Involvement of Bcl-2 Family Proteins in Tetraploidization-Related Senescence

Abstract

The B-cell lymphoma 2 (Bcl-2) family of proteins is the main regulator of apoptosis. However, multiple emerging evidence has revealed that Bcl-2 family proteins are also involved in cellular senescence. On the one hand, the different expression of these proteins determines the entry into senescence. On the other hand, entry into senescence modulates the expression of these proteins, generally conferring resistance to apoptosis. With some exceptions, senescent cells are characterized by the upregulation of antiapoptotic proteins and downregulation of proapoptotic proteins. Under physiological conditions, freshly formed tetraploid cells die by apoptosis due to the tetraploidy checkpoint. However, suppression of Bcl-2 associated x protein (Bax), as well as overexpression of Bcl-2, favors the appearance and survival of tetraploid cells. Furthermore, it is noteworthy that our laboratory has shown that the joint absence of Bax and Bcl-2 antagonist/killer (Bak) favors the entry into senescence of tetraploid cells. Certain microtubule inhibitory chemotherapies, such as taxanes and vinca alkaloids, induce the generation of tetraploid cells. Moreover, the combined use of inhibitors of antiapoptotic proteins of the Bcl-2 family with microtubule inhibitors increases their efficacy. In this review, we aim to shed light on the involvement of the Bcl-2 family of proteins in the senescence program activated after tetraploidization and the possibility of using this knowledge to create a new therapeutic strategy targeting cancer cells.

Keywords: Bcl-2 family proteins; apoptosis; cancer; senescence; tetraploidy.

Figure 1

Apoptosis and senescence: two ways to suppress cell proliferation. Under physiological conditions, such as embryonic development or natural aging, healthy cells can enter apoptosis or senescence, respectively. However, apoptosis and senescence also play a role in pathophysiological situations. Certain stimuli can provoke stress signals, such as DNA damage or increased reactive oxygen species (ROS), leading to the suppression of cell proliferation by apoptosis or senescence. B-cell lymphoma 2 (Bcl-2) family proteins are differentially expressed in each case. Apoptosis is characterized by the increased expression of proapoptotic proteins and decreased expression of antiapoptotic proteins, whereas, in senescence, there is generally an increase in antiapoptotic proteins and a decrease in proapoptotic proteins. Likewise, senescent cells show an increase in senescent markers, such as activation of the p53/p21^Waf/Cip1 (p53/p21), p16^INK4a/retinoblastoma protein (p16/pRb) and senescence-associated secretory phenotype (SASP) pathways. Up arrows mean increase, down arrows mean decrease. Black arrows are related to Bcl-2 family proteins, while red arrows are related to senescence hallmarks.

Figure 2

Role of the antiapoptotic proteins of the Bcl-2 family. (A) High levels of Bcl-2 trigger senescence in response to several stimuli (1), while Bcl-2 overexpression confers the resistance of senescent cells to apoptosis (2). Overexpression of Bcl-2, inhibition of cyclin-dependent kinase (Cdk)2 and induction of p27^Kip1 (p27) lead to cell cycle arrest in G1. Moreover, Bcl-2 upregulates p27 ^Kip1 and p130, which forms repressive complexes with transcriptor factor E2F4, inhibiting its release and preventing cell cycle progression. Oncogene-induced senescence (OIS) and therapy-induced senescence (TIS) increase Bcl-2 expression via p38 mitogen-activated protein kinase (p38MAPK) and via p53/p16^INK4a (p53/p16), respectively. Jun N-terminal kinase (JNK) inhibition leads to the dephosphorylation of Bcl-2, accumulation of ROS, induction of DNA damage response (DDR) and G2/M arrest, which is characterized by an increase in the p53/p21 pathway, inactivation of M-phase inducer phosphatase 3 (Cdc25C, P-Cdc25C) and a reduction of cyclin B/Cdk2. ROS, reactive oxygen species; DDR, DNA damage response. (B) Overexpression of B-cell lymphoma extra-large (Bcl-xL) or Induced myeloid leukemia cell differentiation protein (Mcl-1) reduces entry into senescence (1). A possibility is that Bcl-xL blocks p38MAPK activation and inhibits senescence induction by preventing p53-induced ROS generation. Treatment with G2/M blocking agents causes translocation of Bcl-xL and/or Mcl-1 to the nucleus, where they bind to Cdk1-stabilizing senescence. Senescent cells showing elevated Bcl-xL or Mcl-1 levels prevent apoptosis (2). Up arrows mean increase, down arrows mean decrease.

Figure 3

Role of the proapoptotic proteins of the Bcl-2 family. (A) Different stimuli activate the H2A histone family member X (γ-H2AX)/p53/p21/Bax pathway at different levels. The Cdk2ap1 knockdown (Cdk2ap1 KD) and combinatorial treatment of AMG-232 with radiation causes an increase in γ-H2AX. Treatment with metformin or phenformin, as well as inhibition of cyclin B1, causes an increase in the expression of p53 and/or p21. Overexpression of Ing5 (Ing5 oe) results in a lower expression of Cdk2/Cdk4 and higher expression of p53/p21. As a result, senescent cells with elevated Bax expression are obtained in all cases. (B) Different senescence conditions result in the differentially modified expression of proapoptotic BH3-only proteins, as well as increased expression of senescent markers. Cdk2ap1, Cdk2-associated protein-1; Ing5, inhibitor of growth protein 5; MEKi, mitogen-activated protein kinase kinase inhibitors; DNMTi, DNA methyltransferase inhibitor; VSMC, vascular smooth muscle cells. Up arrows mean increase, down arrows mean decrease.

Figure 4

Involvement of Bcl-2 family proteins and senescence in the emergence of tetraploid cells. (A) The generation of tetraploid cells is favored by the absence of p53, retinoblastoma protein (pRb), p21^Waf1/Cip1 (p21) and Bax, as well as the overexpression of Bcl-2 and the presence of Bcl-xL. (B) Cells deficient in Bax and Bak enter senescence when tetraploidized with microtubule inhibitors and show an increase in p21^Waf1/Cip1, p16^INK4a (p16) and p27^Kip1 (p27). (C) Some chemotherapies, such as taxanes and vinca alkaloids, induce tumor cell polyploidization and cell cycle arrest. The table summarizes the relationship found between tetra- or polyploidy-inducing chemotherapies, Bcl-2 family proteins and their effects on tumor cells. Up arrows mean increase, down arrows mean decrease.