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Cell anchorage regulates apoptosis through the retinoblastoma tumor suppressor/E2F pathway
1Department of Surgery, Division of Urology, and the University of Michigan Comprehensive Cancer Center, University of Michigan, Ann Arbor, Michigan 48109, USA. mday@umich.edu
Abstract:
Epithelial cells are dependent upon adhesion to extracellular matrix for survival. We show that loss of beta1 integrin receptor contact with extracellular matrix signals the inhibition of G1 cyclin-dependent kinase activity. This loss of cyclin-dependent kinase activity leads to accumulation of the hypophosphorylated (active) form of the retinoblastoma tumor suppressor protein (Rb). We present evidence that in epithelial cells deprived of matrix contact, the growth suppression signal elicited by hypophosphorylated Rb opposes stimulatory signals from serum growth factors, leading to a cell cycle conflict that triggers apoptosis. This apoptotic pathway is modulated by Bcl-2 through a novel mechanism that regulates Rb phosphorylation. We present evidence that the Rb-dependent apoptotic pathway functions in vivo in the apoptosis of the prostate glandular epithelium following castration.
Insights
Epithelial cells require matrix adhesion for survival. Loss of beta1 integrin signaling triggers retinoblastoma protein (Rb) activation, leading to apoptosis, a process modulated by Bcl-2.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- Epithelial cell survival is contingent on extracellular matrix (ECM) interactions.
- Integrins are key mediators of cell-ECM adhesion and signaling.
- Disruptions in cell adhesion can trigger cell cycle arrest and apoptosis.
Purpose of the Study:
- To elucidate the molecular mechanisms linking loss of cell-ECM adhesion to apoptosis in epithelial cells.
- To investigate the role of beta1 integrin, retinoblastoma protein (Rb), and Bcl-2 in this process.
- To explore the in vivo relevance of this pathway in prostate epithelium following castration.
Main Methods:
- Investigated the impact of matrix detachment on G1 cyclin-dependent kinase (CDK) activity.
- Assessed the phosphorylation status and activity of the retinoblastoma tumor suppressor protein (Rb).
- Examined the role of Bcl-2 in modulating Rb phosphorylation and apoptosis.
- Studied the in vivo apoptosis of prostate glandular epithelium post-castration.
Main Results:
- Loss of beta1 integrin-ECM contact inhibits G1 cyclin-dependent kinase activity.
- This inhibition causes accumulation of hypophosphorylated (active) Rb.
- Hypophosphorylated Rb generates a growth-suppressive signal that conflicts with growth factors, inducing apoptosis.
- Bcl-2 modulates this apoptotic pathway via a novel mechanism regulating Rb phosphorylation.
- The Rb-dependent apoptotic pathway is active in vivo during prostate epithelial cell death after castration.
Conclusions:
- Matrix detachment-induced apoptosis in epithelial cells is mediated by the retinoblastoma protein (Rb) pathway.
- Bcl-2 plays a regulatory role in this Rb-dependent apoptotic cascade.
- This pathway is functionally relevant in vivo, as demonstrated in prostate cancer following castration.