Background: Osteopontin (OPN) has been shown to play many roles in the progression of cancer. We have recently demonstrated the activation of Akt by OPN. Integrin-linked kinase and PI3-kinase are integral proteins in OPN/AKT pathway in PC3 cells. To investigate the role of the extracellular receptors in OPN signaling, we have examined the spatio-temporal regulation of CD44 and integrin αvβ3 receptor in OPN-induced Akt activation in PC3 cells.
Results: Here, our studies demonstrate that OPN can activate Akt either through the αVβ3 integrin or the CD44 cell surface receptor. Members of the Mitogen Activated Protein Kinase (MAPK) family have been shown to be up-regulated in a variety of human cancers and have been implicated in the metastatic behavior. Our studies have demonstrated an increase in the phosphorylation of c-Raf at Ser259 and Ser338 in PC3 cells over-expressing OPN. This increase matches up with the Erk1/2 phosphorylation at Thr202/204 and activation. However, the inhibition of Akt activity augments the phosphorylation state of ERK1/2 to two to three fold with a concomitant reduction in the phosphorylation state of c-Raf at Ser259.
Conclusions: Regulation c-Raf phosphorylation at Ser259 has a role in the anti-apoptotic pathways mediated by Akt or Raf/MEK/ERK proteins. OPN may have dual effects in the activation of Erk1/2. We propose this based on the observations that while OPN activates c-Raf and Erk1/2; it also acts to inhibit c-Raf and Erk1/2 activation through Akt pathway. Our observations suggest that the activation of c-Raf-ERK cascade may promote cell cycle arrest in prostate cancer cells and OPN signaling has a role in the anti-apoptotic mechanism.