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Vitamin D Receptor–Dependent 1α,25(OH) 2 Vitamin D 3 –Induced Anti‐Apoptotic PI3K/AKT Signaling in Osteoblasts
Author(s) -
Zhang Xiaoyu,
Zanello Laura P
Publication year - 2008
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1359/jbmr.080326
Subject(s) - protein kinase b , pi3k/akt/mtor pathway , calcitriol receptor , apoptosis , osteoblast , phosphorylation , chemistry , microbiology and biotechnology , endocrinology , medicine , staurosporine , cancer research , kinase , signal transduction , vitamin d and neurology , biology , protein kinase a , biochemistry , in vitro
Abstract Osteoblast apoptosis plays a crucial role in bone remodeling. Physiological doses of 1α,25(OH) 2 ‐vitamin D 3 (1,25D) protect osteoblasts against apoptosis by means of mechanisms only partially understood. We studied activation of an Akt survival cascade downstream of 1,25D nongenomic stimulation of phosphatidylinositide‐3′‐kinase (PI3K) in osteoblastic cells. We measured a dose‐ and time‐dependent 1,25D induction of Akt phosphorylation (p‐Akt) in cultured osteoblastic cells. Maximal response was achieved with 10 nM 1,25D after 5 min. We found that staurosporine (STSP)‐induced apoptosis was significantly reduced in 1,25D‐pretreated osteoblasts. 1,25D prosurvival effects were abolished when cells were preincubated with inhibitors of PI3K activation. By means of siRNA silencing, we proved that 1,25D induction of p‐Akt requires a classic vitamin D receptor (VDR) in osteoblasts. Furthermore, non‐osteoblastic CV‐1 cells transfected with an enhanced green fluorescent protein (EGFP)‐VDR construct responded to 1,25D treatment with a rapid p‐Akt response associated with increased cell survival not detected in native, nontransfected cells. We measured increased levels of p‐Akt substrates p‐Bad and p‐FKHR and significantly reduced activity of caspases 8 and 3/7 after 1,25D treatment. In addition, 1,25D‐induced protection against apoptosis was abolished when osteoblasts were preincubated with pertussis toxin. We conclude that anti‐apoptotic effects of 1,25D in osteoblasts occur through nongenomic activation of a VDR/PI3K/Akt survival pathway that includes phosphorylation of multiple p‐Akt substrates and reduction of caspase activities.