Platinum-based chemotherapies are effective agents in the treatment of a wide variety of human cancers. However, patients with recurrent disease can become resistant to platinum-based chemotherapy, leading to low overall survival rates. Activating transcription factor 3 (ATF3) is a stress-inducible gene that is a regulator of cisplatin-induced cytotoxicity. ATF3 protein expression was upregulated after cytotoxic doses of cisplatin treatment in a panel of cell lines. A chromatin immunoprecipitation assay showed that upon treatment with cisplatin, ATF3 directly bound to the CHOP gene promoter and this correlated with an increase in CHOP protein expression. In a 1200 compound library screen performed on cancer cell lines, disulfiram, a dithiocarbamate drug, was identified as an enhancer of the cytotoxic effects of cisplatin. This increased cytotoxic action was likely due to disulfiram and cisplatin’s ability to induce ATF3 independently through two separate mechanisms, namely the MAPK and integrated stress pathways. Furthermore, ATF3 protein and mRNA levels were variable amongst human ovarian and lung cancer tissues, suggesting the potential for basal expression of ATF3 to be predictive of cisplatin treatment response. Thus, understanding ATF3’s role in cisplatin-induced cytotoxicity will lead to novel therapeutic approaches that could improve this drug’s efficacy.

Iroquois homeobox (Irx) family members, a group of highly conserved homeodomain containing transcription factors, are involved in the patterning and the proper functions of vertebrate organs. They can act as transcriptional activators or repressors in a context-dependent manner. Preliminary data indicated that Irx3 and Irx5 are functionally redundant during cardiac morphogenesis, and they physically interact with other cardiac transcription factors. At E14.5, outflow tract septation failure and ventricular septation failure were observed in Irx3/5DKO mouse hearts. Loss of Irx3/5 in neural crest and endothelial cell lineages led to outflow tract septation failure and ventricular septal defect. In adult mice, Irx3 is expressed in the atrioventricular conduction system, and loss of Irx3 leads to slower ventricular conduction velocity. qRT-PCR analysis and immunofluorescence staining revealed that the expression of gap junction proteins, Cx40 and Cx43, are affected by the loss of Irx3. Over-expression of Irx3 and a dominant repressor form of Irx3, Irx3-EnR, resulted in Cx40 upregulation, indicating that Irx3 acts as an indirect positive regulator of Cx40. Irx3-EnR over-expression in vivo resulted in postnatal onset of atrial enlargement, ventricular hypertrophy, and conduction failure. Taken together, this study demonstrates the significance of Irx3/5 in both cardiovascular development and cardiac electrophysiology.

The specialized myocytes of the ventricular conduction system (VCS) coordinate ventricular contraction and are critical for efficient pumping by the heart. Impaired VCS conduction is characteristic of inherited forms of cardiac conduction disorders. Here we show that the Iroquois homeobox 3 (Irx3) transcription factor is preferentially expressed in the developing and mature VCS. Loss of Irx3 in mice results in slowed VCS conduction and prolonged QRS duration with right bundle branch block, caused by reduction (42%) in VCS-specific connexin 40 (Cx40) expression and VCS fiber hypoplasia, absent in littermate controls. Therefore, we show that the role of Irx3 in the heart is two-fold, whereby Irx3 (1) indirectly regulates Cx40 gene expression, by repressing a repressor of Cx40 transcript, and (2) controls VCS maturation, possibly in an Nkx2-5-dependent manner. To our knowledge, this is the first report of a role for Irx3 in regulating the development and function of the VCS.

