Potential Implication of Paxillin in Cancer Establishment Within the Bone Environment

Bone metastases are a common feature of advanced prostatic malignancies. They are characterised by a unique prevalence of osteoblastic phenotype and a poor prognosis. Paxillin is a 68-kDa signal transduction adaptor and scaffold protein that contains motifs involved in the mediation of protein-protein interactions. The state of paxillin phosphorylation is central to determining a cell's ability to adhere, detach and migrate and hence has been linked to processes such as wound repair and tumour metastasis. The current study explored the impact of paxillin suppression on prostate and breast cancer cell function and their responsiveness to hepatocyte growth factor (HGF) and bone matrix extract (BME) in order to assess its potential to influence bone colonization and homing.

Hammerhead ribozyme transgenes were used to knockdown the expression of paxillin in breast and prostate cancer cell lines. The impact on the cell growth, migration, adhesion and invasion was assessed using in vitro functional assays. In order to explore potential mechanisms, focal adhesion kinase (FAK) inhibitor was also used.

Knockdown of paxillin expression was observed in all tested cell lines following transfection with the ribozyme transgene. The knockdown of paxillin increased proliferation and invasiveness of LNCaP cells, with no effect on their attachment abilities. The opposite, however, is true for PC-3 cells where, following knockdown, cellular attachment was significantly reduced, while no significant changes in growth and invasiveness were detected. In the MDA-MB-231 breast cancer knockdown model, cells had little difference in proliferative rates and generally increased attachment and reduced invasive abilities. Treatments with HGF and BME had differential effects on targeted cells when compared to controls.

These data suggest that paxillin appears to influence major cell functions in a diverse range of prostate and breast cancer models. The responsiveness of cells to environmental factors such as HGF or BME may be influenced by paxillin status, although this seems to be dependent on cell type.