Nonclassical Activation of Hedgehog Signaling Enhances Multidrug Resistance and Makes Cancer Cells Refractory to Smoothened-targeting Hedgehog Inhibition

The Hedgehog (Hh) pathway is critical in normal development. However, it has been reported to be up-regulated in numerous cancers and implicated in tumorigenicity and metastasis. Classical activation of Hh signaling initiated by Hh ligands results in activation of Smoothened (SMOH) and culminates in the activation of the GLI transcription factors. Classical Hh signaling is autocrine or paracrine (involving interaction between tumor cells and their stroma/microenvironment). The tumor milieu is rich in inflammatory cytokines that can modulate tumor cell behavior. Here, we show for the first time that the Hh pathway can be nonclassically up-regulated by the inflammatory cytokine, osteopontin (OPN). OPN-initiated Akt-GSK3 beta signaling mediates the subcellular distribution and activation of GLI1 resulting in the modulation of epithelial mesenchymal plasticity and drug resistance. Interestingly, the SMOH inhibitor cyclopamine was unable to uncouple the effects of OPN on Hh signaling, indicating that OPN nonclassically activates GLI-mediated transcription. Given the fact that OPN is itself transcriptionally activated upon Hh signaling, our current findings highlight the possibility of a feedforward vicious cycle such that the Hh pathway might be turned on nonclassically by stimuli from the tumor milieu. Thus, drugs that target the classical Hh ligand-mediated activation of Hh signaling may be compromised in their ability to interfere with the functioning of the pathway.