The Escherichia coli transcription factor NikR is responsible for nickel-mediated repression of the nik operon. The crystal structure of NikR in complex with its operator sequence provided insight into the mechanistic details of nickel-activated NikR-DNA complex formation. The crystal structure revealed that the α3 helix and its preceding loop (residues 63-79) in two of the metal-binding domains—which become structurally ordered after high-affinity nickel binding—make non-specific contacts with the DNA phosphodiester backbone. The proposed mechanism of NikR binding to DNA suggests that the non-specific interactions between the DNA phosphodiester backbone and the positively-charged residues Lys64 and Arg65 anchor NikR to the DNA, thereby allowing the protein to initiate a one-dimensional search for its recognition sequence. The DNA-binding studies presented here strongly support an important role for Lys64 and Arg65 in NikR-DNA complex formation which is in agreement with the proposed model of NikR binding to DNA.

Iroquois homeobox (Irx) family members, a group of highly conserved homeodomain containing transcription factors, are involved in the patterning and the proper functions of vertebrate organs. They can act as transcriptional activators or repressors in a context-dependent manner. Preliminary data indicated that Irx3 and Irx5 are functionally redundant during cardiac morphogenesis, and they physically interact with other cardiac transcription factors. At E14.5, outflow tract septation failure and ventricular septation failure were observed in Irx3/5DKO mouse hearts. Loss of Irx3/5 in neural crest and endothelial cell lineages led to outflow tract septation failure and ventricular septal defect. In adult mice, Irx3 is expressed in the atrioventricular conduction system, and loss of Irx3 leads to slower ventricular conduction velocity. qRT-PCR analysis and immunofluorescence staining revealed that the expression of gap junction proteins, Cx40 and Cx43, are affected by the loss of Irx3. Over-expression of Irx3 and a dominant repressor form of Irx3, Irx3-EnR, resulted in Cx40 upregulation, indicating that Irx3 acts as an indirect positive regulator of Cx40. Irx3-EnR over-expression in vivo resulted in postnatal onset of atrial enlargement, ventricular hypertrophy, and conduction failure. Taken together, this study demonstrates the significance of Irx3/5 in both cardiovascular development and cardiac electrophysiology.

The specialized myocytes of the ventricular conduction system (VCS) coordinate ventricular contraction and are critical for efficient pumping by the heart. Impaired VCS conduction is characteristic of inherited forms of cardiac conduction disorders. Here we show that the Iroquois homeobox 3 (Irx3) transcription factor is preferentially expressed in the developing and mature VCS. Loss of Irx3 in mice results in slowed VCS conduction and prolonged QRS duration with right bundle branch block, caused by reduction (42%) in VCS-specific connexin 40 (Cx40) expression and VCS fiber hypoplasia, absent in littermate controls. Therefore, we show that the role of Irx3 in the heart is two-fold, whereby Irx3 (1) indirectly regulates Cx40 gene expression, by repressing a repressor of Cx40 transcript, and (2) controls VCS maturation, possibly in an Nkx2-5-dependent manner. To our knowledge, this is the first report of a role for Irx3 in regulating the development and function of the VCS.

The Escherichia coli transcription factor NikR is responsible for nickel-mediated repression of the nik operon. The crystal structure of NikR in complex with its operator sequence provided insight into the mechanistic details of nickel-activated NikR-DNA complex formation. The crystal structure revealed that the α3 helix and its preceding loop (residues 63-79) in two of the metal-binding domains—which become structurally ordered after high-affinity nickel binding—make non-specific contacts with the DNA phosphodiester backbone. The proposed mechanism of NikR binding to DNA suggests that the non-specific interactions between the DNA phosphodiester backbone and the positively-charged residues Lys64 and Arg65 anchor NikR to the DNA, thereby allowing the protein to initiate a one-dimensional search for its recognition sequence. The DNA-binding studies presented here strongly support an important role for Lys64 and Arg65 in NikR-DNA complex formation which is in agreement with the proposed model of NikR binding to DNA.

