Ras protects Rb family null fibroblasts from cell death: a role for AP-1

Area of Science:

• Molecular Biology
• Cell Biology
• Oncogenesis

Background:

• The retinoblastoma protein (Rb) is a crucial tumor suppressor controlling cell proliferation, differentiation, and senescence.
• Elimination of Rb function is necessary for cells to achieve unlimited proliferative potential, but this loss normally triggers apoptosis.
• Tumorigenic cells must therefore overcome both Rb's suppressive function and the subsequent apoptotic response.

Purpose of the Study:

• To investigate how oncogenic Ras overcomes the apoptotic response triggered by the loss of Rb function.
• To identify the specific Ras effector pathways involved in promoting cell survival in the absence of Rb.
• To elucidate the role of Raf, MEK, c-Jun, and AP-1 in Ras-mediated cell death blockade.

Main Methods:

• Utilized Rb family member knockout mouse embryo fibroblasts (TKO cells).
• Introduced oncogenic Ras (RasV12) to TKO cells.
• Investigated the roles of phosphatidylinositol 3-kinase, Raf, and MEK pathways in cell survival.
• Assessed the involvement of c-Jun and AP-1 activation in Ras-mediated protection.

Main Results:

• Oncogenic Ras (RasV12) potently blocked cell death in TKO cells.
• Activation of phosphatidylinositol 3-kinase and Raf by Ras contributed to this pro-survival signal.
• A Raf-dependent, MEK-independent pathway, involving c-Jun and AP-1 activation, was critical for Ras-mediated protection.
• Pharmacologic inhibition of MEK had minimal impact on RasV12-induced cell death blockade.

Conclusions:

• Oncogenic Ras employs multiple effector pathways, including a novel Raf-dependent, MEK-independent route, to suppress apoptosis induced by Rb loss.
• Activation of c-Jun and AP-1 is a key mechanism by which Ras promotes survival, explaining its role in oncogenesis.
• Tumorigenesis requires cooperation between distinct oncogenic events to achieve both immortalization and survival.