused the accumulation of prepro-PTH and pro-PTH. Compared with mock-treated cells, the processing of pro-PTH to PTH was delayed, and the secretion of intact PTH decreased in proteasome inhibitor-treated cells. Relieving the inhibition of the proteasome by chasing MG132-treated cells in medium without the inhibitor led to the rapid disappearance of the accumulated prepro-PTH, and the rate of PTH secretion was restored to levels comparable to those in mock-treated cells. Furthermore, overexpression of the Hsp70 family of molecular chaperones was observed in proteasome inhibitor-treated cells, and we show that PTH/PTH precursors interact with these molecular chaperones. These data suggest the involvement of parathyroid cell proteasomes in the quality control of PTH biosynthesis.

* This work was supported in part by grants from the Swedish Medical Research Council.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Dagger Supported by a guest research fellowship from the Swedish Institute. To whom correspondence should be addressed: Dept. of Medical Biochemistry and Microbiology, Uppsala Biomedical Center, Uppsala University, P. O. Box 582, SE-751 23 Uppsala, Sweden. Tel.: 46-18-471-4567; Fax: 46-18-471-4975; E-mail: Amos.Sakwe@imbim.uu.se.

Copyright © 2002 by The American Society for Biochemistry and Molecular Biology, Inc.
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