Hormonal regulation of IGFBP-2 proteolysis is attenuated with progression to androgen insensitivity in the LNCaP progression model

The identification of molecular determinants involved in the promotion of metastasis and development of androgen insensitive prostate cancer (AI-PCa) is necessary to discriminate aggressive from indolent disease and to identify therapeutic targets for advanced disease. Overexpression of one particular member of the insulin like growth factor (IGF) axis, IGFBP-2, is implicated in the development of AI-PCa and other cancers. Using the LNCaP human PCa progression model, we show that the AI and metastatic prostate cancer cell line C4-2B4 expresses greater amounts of secreted IGFBP-2 than the androgen sensitive (AS), non-metastatic LNCaP progenitor cell line. Further, the ability of androgens to decrease extracellular IGFBP-2 levels is attenuated in the AI and metastatic C4-2 cell line. The ability of androgen to negatively regulate extracellular IGFBP-2 levels was blocked by Casodex in a dose-dependent manner. The mechanism underlying the androgen-induced downregulation of secreted IGFBP-2 appears to involve extracellular proteolysis, resulting in the production of IGFBP-2 fragments lacking the ability to bind IGF-I and IGF-II. As C4-2 cells have an attenuated ability to proteolyze IGFBP-2 in response to androgen and C4-2B4 cells express greater amounts of IGFBP-2, our data implies that the diminished regulation of IGFBP-2 and loss of associated proteolytic fragments play a role in the increased metastatic behavior of these cells in vivo. Furthermore, our results suggest that either increased levels of intact IGFBP-2 or decreased levels of IGFBP-2 proteolytic fragments could serve as a biomarker to monitor for progression to AI-PCa.