Irx5 is a homeodomain transcription factor that negatively regulates cardiac fast transient outward K+ currents (Ito,f) via the KV4.2 gene and is thereby a major determinant of the transmural repolarization gradient. While Ito,f is invariably reduced in heart disease and changes in Ito,f can modulate both cardiac contractility and hypertrophy, less is known about a functional role of Irx5, and its relationship with Ito,f, in the normal and diseased heart. Here I show that Irx5 plays crucial roles in the regulation of cardiac contractility and proper adaptive hypertrophy. Specifically, Irx5-deficient (Irx5-/-) hearts had reduced cardiac contractility and lacked the normal regional difference in excitation-contraction with decreased action potential duration, Ca2+ transients and myocyte shortening in sub-endocardial, but not sub-epicardial, myocytes. In addition, Irx5-/- mice showed less cardiac hypertrophy, but increased interstitial fibrosis and greater contractility impairment following pressure overload. A defect in hypertrophic responses in Irx5-/- myocardium was confirmed in cultured neonatal mouse ventricular myocytes, exposed to norepinephrine while being restored with Irx5 replacement. Interestingly, studies using mice virtually lacking Ito,f (i.e. KV4.2-deficient) showed that reduced contractility in Irx5-/- mice was completely restored by loss of KV4.2, whereas hypertrophic responses to pressure-overload in hearts remained impaired when both Irx5 and Ito,f were absent. These findings suggest that Irx5 regulates cardiac contractility in an Ito,f-dependent manner while affecting hypertrophy independent of Ito,f. On the other hand, Irx5-ablation attenuated calcineurin (Cn)-induced hypertrophy in hearts and cultured cardiomyocytes, suggesting that the effect of Irx5 on hypertrophy involves the Cn-NFAT signalling cascade. Biochemical assessments further revealed that Irx5 can positively mediate Cn-NFAT activities as well as Nfatc3 and Gata4 expression, and interacts with Nfatc3 and Gata4, suggesting the formation of a transcription complex for hypertrophic gene regulation. Taken together, these studies have identified Irx5 as a vital cardiac transcription factor, important for contractile function of the heart by regulating Ito,f, and compensatory hypertrophic response to biomechanical stress in the heart by affecting the Cn-NFAT (and Gata4) signaling pathway.

Understanding the specific mechanisms regulating chemotherapeutic drug anti-cancer activities will uncover novel strategies to enhance the efficacy of these drugs in clinical settings. Activating Transcription Factor 3 (ATF3) is a stress inducible gene whose expression has been associated with survival outcomes in cancer models. This study characterizes the chemotherapeutic drugs, cisplatin and Histone Deacetylase Inhibitor (HDACi), M344 as novel inducers of ATF3 expression. Cisplatin is a DNA damaging agent widely used in various tumour types including lung, head and neck, and ovarian carcinomas. The HDAC inhibitor, SAHA, has recently been approved as a single agent in the treatment of subcutaneous T-cell lymphoma and HDACis themselves show potential for synergistic anti-cancer effects when used in combination with established chemotherapeutic drugs, including cisplatin. This study evaluates the mechanisms by which cisplatin and HDACi induce ATF3, as well as the role ATF3 plays as a mediator of cisplatin-induced cytotoxicity and the enhanced cytotoxicity between HDACi and cisplatin in combination. In this study, we demonstrate that cytotoxic doses of cisplatin and carboplatin consistently induced ATF3 expression in a panel of human tumour derived cell lines. Characterization of this induction revealed a p53, BRCA1, and integrated stress response (ISR) independent mechanism, all previously implicated in stress mediated ATF3 induction. Analysis of MAPKinase pathway involvement in ATF3 induction by cisplatin revealed a MAPKinase dependent mechanism. Cisplatin treatment, in combination with specific inhibitors to each MAPKinase pathway (JNK, ERK and p38) resulted in decreased ATF3 induction at the protein level. MAPKinase pathway inhibition led to decreased ATF3 mRNA expression and a reduction in the cytotoxic effects of cisplatin as measured by MTT cell viability assay. In A549 lung carcinoma cells, targeting ATF3 with specific shRNAs also attenuated the cytotoxic effects of cisplatin. Similarly, ATF3 -/- MEFs were shown to be less sensitive to cisplatin induced cytotoxicity as compared with ATF3+/+ MEFs. Taken together, we identified cisplatin as a MAPKinase pathway dependent inducer of ATF3 whose expression regulates in part cisplatin’s cytotoxic effects. Furthermore, we demonstrated that the HDAC inhibitor M344 was also an inducer of ATF3 expression at the protein and mRNA level in the same human derived cancer cell lines. Combination treatment with M344 and cisplatin lead to increased induction of ATF3 compared with cisplatin alone. Utilizing the MTT cell viability assay, M344 treatment was also shown to enhance the cytotoxic effects of cisplatin in these cancer cell lines. Unlike cisplatin, the mechanism of ATF3 induction by M344 was found to be independent of MAPKinase pathways. Utilizing ATF4 heterozygote (+/-) and knock out (-/-) mouse embryonic fibroblast (MEF) M334 induction of ATF3 was shown to depend on the presence of ATF4, a known regulator of ATF3 expression as part of the ISR pathway. HDACi treatment did not affect the level of histone acetylation associated with the ATF3 promoter as determined through Chromatin immunoprecipitation (ChIP) analysis, suggesting that ATF3 induction was not a direct effect of HDACi mediated histone acetylation. We also demonstrated that ATF3 regulates the enhanced cytotoxicity of M344 in combination with cisplatin as evidenced by attenuation of cytotoxicity in shRNAs targeting ATF3 expressing cells. This study identifies the pro-apoptotic factor, ATF3 as a novel target of M344, as well as a mediator of the co-operative effects of cisplatin and M344 induced tumour cell cytotoxicity.

