Implications of cellular senescence in tissue damage response, tumor suppression, and stem cell biology

Abstract

Cellular senescence is characterized by an irreversible cell cycle arrest that, when bypassed by mutation, contributes to cellular immortalization. Activated oncogenes induce a hyperproliferative response, which might be one of the senescence cues. We have found that expression of such an oncogene, Akt, causes senescence in primary mouse hepatoblasts in vitro. Additionally, AKT-driven tumors undergo senescence in vivo following p53 reactivation and show signs of differentiation. In another in vivo system, i.e., liver fibrosis, hyperproliferative signaling through AKT might be a driving force of the senescence in activated hepatic stellate cells. Senescent cells up-regulate and secrete molecules that, on the one hand, can reinforce the arrest and, on the other hand, can signal to an innate immune system to clear the senescent cells. The mechanisms governing senescence and immortalization are overlapping with those regulating self-renewal and differentiation. These respective control mechanisms, or their disregulation, are involved in multiple pathological conditions including fibrosis, wound healing, and cancer. Understanding extracellular cues that regulate these processes may enable new therapies for these conditions.

Figure 1

AKT induces cellular senescence and differentiation in cooperation with p53. (A) Wild-type (WT) but not p53^−/− primary hepatoblasts senesce in response to AKT. (B) AKT-driven tumor cells senesce in vivo in response to p53 restoration (+DOX), but not in the absence of p53 (−DOX). (C) Expression of the differentiation marker CK8 was increased in tumors following p53 restoration.

Figure 2

Senescent cells are present in mouse fibrotic liver. (A) Mice were treated with CCl₄ twice weekly for 6 weeks. (B) CCl₄-treated livers (fibrotic) but not control livers exhibit fibrotic scars (evaluated by H&E and Sirius Red staining). Multiple cells adjacent to the scar stain positively for the senescence marker SA-β-gal.

Figure 3

p53^−/−;Ink4a/ARF^−/− activated stellate cells are immortalized and contribute to fibrosis progression. (A) p53^−/−;Ink4a/ARF^−/− (DKO) but not wild-type-activated stellate cells form colonies following a 10-day colony-formation assay in vitro as evaluated by crystal violet staining. Numbers indicate amount of cells seeded per well. (B) Immunostaining identified higher expression of the activated stellate cell marker αSMA in fibrotic livers from DKO mice compared to wild-type mice.

Figure 4

AKT signaling might contribute to the senescence of activated stellate cells. pAKT was detected in cells expressing the activated stellate cell marker αSMA in mouse fibrotic livers. Nuclei were identified by DAPI.

Figure 5

The immune system facilitates the clearance of senescent activated stellate cells in vivo. (A) Electron microscopy revealed that immune cells ([lp] lymphocytes; [np] neutrophil) are adjacent to activated HSCs in fibrotic mouse livers but not in normal mouse livers. (B) Mice treated with CCl₄ were treated with an anti-NK antibody (to deplete NK cells), polyI:C (as an interferon-γ activator), or saline (as a control) for 10 days. More activated stellate cells, identified by αSMA, are retained in mouse livers following depletion of NK cells.

Figure 6

The eventual senescence of activated stellate cells limits fibrosis through a coordinated program involving cell cycle exit, down-regulation of ECM components, up-regulation of ECM-degrading enzymes, and enhanced immunosurveillance. (Reprinted, with permission, from Krizhanovsky et al. 2008 Supplemental Data [© Elsevier].)

Figure 7

Molecular pathways driving cell fate decisions. Common regulators are illustrated.