The role of osteoblast IGF1-R signaling in matrix mineralization in a model of prostate cancer bone metastasis

KREIMANN EL; YANG J; SIKES CR; OLIVE M; PELEG S; LOGOTHETIS CJ; KARSENTY G; NAVONE NM

Washington Convention Center, Washington. DC, USA

Congreso; 94th Annual Meeting of the American Association for Cancer Research; 2003

American Association for Cancer Research

IGF-1 is abundant in human bones and has mitogenic, chemotactic, and antiapoptotic effects on a wide variety of cells, including prostate cancer (PCa) cells. IGF-I influences osteoblast growth and differentiation and induces type I collagen expression in differentiated fetal rodent osteoblasts.  It exerts its action through the type 1 IGF receptor (IGF1R), which is a tyrosine kinase that favors differentiation in several cell types.  We previously reported that two PCa cell lines derived from a bone metastasis of prostate cancer (MDA PCa 2a and MDA PCa 2b) induced primary mouse osteoblasts (PMOs) to differentiate more and produce more bone matrix in our co-culture system.  We then decided to test the effect of blocking IGF1R signaling on osteoblast differentiation in this co-culture system.  We used a specific antibody that blocks the binding of IGF-1 to its receptor.  PMOs grown alone were used as controls.  After 4 days of co-culture, we placed the PMOs in differentiation medium for 21 days and assessed the formation of mineralized nodules using von Kossa’s staining.  We found that calcified matrix deposition was increased in PMOs grown with MDA PCa 2b cells, indicating that these cells affected the ability of PMOs to differentiate.  Bone matrix secreted by PMOs grown in the presence of IGF1R antibody mineralized poorly compared with that secreted by control PMOs. Also, less mineralized nodules were found in PMOS grown with MDA PCa 2b cells plus IGF1R antibody when compared with controls. Taken together, these results indicate that bone-derived MDA PCa 2b cells favor molecular events that lead to osteoblast differentiation through the IGF1R pathway and suggest that IGF1R signaling in osteoblasts may be implicated in the high bone mass of bone metastases of PCa.  In contrast with the effect on differentiation, IGF1R blockade had no effect on PMOs proliferation at the IGF1R concentration tested. To assess whether IGF1R signaling pathways play a role in the pathophysiology of osteoblastic bone metastases from PCa, we will use transgenic mice with an osteoblast-specific inactivating mutation in the Igf1r gene (cre/loxP-Igf1r transgenic mice).  Mice carrying a null mutation of the Igf1r gene are not informative because they die too early.  We have already produced the Igf1r conditional knockout mice and assessed the bone mineral density of the femur of cre/loxP-Igf1r transgenic mice using a MicroCT Scanner.  The transgenic mice had a lower bone mineral density than that of wild-type litter mates; the combination of both cancellous and cortical bone density in the Igf1r-null mice was 86% of that in the wild-type mice.  These results support our finding that IGF1R signaling in osteoblasts impacts bone matrix mineralization and suggest that these mice are suitable to assess the role of osteoblast IGF1R signaling in the interaction between PCa  cells and osteoblasts.