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To elucidate the mechanism by which TPA induces activation of the MAP kinase pathway, we asked whether trans-activation of EGF receptor could be involved. AG1478 and PD153035 are potent and selective inhibitors of EGF receptor kinase and can block FUT 175 of EGF receptor induced by several substances. As expected, AG1478 and PD153035, as well as the MEK inhibitor U0126, completely counteracted EGF-induced inhibition of GJIC (Fig. 1C). Interestingly, AG1478 and PD153035 counteracted the TPA-induced inhibition of GJIC by approximately 20% (Fig. 1A). This suggests that TPA activates the MAP kinase pathway partly by transactivating EGF receptor.
We next examined the role of Src in TPA-induced inhibition of GJIC, since PKCδ has been shown to be able to activate EGF receptor via activation of Src in response to TPA treatment [20]. However, the selective inhibitor of Src activity, PP2, was found to have no effect on the TPA-induced inhibition of GJIC, suggesting that Src is not involved in this process (Fig. 1A).