Abstract
Targeting the PI3K pathway has achieved limited success in cancer therapy. One reason for the disappointing activity of drugs that interfere with molecules that are important player in this pathway is the induction of multiple feedback loops that have been only partially understood. To understand these limitations and develop improved treatment strategies, we comprehensively characterized molecular mechanisms of PI3K pathway signaling in bladder cancer cell lines upon using small molecule inhibitors and RNAi technologies against all key molecules and protein complexes within the pathway and analyzed functional and molecular consequences. When targeting either mTORC1, mTOR, AKT or PI3K, only S6K1 phosphorylation was affected in most cell lines examined. Dephosphorylation of 4E-BP1 required combined inhibition of PI3K and mTORC1, independent from AKT, and resulted in a robust reduction in cell viability. Long-term inhibition of PI3K however resulted in a PDK1-dependent, PIP3 and mTORC2 independent rephosphorylation of AKT. AKT rephosphorylation could also be induced by mTOR or PDK1 inhibition. Combining PI3K/mTOR inhibitors with AKT or PDK1 inhibitors suppressed this rephosphorylation, induced apoptosis, decreased colony formation, cell viability and growth of tumor xenografts. Our findings reveal novel molecular mechanisms that explain the requirement for simultaneous targeting of PI3K, AKT and mTORC1 to achieve effective tumor growth inhibition.
Highlights
Frequent hyperactivation and deregulation of the phosphoinositide 3-kinase (PI3K)/ AKT/ mammalian target of rapamycin pathway in cancer has made it one of the most investigated therapeutic targets in tumor therapy
By using different types of PI3K pathway inhibitors and knockdown of selected molecules in the pathway we demonstrate that parallel inhibition of mTORC1 and PI3K is crucial for 4E-BP1 dephosphorlyation, independently of AKT signaling
When using the dual PI3K/mTOR inhibitor NVP-BEZ235, which targets the kinase activities of both PI3K (IC50 4–75 nM) as well as mTOR (IC50 20 nM), dephosphorylation of S6 kinase beta-1 (S6K1) was achieved with 5 Nm S3A Fig [23,31]
Summary
Frequent hyperactivation and deregulation of the phosphoinositide 3-kinase (PI3K)/ AKT/ mammalian target of rapamycin (mTOR) pathway in cancer has made it one of the most investigated therapeutic targets in tumor therapy. Class IA PI3Ks, consisting of a p85 regulatory subunit and a p110 catalytic subunit, with the isoforms p110α, p110β, p110γ and p110δ, phosphorylate phosphatidylinositol-4, 5 bisphosphate (PI-4,5-P2) to phosphatidylinositol3,4,5-trisphosphate (PIP3). This reaction is reversed by the protein phosphatase and tensin homolog (PTEN) [1,2]). AKT, a serine-threonine kinase, is functionally activated by phosphorylation at two distinct amino acid residues, threonine 308 and serine 473, by PDK1 and mTORC2, respectively. Phosphorylated AKT in turn has the potential to regulate multiple downstream effectors and signaling pathways that are involved for example in cell proliferation, apoptosis, migration, and metabolism [4]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
