Phosphorylation by DNA-dependent protein kinase is critical for apoptosis induction by insulin-like growth factor binding protein-3.

Abstract

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) promotes apoptosis of cancer cells by both IGF-dependent and IGF-independent mechanisms. In vitro phosphorylation of IGFBP-3 by DNA-dependent protein kinase (DNA-PK) has been reported but with unknown functional relevance. Using a chemical inhibitor for DNA-PK in prostate cancer cells and a paired system of glioblastoma cell lines that either lack or express DNA-PK, we show that the apoptosis-promoting and growth-inhibitory actions of IGFBP-3 are completely abrogated in the absence of catalytically active DNA-PK. In the absence of DNA-PK activity, IGFBP-3 has reduced nuclear accumulation and is unable to bind its nuclear binding partner retinoid X receptor (RXR) alpha. We assessed the importance of the three potential DNA-PK phosphorylation sites in IGFBP-3 using PCR-based site-directed mutagenesis. When transfected into 22RV1 cells, IGFBP-3-S165A and IGFBP-3-T170A functioned in an identical manner to wild-type IGFBP-3 to induce apoptosis. In contrast, IGFBP-3-S156A was unable to promote apoptosis and exhibited reduced nuclear accumulation, suggesting a key role for DNA-PK-dependent phosphorylation in the regulation of IGFBP-3 action. These studies reveal a novel regulatory mechanism for the actions of IGFBP-3 in prostate cancer and show phosphorylation of Ser(156) to be functionally critical in its apoptosis-inducing actions.

• Authors: Laura J Cobb, Bingrong Liu, Kuk-Wha Lee, Pinchas Cohen
• Journal: Cancer research (Cancer Res) Vol. 66 Issue 22 Pg. 10878-84 (Nov 15 2006) ISSN: 0008-5472 [Print] United States
• PMID: 17108124 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, U.S. Gov't, Non-P.H.S.)

Chemical References

• Insulin-Like Growth Factor Binding Protein 3
• Retinoid X Receptor alpha
• DNA-Activated Protein Kinase

Topics

• Apoptosis (physiology)
• Binding Sites
• Cell Growth Processes (physiology)
• Cell Line, Tumor
• Cell Nucleus (metabolism)
• DNA-Activated Protein Kinase (metabolism)
• Humans
• Insulin-Like Growth Factor Binding Protein 3 (metabolism)
• Male
• Phosphorylation
• Prostatic Neoplasms (enzymology, metabolism, pathology)
• Retinoid X Receptor alpha (metabolism)