Abstract B15: The homeobox patterning gene DLX4 confers resistance to transforming growth factor-β signaling in tumors

Abstract

Abstract Rationale: Transforming growth factor-β (TGF-β) inhibits cell proliferation and also induces epithelial-to-mesenchymal transition (EMT). The antiproliferative effect of TGF-β is orchestrated by a cytostatic program of gene responses that induce G1 arrest and are controlled by Smad-dependent mechanisms, whereas TGF-β induces EMT by both Smad and non-Smad mechanisms. In contrast to normal cells, many tumors are resistant to the antiproliferative effect of TGF-β. This resistance has been attributed to TGF-β receptor or Smad mutations in several types of tumors, particularly those of gastrointestinal origin. However, the rareness of these mutations in many other tumors (e.g., prostate, lung cancers) indicates that resistance to TGF-β also stems from other aberrations. The transcription factor encoded by the homeobox gene DLX4 is aberrantly expressed in a wide range of malignancies. Increasing evidence indicates that crosstalk between members of the TGF super-family and DLX genes is important for controlling normal embryonic patterning. The objective of this study is to determine whether and how DLX4 modulates the TGF-β signaling pathway in tumors. Experimental procedure: The effect of DLX4 on TGF-β-mediated growth inhibition and EMT was investigated by cell cycle and morphological analyses in different types of nontumorigenic and malignant epithelial cells. Molecular mechanisms of DLX4 in modulating TGF-β signaling were determined by protein and chromatin immunoprecipitation and analysis of transcriptional activity of TGF-β target genes. Results: Our studies demonstrate that DLX4 blocks the antiproliferative effect of TGF-β by disabling key transcriptional control mechanisms of the TGF-β cytostatic program. In particular, DLX4 blocked the ability of TGF-β to induce p15Ink4B and p21WAF1/Cip1 transcription by directly interacting with Smad4 and Sp1. Binding of DLX4 to Smad4 prevented Smad4 from forming transcriptional complexes with Smad2 and Smad3, whereas binding of DLX4 to Sp1 inhibited DNA-binding activity of Sp1. DLX4 also induced expression of c-myc, a repressor of p15Ink4B and p21WAF1/Cip1 transcription. On the other hand, DLX4 enabled epithelial cells to retain epithelial morphology and partially prevented TGF-β-mediated downregulation of E-cadherin and induction of N-cadherin. Conclusion: Our studies demonstrate that DLX4 blocks the antiproliferative effect of TGF-β and partially inhibits TGF-β-mediated EMT. The ability of DLX4 to counteract key transcriptional control mechanisms of the TGF-β cytostatic program could explain in part the resistance of tumors to the anti-proliferative effect of TGF-β in the absence of mutations in core components of the TGF-β signaling pathway. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr B15.