The inhibitor of apoptosis (IAP) proteins traditionally regulate programmed cell death by binding to and inhibiting caspases. Recent studies have uncovered a variety of alternate cellular roles for several IAP family members. The cellular inhibitor of apoptosis 1 (cIAP1) protein, for instance, regulates different axes of the NF-κB signalling pathway. Given the extensive functions of NF-κB signalling in muscle differentiation and regeneration, I asked if cIAP1 also plays critical roles in skeletal muscle myogenesis. In a primary myoblast cell-culture system, genetic and pharmacological approaches revealed that loss of cIAP1 dramatically increases the fusion of myoblasts into myotubes. NF-κB signalling occurs along a classical and an alternative pathway, both of which are highly active in cIAP1-/- myoblasts. Suppression of the alternative pathway attenuates myotube fusion in wildtype and cIAP1-/- myoblasts. Conversely, constitutive activation of the alternative pathway increases myoblast fusion in wildtype myoblasts. cIAP1-/- mice have greater muscle weight and size than wildtypes, as well as an increased number of muscle stem cells. These results identify cIAP1 as a regulator of myogenesis through its modulation of classical and alternative NF-κB signalling pathways. Loss of the structural protein dystrophin in the mdx mouse model of Duchenne muscular dystrophy leads to chronic degeneration of skeletal muscle. The muscle pathology is strongly influenced by NF-κB signaling. Given the roles demonstrated for cIAP1 in cell culture and in vivo, I asked whether loss of cIAP1 would influence muscle pathology in the mdx mouse. To address this question, double-mutant mice were bred lacking both cIAP1 and dystrophin (cIAP1-/-;mdx). Histological analyses revealed that double-mutant mice exhibited reduced indications of damage on several measures, as compared to single-mutant (cIAP1+/+;mdx) controls. Unexpectedly, these reductions were seen in the “slow-twitch” soleus muscle but not in the “fast-twitch” extensor digitorum longus (EDL) muscle. The improvements in pathology of double-mutant solei were associated with reductions in muscle infiltration by CD68-expressing macrophages. Finally, the double-mutant mice exhibited improved endurance and resistance to damage during treadmill-running exercise. Taken together, these results suggest that loss of cIAP1, through its multiple regulatory functions, acts to improve myogenesis and increase muscle resistance to damage.