The Retinoblastoma Protein Binds the Promoter of the Survival Gene bcl-2 and Regulates Its Transcription in Epithelial Cells through Transcription Factor AP-2

Abstract

The retinoblastoma (RB) gene product has been shown to restrict cell proliferation, promote cell differentiation, and inhibit apoptosis. Loss of RB function can induce both p53-dependent apoptosis and p53-independent apoptosis; little is known about the mechanisms of RB-regulated p53-independent apoptosis. Here we show that RB specifically activates transcription of the survival gene bcl-2 in epithelial cells but not in NIH 3T3 mesenchymal cells. This transcriptional activity is mediated by the transcription factor AP-2. By monitoring protein-DNA interactions in living cells using formaldehyde cross-linking and chromatin immunoprecipitation, we show that endogenous RB and AP-2 both bind to the same bcl-2 promoter sequence. In addition, we demonstrate that RB and AP-2 also bind to the E-cadherin gene promoter in vivo, consistent with regulation of this promoter by both AP-2 and RB in epithelial cells. This study provides evidence that RB activates bcl-2 and E-cadherin by binding directly to the respective promoter sequences and not indirectly by repressing an inhibitor. This recruitment is mediated by a transcription factor, in this case AP-2. For the first time, our results suggest a direct molecular mechanism by which RB might inhibit apoptosis independently of p53. The results are discussed in a context where RB and Bcl-2 contribute under nonpathological conditions to the maintenance of cell viability in association with a differentiated phenotype, contributing to the tumor suppressor function of RB and playing important roles in normal development.The harmonious development of any organism requires a fine equilibrium between the processes of cell proliferation, differentiation, and apoptosis. These different functions are highly regulated and interdependent. Many of the genes involved in the regulation of these functions function as oncogenes or tumor suppressor genes.The retinoblastoma (RB) gene is an important tumor suppressor, and its protein product has been shown to restrict cell proliferation, promote cell differentiation, and inhibit apoptosis (for reviews, see references 15, 28, and 53).RB can act as either a negative or a positive regulator of transcription. In the context of cell proliferation, it acts as a negative regulator. The most widely accepted hypothesis proposes that RB represses transcription through the E2F family of transcription factors, partly through masking their activation domains and partly by recruiting a histone deacetylase to promoters that are repressed during the G 1 phase of the cell cycle (for a review, see reference 37). It has also been suggested that repression and activation of E2F-responsive genes may occur through distinct E2F heterodimers (46). In contrast to cell proliferation, when RB promotes differentiation, it regulates the activity of several transcription factors in a positive manner (for a review, see reference 35). However, the precise molecular mechanism for this activity of RB has not yet been elucidated. Similarly, RB also inhibits apoptosis, but it is not...

Transcription factor AP-2α triggers apoptosis in cardiac myocytes

“…AP-2a has been shown to inhibit the activation of apoptosis mediated by the proto-oncogene c-myc, 42 and recently, AP-2a has been implicated in the antiapoptotic effects of the retinoblastoma gene product and in the induction of Bcl-2 in epithelial but not in NIH 3T3 mesenchymal cells. 43 AP-2a has also been recently reported to interact directly with p53 and to augment p53 transcriptional activation. 33 This finding would be in line with our data that Bax, a classical proapoptotic target gene of p53, is increased after overexpression of AP-2a.…”

Androgens repress Bcl-2 expression via activation of the retinoblastoma (RB) protein in prostate cancer cells

“…Importantly, we demonstrated that androgen-induced suppression of Bcl-2 expression was abolished by specific inactivation of RB function by the mutated E1A. Recently, Decary et al (2002) demonstrated that overexpression of RB protein transactivates expression of the Bcl-2 gene in kidney and breast epithelial cells and that this activation is mediated the P2 promoter. The transactivation of Bcl-2 by RB relies completely on expression of the AP2 transcription factor, since no effect of RB on Bcl-2 expression has been found in cells without AP2 expression (Decary et al, 2002).…”

“…Recently, Decary et al (2002) demonstrated that overexpression of RB protein transactivates expression of the Bcl-2 gene in kidney and breast epithelial cells and that this activation is mediated the P2 promoter. The transactivation of Bcl-2 by RB relies completely on expression of the AP2 transcription factor, since no effect of RB on Bcl-2 expression has been found in cells without AP2 expression (Decary et al, 2002). Indeed, no AP2 mRNA has been detected by Northern blot analyses in either LNCaP or PC-3 prostate cancer cells (Butler et al, 2000).…”

AP2 transcription factor induces apoptosis in retinoblastoma cells

“…In support of this hypothesis, TFAP2A has already been shown to physically interact with pRB in vitro and associates with pRB in vivo (Wu and Lee, 1998). Furthermore, pRB regulates the transcription of E-cadherin and BCL2 in epithelial cells through interaction with TFAP2 (Batsche et al, 1998;Decary et al, 2002).…”

“…For example, TFAP2A and TFAP2B interact with pRB in vitro (Wu and Lee, 1998), and transcription of genes such as BCL2 and E-cadherin is mediated by pRB interacting with TFAP2 (Batsche et al, 1998;Decary et al, 2002). To address a possible role for TFAP2 in retinoblastoma, we have studied the expression of TFAP2A and TFAP2B in human retina and in retinoblastoma tumors and cell lines.…”