| Summary: | 碩士 === 國立臺灣大學 === 藥理學研究所 === 99 === Phorbol 12-myristate 13-acetate (PMA) is a PKC activator known for triggering NF-kappaB activation and c-fos expression. To further understand the functional link of both events, we compared the actions of TNF-alpha and PMA in various cell types. We found that PMA- but not TNF-alpha-induced p65 Ser536 phosphorylation in mouse embryonic fibroblast (MEF) is IKKalpha/beta dependent. Additionally, compared to TNF-alpha, which induced dramatically IKKalpha/beta phosphorylation at Ser176/177, PMA preferentially induced IKKalpha/beta phosphorylation at Ser180/181 within its active loop, and this phosphorylation of IKKalpha/beta by PMA is through a novel mechanism depending on PKC-MEK-ERK axis but not RSK or MSK. Moreover, this phosphorylation does not contribute to PMA-induced IkappaBalpha degradation, p65 Ser536 phosphorylation, p65 nuclear translocation and NF-kappaB activation. Surprisingly, we found that PMA-induced immediately early gene c-fos expression is suppressed in IKKalpha-/-, IKKbeta-/-, IKKgamma-/- and p65-/- MEF, suggesting the involvement of NF-kappaB in c-fos gene transcription. Furthermore, we indentified a previously uncharacterized NF-kappaB binding site located in the mouse c-fos promoter. Under PMA stimulation, p65 homodimer in association with CBP/p300 is capable of binding to this site, and this action is independent of TCF-SRF-SRE ternary complex regulation. In summary, this study provides different views of IKK phosphorylation mechanism. Importantly, the requirement of IKK complex in PMA-induced c-fos expression and the NF-kappaB binding site found in c-fos promoter shed light on the crosstalk between NF-kappaB and AP-1 transcription factors.
|
|---|
