Breast Cancer Cell Proliferation Is Inhibited by BAD

Abstract

Recent studies on BCL2 family proteins suggest that members of the proapoptotic arm have functions distinct from apoptosis induction. BID has been shown to have a role in the DNA damage response (1), and BAX and BID take part in homologous recombination (2). A prosurvival role for BAD was demonstrated in murine neuronal cells (2), and metabolic functions of BAD were described in mice (3). Growth regulation by BCL2 family proteins has been previously reported, although the mechanisms are poorly defined (4 -6). BAD is devoid of the membrane-anchoring hydrophobic region present in most other BCL2 proteins and it resides primarily in the cytoplasm. Under apoptotic conditions, BAD is found in the mitochondria, binding to BCL2 and BCLxl (7,8 , respectively) and promote its sequestration by 14-3-3 proteins. This mechanism obliterates the inhibitory effects of BAD on prosurvival BCLxl/BCL2 (9).We previously showed in MCF7 breast cancer epithelial cells that estradiol inhibits apoptosis by inducing phosphorylation of BAD (10), and a role for BAD in regulating apoptosis in murine breast epithelial cells has been described in vivo (9). In our studies, we consistently observed that mere overexpression of BAD in MCF7 cells did not cause apoptosis; rather there was an enhanced response once the cells were exposed to an apoptotic stimulus. This suggests levels of regulation between cytoplasmic BAD and its binding and inhibition of BCL2/BCLxL in mitochondria. Similar to our work in MCF7 cells, studies in BAD knock-out mice demonstrated the importance of BAD in survival promotion by growth factors (9). Furthermore, cleavage of long form BAD to a short form was required for its apoptotic role (3). These studies taken together indicate that BAD, by itself, does not necessarily cause apoptosis.In our previous study (10) and in unpublished work, 4 we observed that some portion of BAD localizes in the nucleus and that BAD overexpression led to decreased cell proliferation. In this study, we have studied in detail the cell cycle regulation of MCF7 cells by BAD. This cell line has been utilized as the premier model of hormone-sensitive (estrogen receptor-positive) breast cancer by many laboratories, including our own. In MCF7 cells, cyclin D1 has a rate-limiting role in G 1 -S phase transition (11-13), and both transgenic mouse models and clin-

Downregulation of Dickkopf-1 is responsible for high proliferation of breast cancer cells via losing control of Wnt/β-catenin signaling

“…cyclin D1 has a rate-limiting role in G 1 -S phase transition. Both transgenic mouse models and clinical studies indicate a pivotal role for cyclin D1 in normal and malignant cell growth, particularly in breast cancer [36] . In our experiments, Western blot analysis showed that DKK-1 was able to downregulate the expression of c-Myc and cyclin D1 (Figure 3), suggesting that c-Myc and cyclin D1 are downstream effectors of DKK-1.…”

AF1q enhancement of γ irradiation-induced apoptosis by up-regulation of BAD expression via NF-κB in human squamous carcinoma A431 cells

“…The alteration of the balance may lead to the leakage of cytochrome c and the formation of apoptosome which subsequently activates the caspase cascade culminating in apoptosis (2). BAD (BCL-2 antagonist of cell death) is a BH3-only proapoptotic BCL-2 family protein and plays a critical role in radiation-induced apoptosis (3)(4)(5). Under physiological conditions, anti-apoptotic BCL-2 family proteins promote cell survival by sequestering the pro-apoptotic BCL-2 proteins such as BAX and BAK (6).…”

The BH3-only protein Bad confers breast cancer taxane sensitivity through a nonapoptotic mechanism

“…On the contrary, paclitaxel treatment did not induce mitochondrial translocation of Bad (Figure 4a, compare green arrows). Furthermore, because nuclear localization of Bad has also been reported (Fernando et al, 2007), we used a more specific subcellular fractionation method to examine whether paclitaxel treatment affected accumulation of Bad to the nucleus. As shown in Figure 4b, again staurosporine, but not paclitaxel treatment, stimulated Bad translocation to mitochondrial-enriched membrane fractions, but there was no detectable nuclear accumulation of Bad under any of the conditions tested.…”