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Dexamethasone Induces Apoptosis in Proliferative Chondrocytes through Activation of Caspases and Suppression of the Akt-Phosphatidylinositol 3'-Kinase Signaling Pathway

Department of Woman and Child Health (D.C., F.Z., A.S.C., L.S.), Pediatric Endocrine Unit, Astrid Lindgren Children’s Hospital, Karolinska Institutet, SE-17176 Stockholm, Sweden; and Department of Biochemistry and Molecular Dentistry (T.M.), Okayama University Graduate School of Medicine and Dentistry, Okayama 700-8525, Japan

Address all correspondence and requests for reprints to: Dionisios Chrysis, Pediatric Endocrinology Unit, Q2: 08, Astrid Lindgren Children’s Hospital, Karolinska Hospital, SE-17176 Stockholm, Sweden. E-mail: Dionisios.Chrysis{at}kbh.ki.se.

Although glucocorticoids are known to induce apoptosis in chondrocytes, the mechanisms for this effect and the potential antiapoptotic role of IGF-I are unknown. To address this, we studied the effects of dexamethasone (Dexa) on apoptosis in the HCS-2/8 chondrocytic cell line. Dexa (25 µM) increased apoptosis (cell death ELISA) by 39% and 45% after 48 and 72 h, respectively (P < 0.01 and P < 0.05, respectively). IGF-I (100 ng/ml) decreased Dexa-induced apoptosis to levels similar to control cells. Apoptosis was associated with cleavage of poly-ADP-ribose polymerase (PARP) and -fodrin and activation of caspases-8, -9, and -3 (Western), an effect that was counteracted when chondrocytes were cocultured with Dexa + IGF-I. Inhibitors for caspases-8, -9, and -3 (50 µM each) equally suppressed Dexa-induced apoptosis (P < 0.01). Time-response experiments showed that caspase-8 was activated earlier (at 12 h) than caspase-9 (at 36 h). We studied the phosphatidylinositol 3'-kinase (PI3K) pathway to further investigate the mechanisms of Dexa-induced apoptosis. Dexa decreased Akt phosphorylation by 93% (P < 0.001) without affecting total Akt and increased the p85 subunit 4-fold. The Akt inhibitor SH-6 (10 µM) increased apoptosis by 54% (P < 0.001). When combining Dexa with SH-6, apoptosis was not further increased, showing that Dexa-induced apoptosis is mediated through inhibition of the PI3K pathway. Addition of IGF-I to SH-6- or Dexa + SH-6-treated cells decreased apoptosis by 21.2% (P < 0.001) and 20.6% (P < 0.001), respectively. We conclude that Dexa-induced apoptosis is caspase dependent with an early activation of caspase-8. IGF-I can rescue chondrocytes from Dexa-induced apoptosis partially through the activation of other pathways than the PI3K signaling pathway. Based on our in vitro data, we speculate that in vivo treatment with glucocorticoids may diminish longitudinal growth by increasing apoptosis of proliferative growth plate chondrocytes.

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