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Polyoma middle T antigen activates the Ser/Thr kinase Akt in a PI3-kinase-dependent manner
S A Summers1, L Lipfert, M J Birnbaum
1Howard Hughes Medical Institute, University of Pennsylvania School of Medicine, Philadelphia 19104, USA.
Abstract:
Polyoma middle T antigen (PMT) was originally identified as the tumorigenic component of the polyomavirus genome. To investigate whether the serine/ threonine kinase Akt/PKB, which is the proto-oncogene transduced by the transforming AKT8 retrovirus, is activated by PMT, 3T3-L1 fibroblasts were stably transfected with wild type PMT. PMT expression accelerated glucose transport and increased phosphorylation of p70 S6-kinase and MAPK. PMT expression also stimulated Akt kinase activity 7 fold as compared to untreated, mock infected cells. This stimulation rivaled that obtained following insulin treatment of both mock and PMT infected cells. Akt activation and phosphorylation were eliminated in a PMT mutant incapable of interacting with PI3-kinase, but not one which does not interact with Shc, and correlated closely to the amount of PI3-kinase activity in anti-phosphotyrosine immunoprecipitates. These results indicate that the PI3-kinase pathway is requisite, but the Shc pathway is dispensable, for Akt activation. The studies further suggest that Akt may participate in PMT and PI3-kinase's regulation of cellular transformation and tumorigenesis.
Insights
Polyoma middle T antigen (PMT) activates the Akt kinase pathway, crucial for cell growth and transformation. This activation requires PI3-kinase signaling, highlighting its role in tumorigenesis.
Area of Science:
- Molecular Biology
- Oncology
- Cell Signaling
Background:
- Polyoma middle T antigen (PMT) is a known oncogenic protein.
- The serine/threonine kinase Akt/PKB is a proto-oncogene implicated in cellular transformation.
Purpose of the Study:
- To investigate the activation of Akt/PKB by PMT.
- To elucidate the signaling pathways involved in PMT-mediated Akt activation.
Main Methods:
- Stable transfection of 3T3-L1 fibroblasts with wild-type and mutant PMT.
- Assays for Akt kinase activity, glucose transport, and protein phosphorylation (p70 S6-kinase, MAPK).
- Analysis of protein interactions using immunoprecipitation.
Main Results:
- PMT expression significantly increased Akt kinase activity (7-fold), glucose transport, and phosphorylation of p70 S6-kinase and MAPK.
- Akt activation by PMT was dependent on PI3-kinase interaction but independent of Shc interaction.
- PMT-induced Akt activation correlated with PI3-kinase activity.
Conclusions:
- The PI3-kinase pathway is essential for Akt activation by PMT.
- The Shc pathway is not required for PMT-mediated Akt activation.
- Akt signaling is implicated in PMT-driven cellular transformation and tumorigenesis.