Figure 2. Schematic showing the syntheses of platinum complexes with a range of axial ligands reported in the literature. It is generally accepted that the platinum(II) congener must itself be capable of effecting cytotoxicity for the platinum(IV) complex to be active. Sample references for each step of the synthetic scheme are as follows. (a) Oxidation of platinum(II) complexes with hydrogen peroxide to yield trans-dihydroxo ligands:

Activating transcription factor 4 (ATF4) is a transcription factor induced under severe hypoxia and a component of the PERK pathway involved in the unfolded protein response (UPR), a process that protects cells from the negative consequences of endoplasmic reticulum (ER) stress. In this study, we have used small interfering RNA (siRNA) and microarray analysis to provide the first whole-genome analysis of genes regulated by ATF4 in cancer cells in response to severe and prolonged hypoxic stress. We show that ATF4 is required for ER stress and hypoxia-induced expansion of autophagy. MAP1LC3B (LC3B) is a key component of the autophagosomal membrane, and in this study we demonstrate that ATF4 facilitates autophagy through direct binding to a cyclic AMP response element binding site in the LC3B promoter, resulting in LC3B upregulation. Previously, we have shown that Bortezomib-induced ATF4 stabilization, which then upregulated LC3B expression and had a critical role in activating autophagy, protecting cells from Bortezomib-induced cell death. We also showed that severe hypoxia stabilizes ATF4. In this study, we demonstrate that severe hypoxia leads to ER stress and induces ATF4-dependent autophagy through LC3 as a survival mechanism. In summary, we show that ATF4 has a key role in the regulation of autophagy in response to ER stress and provide a direct mechanistic link between the UPR and the autophagic machinery.

The ubiquitin-proteasome system plays a key regulatory role in cellular homeostasis. The inhibition of the 26S proteasome by Bortezomib leads to the accumulation of misfolded proteins, resulting in endoplasmic reticulum stress followed by a coordinated cellular response called unfolded protein response (UPR). Endoplasmic reticulum stress is also a potent inducer of macroautophagy. Bortezomib is a selective and potent inhibitor of the 26S proteasome and is approved for the treatment of multiple myeloma. Clinical trials with Bortezomib have shown promising results for some types of cancers, but not for some others, including those of the breast. In this study, we show that Bortezomib induces the UPR and autophagy in MCF7 breast cancer cells. Surprisingly, Bortezomib did not induce phosphorylation of PERK, a key initial step of the UPR. We show that induction of autophagy by Bortezomib is dependent on the proteasomal stabilisation of ATF4 and up-regulation of LC3B by ATF4. We show that ATF4 and LC3B play a critical role in activating autophagy and protecting cells from Bortezomib-induced cell death. Our experiments also reveal that HDAC6 knockdown results in decreased LC3B protein and reduced autophagy. Our work shows that the induction of autophagy through ATF4 may be an important resistance mechanism to Bortezomib treatment in breast cancer, and targeting autophagy may represent a novel approach to sensitize breast cancers to Bortezomib.

Aberrant signaling by transforming growth factor-b (TGF-b) and its type I (ALK5) receptor has been implicated in a number of human diseases and this pathway is considered a potential target for therapeutic intervention. Transforming growth factor-b signaling via ALK5 plays a critical role during heart development, but the role of ALK5 in the adult heart is poorly understood. In the current study, the preclinical toxicology of ALK5 inhibitors from two different chemistry scaffolds was explored. Ten-week-old female Han Wistar rats received test compounds by the oral route for three to seven days. Both compounds induced histopathologic heart valve lesions characterized by hemorrhage, inflammation, degeneration, and proliferation of valvular interstitial cells. The pathology was observed in all animals, at all doses tested, and occurred in all four heart valves. Immunohistochemical analysis of ALK5 in rat hearts revealed expression in the valves, but not in the myocardium. Compared to control animals, protein levels of ALK5 were unchanged in the heart valves of treated animals. We also observed a physeal dysplasia in the femoro-tibial joint of rats treated with ALK5 inhibitors, a finding consistent with a pharmacological effect described previously with ALK5 inhibitors. Overall, these findings suggest that TGF-b signaling via ALK5 plays a critical role in maintaining heart valve integrity.

No abstract

Cardiotoxicity, also referred to as drug-induced cardiac injury, is an issue associated with the use of some small-molecule kinase inhibitors and antibody-based therapies targeting signaling pathways in cancer. Although these drugs have had a major impact on cancer patient survival, data have implicated kinase-targeting agents such as sunitinib, imatinib, trastuzumab, and sorafenib in adversely affecting cardiac function in a subset of treated individuals. In many cases, adverse cardiac events in the clinic were not anticipated based on preclinical safety evaluation of the molecule. In order to support the development of efficacious and safe kinase inhibitors for the treatment of cancer and other indications, new preclinical approaches and screens are required to predict clinical cardiotoxicity. Laboratory investigations into the underlying molecular mechanisms of heart toxicity induced by these molecules have identified potentially common themes including mitochondrial perturbation and modulation of adenosine monophosphate-activated protein kinase activity. Studies characterizing cardiac-specific kinase knockout mouse models have developed our understanding of the homeostatic role of some of these signaling mediators in the heart. Therefore, when considering kinases as potential future targets or when examining secondary pharmacological interactions of novel kinase inhibitors, these models may help to inform us of the potential adverse cardiac effects in the clinic.

No abstract

The quiescent cell population of tumours poses a barrier to the success of many cancer therapies. Most chemotherapeutic drugs target proliferating cells, but the growth fraction of many tumours is low. Based on the multicellular tumour spheroid model, a system was developed using human colon adenocarcinoma (DLD-1) cells to mimic the microenvironment of quiescent microregions of solid tumours. The quiescent tumour spheroids (TS Q ) showed decreased expression of the proliferation marker Ki-67 and increased expression of the quiescence marker p27 kip1 compared to proliferating spheroids (TS P ). The quiescent status of the TS Q was confirmed by long-term growth assessment. The quiescence was completely reversible demonstrating that the TS Q retained the ability to proliferate and morphological assessment by light microscopy confirmed the absence of significant apoptosis. When the efficacy of widely used chemotherapeutic drugs was determined, vinblastine, doxorubicin, cisplatin and 5-fluorouracil (5-FU) all produced significant cell death in the TS P . However, while still effective, the potencies of doxorubicin and cisplatin were significantly reduced in TS Q . In contrast, 5-FU and vinblastine did not produce cell death in the TS Q . In summary, TS Q show considerable resistance to a panel of established chemotherapeutic agents and represent a useful model for evaluating the efficacy of drugs and other cancer therapies in quiescent tumours.