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Dissecting the Roles of TGF-b Receptors in TGF-b-Induced PI3K-Akt Activation


TGF-β plays many roles in growth and development, and aberrant TGF-β signaling contributes to the pathogenesis and progression of various diseases, including fibrosis and the initiation and progression of cancer toward metastasis. TGF-β activates many downstream effectors, including Smad signaling, which regulates gene transcription, and PI3K-Akt signaling, which has roles in regulating cell proliferation and survival among other cellular processes. Both pathways are highly targeted in development of drug therapies.

It has been shown that the kinase activity of TβRI is needed for Akt activation through PI3K and that TGF-β receptors can associate with PI3K, but the mechanism of how TGF-β activates PI3K-Akt is yet to be determined. The PI3K-Akt pathway is known to be activated downstream of RTKs. Given that TGF-β receptors are dual-specificity kinases, we can reason that TGF-β receptors may activate PI3K-Akt similarly to RTKs, in that phosphorylated tyrosines on activated TGF-β receptors play key roles in recruitment and activation of PI3K.

In this study, we interrogate the mechanism of TGF-β-induced PI3K-Akt activation at the receptor level. We find that the PI3K p85α regulatory and the p110α catalytic subunits have major roles in TGF-β-induced Akt activation and EMT. We also find that the C-terminal SH2 domain of p85α, along with the iSH2 domain, is needed for interaction with TβRII. Efforts have been put in to exploring the role of TβRII tyrosine phosphorylation in the TβRII-p85α interaction, and it appears that loss of tyrosine phosphorylation sites attenuate TβRII-p85α interaction. While the kinase activity of TβRI is needed for activation of Akt, it seems neither the kinase activity of TβRI nor TβRII is needed for PI3K interaction.

The TβRII-p85α interaction seems to be constitutive, with subtle fluctuations in interaction upon TGF-β stimulation or inhibition of the TβRI kinase. However, this fluctuation is too subtle to be conclusive. Some data suggest that the TβRII-p85α interaction could be one that is inhibitory. Data also suggest that the adaptor proteins ShcA and/or Grb2 may have a role in TGF-β-induced Akt activation. These theories have yet to be fully examined, but preliminary data and speculations are discussed in this dissertation.

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