Abstract
Purpose: :
Our previous studies show that the canonical mitochondrial death pathway has a requisite role in signaling lens differentiation initiation through the activation of the caspase-3 protease. The ability of this pathway to function as a molecular switch in lens differentiation depends on the concurrent induction of the survival molecules that regulate it, including proteins in the Bcl-2 and the Inhibitor of Apoptosis Protein (IAP) families. Here, we studied the upstream signaling pathway that induces expression of these survival molecules, focusing our investigation on α6 integrin as the transactivator of an IGF-1R-mediated survival-signaling pathway involving Nuclear Factor Kappa B (NFΚB/RelA), the transcription factor for the Bcl-2 and IAP molecules induced at the time of lens differentiation initiation.
Methods: :
α6 integrin receptor was knocked down in primary quail lens cell cultures with a siRNA approach and activated by exposure to PMA. Expression of α6 integrin, activation of IGF-1R, and expression of its downstream effectors including NFΚB, Bcl 2, survivin, and ch IAP1/2 were determined by Western blot analysis. Receptor complexes in differentiation-specific fractions of the E10 chick lenses were analyzed by co-immunoprecipitation.
Results: :
As lens cells initiate their differentiation program in the equatorial zone IGF-1R was recruited to and activated in α6 integrin receptor signaling complexes. The suppression of IGF-1R activation following siRNA knockdown of α6 integrin showed that activation of IGF-1R was dependent on α6 integrin. In support of this finding the activation of α6 integrin by PMA induced activation of IGF-1R. The expression of Bcl-2 and IAP survival proteins in lens cultures is dependent on an IGF-1R/NFΚB signaling pathway. This survival pathway now was found to be dependent on expression of α6 integrin. Knockdown of α6 integrin suppressed expression of NFΚB/RelA and its effectors Bcl-2, ch-IAP1/2 and survivin, and these lens cells failed to differentiate, as evidenced by inhibition of the expression of filensin, MP28 and p27.
Conclusions: :
α6 integrin transactivates an IGF-1R-mediated survival-signaling pathway that is crucial for the initiation of lens cell differentiation.
Keywords: cell survival • differentiation • signal transduction