THE ROLE AND MECHANISM OF THE HOMEOBOX GENE DLX4 IN TRANSFORMING GROWTH FACTOR-B RESISTANCE IN CANCER

Transforming growth factor-b (TGF-b) is a cytokine that plays essential roles in regulating embryonic development and tissue homeostasis. In normal cells, TGF-b exerts an anti-proliferative effect. TGF-b inhibits cell growth by controlling a cytostatic program that includes activation of the cyclin-dependent kinase inhibitors p15Ink4B and p21WAF1/Cip1 and repression of c-myc. In contrast to normal cells, many tumors are resistant to the anti-proliferative effect of TGF-b. In several types of tumors, particularly those of gastrointestinal origin, resistance to the anti-proliferative effect of TGF-b has been attributed to TGF-b receptor or Smad mutations. However, these mutations are absent from many other types of tumors that are resistant to TGF-b-mediated growth inhibition. The transcription factor encoded by the homeobox patterning gene DLX4 is overexpressed in a wide range of malignancies. In this study, I demonstrated that DLX4 blocks the anti-proliferative effect of TGF-b by disabling key transcriptional control mechanisms of the TGF-b cytostatic program. Specifically, DLX4 blocked the ability of TGF-b to induce expression of p15Ink4B and p21WAF1/Cip1 by directly binding to Smad4 and to 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. In addition, DLX4 induced expression of c-myc, a repressor of p15Ink4B and p21WAF1/Cip1 transcription, independently of TGF-b signaling. The ability of DLX4 to counteract key transcriptional control mechanisms of the TGF-b cytostatic program could explain in part the resistance of tumors to the anti-proliferative effect of TGF-b. This study provides a molecular explanation as to why tumors are resistant to the anti-proliferative effect of TGF-b in the absence of mutations in the TGF-b signaling pathway. Furthermore, this study also provides insights into how aberrant activation of a developmental patterning gene promotes tumor pathogenesis.

Cotransfer of antibiotic resistance genes and a hylEfm-containing virulence plasmid in Enterococcus faecium.

The hyl(Efm) gene (encoding a putative hyaluronidase) has been found almost exclusively in Enterococcus faecium clinical isolates, and recently, it was shown to be on a plasmid which increased the ability of E. faecium strains to colonize the gastrointestinal tract. In this work, the results of mating experiments between hyl(Efm)-containing strains of E. faecium belonging to clonal cluster 17 and isolated in the United States and Colombia indicated that the hyl(Efm) gene of these strains is also carried on large plasmids (>145 kb) which we showed transfer readily from clinical strains to E. faecium hosts. Cotransfer of resistance to vancomycin and high-level resistance (HLR) to aminoglycosides (gentamicin and streptomycin) and erythromycin was also observed. The vanA gene cluster and gentamicin resistance determinants were genetically linked to hyl(Efm), whereas erm(B) and ant(6)-I, conferring macrolide-lincosamide-streptogramin B resistance and HLR to streptomycin, respectively, were not. A hyl(Efm)-positive transconjugant resulting from a mating between a well-characterized endocarditis strain [TX0016 (DO)] and a derivative of a fecal strain of E. faecium from a healthy human volunteer (TX1330RF) exhibited increased virulence in a mouse peritonitis model. These results indicate that E. faecium strains use a strategy which involves the recruitment into the same genetic unit of antibiotic resistance genes and determinants that increase the ability to produce disease. Our findings indicate that the acquisition of the hyl(Efm) plasmids may explain, at least in part, the recent successful emergence of some E. faecium strains as nosocomial pathogens.

BIM MEDIATES IMATINIB-INDUCED APOPTOSIS OF GASTROINTESTINAL STROMAL TUMORS: TRANSLATIONAL IMPLICATIONS

Gastrointestinal stromal tumors (GISTs) are oncogene-addicted cancers driven by activating mutations in the genes encoding receptor tyrosine kinases KIT and PDGFR-α. Imatinib mesylate, a specific inhibitor of KIT and PDGFR-α signaling, delays progression of GIST, but is incapable of achieving cure. Thus, most patients who initially respond to imatinib therapy eventually experience tumor progression, and have limited therapeutic options thereafter. To address imatinib-resistance and tumor progression, these studies sought to understand the molecular mechanisms that regulate apoptosis in GIST, and evaluate combination therapies that kill GISTs cells via complementary, but independent, mechanisms. BIM (Bcl-2 interacting mediator of apoptosis), a pro-apoptotic member of the Bcl-2 family, effects apoptosis in oncogene-addicted malignancies treated with targeted therapies, and was recently shown to mediate imatinib-induced apoptosis in GIST. This dissertation examined the molecular mechanism of BIM upregulation and its cytotoxic effect in GIST cells harboring clinically-representative KIT mutations. Additionally, imatinib-induced alterations in BIM and pro-survival Bcl-2 proteins were studied in specimens from patients with GIST, and correlated to apoptosis, FDG-PET response, and survival. Further, the intrinsic pathway of apoptosis was targeted therapeutically in GIST cells with the Bcl-2 inhibitor ABT-737. These studies show that BIM is upregulated in GIST cells and patient tumors after imatinib exposure, and correlates with induction of apoptosis, response by FDG-PET, and disease-free survival. These studies contribute to the mechanistic understanding of imatinib-induced apoptosis in clinically-relevant models of GIST, and may facilitate prediction of resistance and disease progression in patients. Further, combining inhibition of KIT and Bcl-2 induces apoptosis synergistically and overcomes imatinib-resistance in GIST cells. Given that imatinib-resistance and GIST progression may reflect inadequate BIM-mediated inhibition of pro-survival Bcl-2 proteins, the preclinical evidence presented here suggests that direct engagement of apoptosis may be an effective approach to enhance the cytotoxicity of imatinib and overcome resistance.