Page 84 - Helicobacter pylori and Gastric Cancer: From Tumor microenvironment to Immunotherapy

Smoothened independent signaling that is required for survival of these cancer stem cells[29]. Tantalizingly, these results open the theoretical possibility to uncouple the anti-cancer effect of Hedgehog signaling on colorectal cancer in general[31] and the trophic effect of Hedgehog signaling on specifically colorectal cancer cells. In the absence, however, of knowledge on the molecular pathways that mediate these non- canonical effects of Patched-dependent but Smoothened independent Hedgehog signaling, this possibility remains hypothetical only.
Apart from Patched-dependent Smoothened-independent non-canonical Hedgehog signaling, Smoothened-dependent Gli-independent non-canonical Hedgehog signaling has also been described and likewise the molecular mechanisms involved are only partly understood. The interaction of Hedgehog with Patched stimulates the translocation of Smoothened to the primary cilium, which is required for the transcriptional Hh response. This translocation involves activation of phospholipase A2 following Smoothened activation and results in the enzymatic release of arachidonic acid from plasma membrane phospholipids. The thus released arachidonic acid binds to Smoothened and this interaction appears essential for cilium translocation of Smoothened and canonical Hedgehog signaling[32]. Arachidonic acid metabolites are powerful actin cytoskeleton remodeling agents[33] and while located outside the primary cilium Smoothened also mediates transcription-independent actin reorganization and chemotactic responses through the production of these metabolites[16, 34]. The physiological importance of this non-canonical response to Hedgehog signaling is illustrated by its pivotal role in Hedgehog effects in directing neurite projection[35], It has become clear, however, that non-canonical Hedgehog effects on axonal guidance involve activation of Src- like kinases[36] but generally speaking it is fair to say that the molecular events underlying Smoothened-dependent non-canonical signaling remain unresolved.
The above-mentioned considerations prompted us to characterize the kinase activities associated with Hedgehog challenge in general as well as those specifically associated with Patched activation and Smoothened activation in isolation. To this end we exploited the power of the peptide array-based kinome profiling in which peptide arrays of kinase-specific substrates are incubated with cell lysates and 33P-γ-ATP thus generating comprehensive descriptions of cellular kinase activities[37, 38]. The results provide a wealth of information on the effects of Hedgehog signal transduction on cellular phosphorylation events and reveal a Patched-dependent Smoothened-independent signal transduction cascade that
Chapter 4
Chapter 4
78
82