Signaling crosstalk in cancers



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Deregulation of cellular signaling contributes to the malignant phenotype of cancer. In this dissertation, cross-talk among oncogenic signaling pathways was investigated in two common human cancers. \r\n Pancreatic cancer has a well known spectrum of genetic alterations, among which KRAS mutation is the most prominent component with a detection frequency as high as 90%. Hedgehog signaling pathway is involved in developmental process and it’s reactivated in the early stage of carcinogenesis of pancreatic cancer. The coincidence of KRAS mutation and abnormal hedgehog activation indicates that there’s a potential link between them. We found that oncogenic KRAS activated hedgehog signaling in pancreatic cancer cells through RAF/MEK/MAPK subpathway, but not PI3K/AKT subpathway. Inactivation of KRAS/MEK pathway activity by a KRAS specific siRNA or MEK inhibitors inhibits GLI transcriptional activity and GLI1 expression, and promotes GLI1 protein degradation in pancreatic cancer cell lines with activating KRAS mutation. Furthermore, Suppressing Gli activity leads to a selective attenuation of the oncogenic transformation activity of mutant KRAS-expressing pancreatic cancer cells. These results indicate that hedgehog pathway activation play an important role in oncogenic KRAS mediated transformation during the pancreatic cancer formation. \r\n In addition to its role in pancreatic cancer, hedgehog signaling was also activated in acute lymphoblastic leukemia (ALL). We found that hedgehog pathway activity was correlated to glucocorticoid (GC) sensitivity status in ALL cells. GC resistant ALL cells had higher hedgehog activity when compared to their sensitive counterpart. Increasing intracellular cAMP levels by forskolin overcomes GC resistance in ALL cells. Our studies demonstrate that PKA is responsible for the observed synergism with GC while Epac isn’t. We find that endogenous PKA activity is higher in the GC-sensitive clone than in the GC-resistant clones. Inhibiting hedgehog pathways activity by specific inhibitors alone leads to cell cycle arrest and apopotosis in ALL cells. These results suggest that Hh activity is critical for leukemia cell growth and survival and that the level of Hh activity is in part responsible for the synergism between cAMP and GC. \r\n To identify the mechanism contributing to the PKA activity difference, we measured PKA subunits expression levels and found that regulatory subunit RIIâ is preferentially expressed in the GC sensitive clone C7-14 cells compared to the GC resistant clone C1-15 cells. Down-regulation of RIIbeta by siRNA transfection leads to enhanced GC resistance in CEM cells, which indicates RIIbeta expression is required for GC sensitivity. We also found that activation of PKA II by selective agonists recapitulated forskolin’s effects of promoting apoptosis and antagonizing Akt activity in both GC sensitive and resistant cells. However, PKA I agonists did not have the same effect. These results demonstrate that PKA II is critical for cAMP-promoted apoptosis in ALL cells and its activation or with GC combination could be a novel approach against lymphoid malignancy. \r\n



Ras, pancreatic, leukemia, hedgehog, glucocoritoid, crosstalk, cancer, cAMP, AKT