Reduced Levels of ATF-2 Predispose Mice to Mammary Tumors

Abstract

Transcription factor ATF-2 is a nuclear target of stress-activated protein kinases, such as p38, which are activated by various extracellular stresses, including UV light. Here, we show that ATF-2 plays a critical role in hypoxia-and high-cell-density-induced apoptosis and the development of mammary tumors. Compared to wild-type cells, Atf-2 ؊/؊ mouse embryonic fibroblasts (MEFs) were more resistant to hypoxia-and anisomycininduced apoptosis but remained equally susceptible to other stresses, including UV. Atf-2 ؊/؊ and Atf-2MEFs could not express a group of genes, such as Gadd45␣, whose overexpression can induce apoptosis, in response to hypoxia. Atf-2 ؊/؊ MEFs also had a higher saturation density than wild-type cells and expressed lower levels of Maspin, the breast cancer tumor suppressor, which is also known to enhance cellular sensitivity to apoptotic stimuli. Atf-2 ؊/؊ MEFs underwent a lower degree of apoptosis at high cell density than wild-type cells. Atf-2 ؉/؊ mice were highly prone to mammary tumors that expressed reduced levels of Gadd45␣ and Maspin. The ATF-2 mRNA levels in human breast cancers were lower than those in normal breast tissue. Thus, ATF-2 acts as a tumor susceptibility gene of mammary tumors, at least partly, by activating a group of target genes, including Maspin and Gadd45␣.

ATF-2 controls transcription of Maspin and GADD45α genes independently from p53 to suppress mammary tumors

“…p53 activates the Gadd45a transcription via the p53-binding site in the third intron. ATF-2 activates Gadd45a transcription independently from p53 in response to hypoxic stress or anisomycin (AM) treatment (Maekawa et al, 2007). Recombinant ATF-2 did not bind to the GADD45a promoter (data not shown).…”

“…In contrast, ATF-2 did not Figure 4 Mammary tumors of Atf-2 þ /À p53 þ /À mice. (a) Kaplan-Meier tumor-free survival curves of Atf-2 þ /À p53 þ /À , p53 þ /À and Atf-2 þ /À (Maekawa et al, 2007) mice. The P-value obtained by the Mantel-Cox log-rank test: Atf-2 þ /À p53 þ /À vs Atf-2 þ /À , P ¼ 0.0004; Atf-2 þ /À p53 þ /À vs p53 þ /À , P ¼ 0.0127.…”

“…By using Atf-2 À/À mouse embryonic fibroblasts (MEFs), we recently showed that ATF-2 plays a role in hypoxiainduced apoptosis (Maekawa et al, 2007). ATF-2 is involved in the apoptosis at the saturation cell density, and the loss of one copy of Atf-2 facilitates the activated ras-mediated transformation of cultured cells.…”

“…In the absence of hypoxia, ATF-2 also activates some genes including Maspin, which encodes a member of the serine protease inhibitor family (Maekawa et al, 2007). Maspin was identified as a tumor suppressor in human breast cancers (Zou et al, 1994).…”

“…The expression of Maspin is regulated by p53 (Zou et al, 2000). In the mammary tumors arisen in Atf-2 þ /À mice, downregulation of both Gadd45a and Maspin was observed (Maekawa et al, 2007). However, it remains unknown how ATF-2 regulates the transcription of those genes.…”