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Calcitonin Receptor-Mediated Growth Suppression of HEK-293 Cells Is Accompanied by Induction of p21^WAF1/CIP1 and G₂/M Arrest

Department of Orthopaedics and Trauma University of Adelaide The Royal Adelaide Hospital and the Hanson Centre for Cancer Research at the Institute for Veterinary and Medical Science Adelaide 5000, South Australia, Australia

We investigated the mechanisms by which calcitonin (CT) suppresses cellular proliferation, using HEK-293 cells stably transfected with either the rat C1a CT receptor (CTR) or the insert-negative form of the human CTR. CT treatment of clonal cell lines expressing either receptor type, but not untransfected HEK-293 cells, strongly suppressed cell growth in a concentration-dependent manner. The reduction in cell growth with CT treatment could not be attributed to cellular necrosis or apoptotic cell death, the latter assessed by both DNA fragmentation analysis and caspase 3 (CPP-32) assay. Growth inhibition was associated with an accumulation of cells in the G₂ phase of the cell cycle. CT treatment of the human and rat CTR-expressing cell lines resulted in a rapid and sustained induction of mRNA encoding the cyclin-dependent kinase inhibitor, p21^WAF1/CIP1, increased levels of which were maintained at least 48 h after initiation of treatment. Western blot analysis showed a rapid corresponding increase in p21^WAF1/CIP1 protein, whereas protein levels of another member of the cyclin-dependent kinase inhibitor family, p27^kip1, were unchanged. In parallel with the induction of p21, CT treatment reduced levels of p53 mRNA and protein. CT treatment resulted in a specific cell cycle block in G₂, which was associated with inhibition of Cdc2/cyclin B kinase activity as measured by histone H1 phosphorylation. There was no evidence for p21 association with this complex despite the inhibition of Cdc2 activity. Evidence that p21 induction was causative of cell growth suppression was obtained from p21 antisense oligonucleotide experiments. Treatment with a p21 antisense oligonucleotide blocked induction of p21 expression and significantly reduced the CT-mediated growth inhibition. These observations suggest that p21 is required for the G₂ arrest in response to CT, but argue against a direct role of p21 in the inhibition of Cdc2 activity. These studies suggest a novel regulation of cell cycle progression by CT and will provide a basis for detailed examination of the molecular mechanisms involved.

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