Inhibition of AP-1 transcription factor causes blockade of multiple signal transduction pathways and inhibits breast cancer growth

Abstract

AP-1 transcription factors play a critical role in signal transduction pathways in many cells. We have investigated the role of AP-1 in controlling proliferative signals in breast cells, and have previously shown that AP-1 complexes are activated by peptide and steroid growth factors in both normal and malignant breast cells. In this study, we investigated the role of AP-1 in transducing proliferative signals induced by peptide and steroid growth factors. We used MCF-7 clones that express a specific inhibitor of AP-1, a dominantnegative cJun mutant (TAM67), under the control of an inducible promoter to investigate the role of AP-1 in regulating breast cancer growth. In the presence of doxycycline (Dox), the AP-1 inhibitor was not expressed, and the MCF-7 clones proliferated normally in response to serum stimulation. However, when Dox was withdrawn, TAM67 was expressed, AP-1 activity was inhibited, and serum-induced proliferation was blocked. We next investigated whether the mitogenic response to specific growth factors also requires AP-1. MCF-7 Tet-Off-TAM67 cells were grown in the presence of increasing concentrations of IGF-1, EGF, heregulin-b, bFGF, or estrogen under un-induced and induced conditions. These studies showed that the AP-1 inhibitor completely blocked proliferation in response to the peptide growth factors (IGF-1, EGF, heregulin-b, and bFGF), and partially blocked the response to estrogen. To investigate the effect of AP-1 blockade on in vivo tumor growth, we injected the MCF-7 TetOff TAM67 cells into nude mice receiving doxycycline to suppress the expression of the AP-1 inhibitor. After the mice developed tumors, they were randomized to either continue to receive Dox or not. In mice not receiving Dox, the expression of TAM67 was induced, and tumor growth was inhibited, while the tumors in mice receiving Dox continued to grow. Analysis of the tumors from these mice showed that the expression of TAM67 caused reduced proliferation of the breast cancer cells without inducing apoptosis. These results demonstrate that AP-1 blockade supresses mitogenic signals from multiple different peptide growth factors as well as estrogen, and inhibits the growth of MCF-7 breast cancer cells both in vitro and in vivo. These results suggest that novel agents specifically targeting AP-1 or its activating kinases could be promising agents for the treatment of breast cancer.

AP-1 blockade in breast cancer cells causes cell cycle arrest by suppressing G1 cyclin expression and reducing cyclin-dependent kinase activity

“…AP-1 blockade causes decreased expression of D and E cyclins, the main cyclins in G1 phase of cell cycle, and These changes in the expression of cell cycle regulators leads to reduced CDK2 and CDK4 activity, which in turn causes hypophosphorylation of Rb and inhibition of E2F activity, ultimately inducing a G1 cell cycle block. In combination with our previous results showing that expression of TAM67 blocks signal transduction by multiple growth factors (Liu et al, 2002), these studies demonstrate that AP-1 blockade can effectively block signal transduction and inhibit the growth of breast cancer cells. We and others have previously shown that AP-1 transcription factors are critical for cell proliferation and transformation of several cell types (Holt et al, 1986;Brown et al, 1994;Chen et al, 1996a;Liu et al, 2002).…”

“…We have also demonstrated that TAM67 inhibited breast cancer growth both in vivo and in vitro (Liu et al, 2002). These studies suggest that AP-1 transcription factor is an important regulator of breast cancer cell growth, invasion, and resistance to antiestrogens.…”

“…We previously showed a cJun dominant-negative mutant, TAM67, suppresses AP-1 activity, inhibits cell growth induced by several different growth factors, (EGF, IGF-1, heregulin-b, bFGF, and estrogen), and MCF7 xenograft tumor growth (Ludes-Meyers et al, 2001;Liu et al, 2002). In this study, we investigated the molecular mechanism by which AP-1 blockade suppresses breast cancer cell growth using MCF7 cells that express TAM67 under the control of an inducible promoter.…”

Modulation of tamoxifen sensitivity by antisense Bcl‐2 and trastuzumab in breast carcinoma cells

“…With respect to the mechanism of TAM resistance, it has been reported that breast carcinoma cells can undergo hormoneindependent growth and, in turn, growth factor-dependent growth activated by the transcription factor AP-1. 38 Overexpression of HER-2 is observed in TAMresistant cells, 39 and treatment with trastuzumab overcomes this resistance. 40 Moreover, TAM resistance also is induced by overexpression of Akt and Bcl-2.…”

Identification of integrins α6 and β7 as c‐Jun‐ and transformation‐relevant genes in highly invasive fibrosarcoma cells

“…1 The expression of c-jun is also constitutively increased in many other transformed cell lines 6,7 and human cancers. [8][9][10] Further, a direct jun amplification and overexpression has recently been demonstrated in aggressive human sarcomas. 11 Additionally, c-Jun is a central mediator of the activity of many oncogenes activated in human tumors.…”