Abstract IA09: Control of PIP3 levels by PI3Kα and PTEN
Authors: Echeverria Ignacia , Liu Yunlong , Maheshwari Sweta , Chakrabarti Mayukh , Miller Michelle , Bolduc David , Cole Philip , Gabelli Sandra B. , Amzel L. Mario 
Published: December 2020

Phosphatidylinositol 3,4,5-triphosphate (PIP3), a lipid second messenger, signals the initiation of cascades that result in, among other processes, cell proliferation, migration, and survival. PIP3 is generated by the phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP2) by phosphoinositide-3-kinase-α (PI3Kα) in response to activation by receptor tyrosine kinases (RTKs) or their substrates. PI3Kα has a residual constitutive activity that is counteracted by the action of PTEN, a phosphatase that is subjected to a large number of post-translational modifications. Thus, levels of PIP3 are controlled by a balance between the opposite actions of PI3Kα, a kinase, and PTEN, a phosphatase. Both enzymes are highly mutated in tumors. Oncogenic mutations have opposite effects on the activities of the two proteins: mutations activate PI3Kα but suppress the activity of PTEN. Both of these effects result in increased levels of PIP3 and provide selective advantage to the tumor cells carrying the mutations.

In the physiologic activation of PI3Kα, the increase in activity is caused by the phosphotyrosine-containing region of the RTK dislodging the nSH2, a PI3Kα inhibitory domain from the regulatory subunit p85α, from its inhibitory interaction with the catalytic subunit p110α. A similar mechanism is used by some oncogenic mutations: they activate the enzyme by reducing the inhibitory action of the nSH2. Importantly, this effect is not a consequence of a transition to a new structure that can be identified as a new ground state. Instead, the mutations affect the dynamics of the protein in such a way that the new landscape favors catalytically competent conformations. Analysis of the interdomain interactions between the p110α and the p85α of the wild-type and the mutants suggests that the tumor-associated mutations effectively weaken the interactions between p110α and p85α by disrupting key stabilizing interactions. These findings support the notion that oncogenic mutations increase the enzymatic activity by enhancing the dynamics of the protein.

Other oncogenic PI3Kα mutations increase the kinase activity by increasing the interaction of the enzyme with the membrane, and therefore augmenting accessibility to the substrate.

In the case of PTEN, activity is controlled, at least in part, by the phosphorylation of four Ser/Thr residues (380, 382, 383, 385) in its 52-residue long C-terminal tail. This multiple phosphorylation results in an inhibition of the membrane-associated phosphatase activity. Using a variety of biochemical and biophysical techniques, we determined structural and physical chemical details of the mechanism controlling this inhibition. Briefly, in contrast to the unphosphorylated form, the tetra phosphorylated PTEN adopts a more compact conformation in which its carboxy terminal tail interacts with the C2 domain in a fashion that reduces its affinity for the cell membrane and diminishes its catalytic activity.

Citation Format: Ignacia Echeverria, Yunlong Liu, Sweta Maheshwari, Mayukh Chakrabarti, Michelle Miller, David Bolduc, Philip Cole, Sandra B. Gabelli, L. Mario Amzel. Control of PIP3 levels by PI3Kα and PTEN [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr IA09.