Understanding the Apoptotic Functions of IGFBP-3 in Prostate Cancer
Abstract
The IGF axis is known to play an important role in the epidemiology of many tumors. IGFBP-3 promotes apoptosis in cancer cells by both IGF-dependent and -independent mechanisms. We have previously shown that IGFBP-3 is rapidly internalized and localized to the nucleus, where its interactions with the nuclear receptor RXR are important in apoptosis induction. We demonstrate that phosphorylation of IGFBP-3 (S156) by DNA-PK enhances its nuclear accumulation, and is essential for its ability to interact with RXR and induce apoptosis in cultured prostate cancer cells. Indeed, IGFBP-3-S156A is completely unable to induce apoptosis in 22RV1 cells. Using specific chemical inhibitors, we investigated the contribution of other protein kinases to the regulation of IGFBP-3-induced apoptosis. Preventing the activation of CK2 enhanced the apoptotic potential of IGFBP-3. We mapped two potential CK2 phosphorylation sites in IGFBP-3: S167 and S175. These sites were mutated to Ala, and the resulting constructs were transfected in to LAPC4 and 22RV1 prostate cancer cells. WtIGFBP-3 and IGFBP-3-S175A induced apoptosis to a comparable extent; however, IGFBP-3-S167A was far more potently aapoptosis-inducing These effects were specific to apoptosis-induction, however, since wtIGFBP-3 and IGFBP-3/S167A had comparable effects on cell growth and proliferation, assessed by MTT and BrdU incorporation assays. Interestingly, IGFBP-3-S167A was able to induce apoptosis even in the absence of active DNA-PK, while the DNA-PK non-phosphorable IGFBP-3-S156A mutant regained the ability to induce apoptosis when CK2 activity was inhibited chemically or by using siRNA. Together, these data reveal two key regulatory phosphorylation sites in the central region of IGFBP-3. Phosphorylation of S156 by DNA-PK promotes apoptosis, whilst phosphorylation of S167 by CK2 limits the ability of IGFBP-3 to induce apoptosis.
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 01, 2007
- Accession Number
- ADA593327
Entities
People
- Laura Cobb
Organizations
- University of California, Los Angeles