Protein Tyrosine Phosphatases PTP-1B, SHP-2, and PTEN Facilitate Rb/E2F-Associated Apoptotic Signaling

To maintain tissue homeostasis, apoptosis is functionally linked to the cell cycle through the retinoblastoma (Rb)/E2F pathway. When the Rb tumor suppressor protein is functionally inactivated, E2F1 elicits an apoptotic response through both intrinsic (caspase-9 mediated) and extrinsic (caspase-8 mediated) apoptotic pathways in order to eliminate hyperproliferative cells. Rb/E2F-associated apoptosis has been demonstrated to be associated with the loss of constitutive transcriptional repression by Rb/E2F complexes and mediated by caspase-8. Protein tyrosine phosphatases (PTPs) PTP-1B and SHP-2 have been previously shown to be directly activated by loss of Rb/E2F repression during Rb/E2F-associated apoptosis. In this current study, we demonstrate that the PTEN tumor suppressor is also directly activated by loss of Rb/E2F repression. We also demonstrate that PTP-1B, SHP-2, and PTEN play a functional role in Rb/E2F-associated apoptosis. Knockdown of PTP1B, SHP2, or PTEN expression with small interfering RNA (siRNA) in apoptotic cells increases cell viability and rescues cells from the Rb/E2F-associated apoptotic response. Furthermore, rescue from apoptosis coincides with inhibition of caspase-8 and caspase-3 cleavage (activation). Our results indicate PTP-1B, SHP-2, and PTEN all play a functional role in Rb/E2F-associated apoptotic signal transduction and provide further evidence that PTP-1B, SHP-2, and PTEN can contribute to tumor suppression through an Rb/E2F-associated mechanism.