Cell-to-cell communication is essential for the organization, coordination, and development of cellular networks and multi-cellular systems. Intercellular communication is mediated by soluble factors (including growth factors, neurotransmitters, and cytokines/chemokines), gap junctions, exosomes and recently described tunneling nanotubes (TNTs). It is unknown whether a combination of these communication mechanisms such as TNTs and gap junctions may be important, but further research is required. TNTs are long cytoplasmic bridges that enable long-range, directed communication between connected cells. The proposed functions of TNTs are diverse and not well understood but have been shown to include the cell-to-cell transfer of vesicles, organelles, electrical stimuli and small molecules. However, the exact role of TNTs and gap junctions for intercellular communication and their impact on disease is still uncertain and thus, the subject of much debate. The combined data from numerous laboratories indicate that some TNT mediate a long-range gap junctional communication to coordinate metabolism and signaling, in relation to infectious, genetic, metabolic, cancer, and age-related diseases. This review aims to describe the current knowledge, challenges and future perspectives to characterize and explore this new intercellular communication system and to design TNT-based therapeutic strategies.

The estrogen receptor alpha (ERalpha) has proven to be the single most important target in breast cancer over the last 30 years. The use of the selective ER modulator (SERM) tamoxifen for the treatment and prevention of breast cancer has changed therapeutics. The SERM raloxifene, approved for the treatment of osteoporosis, lacks tamoxifen's increased risk for endometrial cancer and is being evaluated for the prevention of breast cancer. Other SERMs approved or under development for use against breast cancer or osteoporosis include toremifene, GW5638, GW7604 (the active metabolite of GW5638), idoxifene, lasofoxifene, arzoxifene, bazedoxifene, EM-800 and acolbifene (the active metabolite of EM-800). Aromatase inhibitors (AIs) have recently proven to be more efficacious than tamoxifen as first-line therapy, efficacious for second-line therapy (e.g. against tamoxifen-resistant disease), and useful for extended adjuvant therapy after tamoxifen. The AIs include the non-steroidal agents letrozole and anastrole, and the steroidal agent exemestane. The pure antiestrogen fulvestrant has proven to be just as effective as AIs. Other pure antiestrogens, ZK-703, ZK-253, RU 58668 and TAS-108 show great promise. The development of resistance to endocrine therapy remains a clinically important problem, and laboratory models based on human breast cancer cells grown as tumors in immune-compromised mice have led to important insights into this problem. Progesterone receptor-negative status of ER-positive breast cancers may reflect altered growth factor receptor signaling, and helps to explain why this subclass of tumors exhibits lower response rates to tamoxifen compared to cancers typed progesterone receptor-positive. Crosstalk among plasma membrane-localized ER, growth factor receptor signaling, and nuclear-localized ER provide further insights into antihormonal-resistant breast cancer.

While estrogen receptor (ER)-targeted therapeutics have clearly been a success in the treatment of breast cancer, the orphan estrogen-related receptors (ERRs) represent novel targets for future development. The ERRs, comprising ERRalpha, ERRbeta and ERRgamma, bind and regulate transcription via estrogen response elements (EREs) and extended ERE half-sites termed ERR response elements (ERREs), but do not bind endogenous estrogens. The emerging role of ERRalpha and ERRgamma in modulating estrogen responsiveness, substituting for ER activities, and serving as prognosticators in breast and other cancers is providing an impetus for the identification of compounds which target these proteins. Moreover, ERRalpha plays a role in energy homeostasis and will likely be targeted for the treatment of metabolic disorders. Multiple classes of synthetic ligands have already been identified. The phytoestrogens genistein, daidzein, biochanin A and 6,3'4'-tryhydroxyflavone have been reported as ERRalpha agonists. The phenolic acyl hydrazones GSK4716 and GSK9089 act as selective agonists of ERRbeta and ERRgamma. The organochlorine pesticides toxaphene and chlordane, and the synthetic compound XCT790 antagonize ERRalpha. The synthetic estrogen diethylstilbestrol antagonizes all three ERRs. The selective estrogen receptor modulators 4-hydroxytamoxifen and 4-hydroxytoremifene antagonize ERRgamma. The rational development of synthetic ligands for the ERRs may soon provide new agents to supplement the repertoire of antihormonal therapies to combat breast cancer. Moreover, expression of ERRs in other cancers and metabolic disorders may provide a targeted treatment strategy for these patients as well.

Chronic post-myocardial infarction treatment with a selective HIF PHD inhibitor (GSK360A) exerts systemic and local effects by stabilizing HIF-1 alpha signaling and improves long-term ventricular function, remodeling, and vascularity in a model of established ventricular dysfunction. These results suggest that HIF-PHD inhibitors may be suitable for the treatment of post-MI remodeling and heart failure.

Decreased erythropoietin (EPO) production, shortened erythrocyte survival, and other factors reducing the response to EPO contribute to anemia in patients who have a variety of underlying pathologies such as chronic kidney disease. Treatment with recombinant human EPO (rHuEPO) at supraphysiologic concentrations has proven to be efficacious. However, it does not ameliorate the condition in all patients, and it presents its own risks, including cardiovascular complications. The transcription factors hypoxia-inducible factor (HIF) 1 and HIF2 control the physiologic response to hypoxia and invoke a program of increased erythropoiesis. Levels of HIF are modulated by oxygen tension via the action of a family of HIF-prolyl hydroxylases (PHDs), which tag HIF for proteasomal degradation. Inhibition of these PHDs simulates conditions of mild hypoxia, leading to a potentially more physiologic erythropoietic response and presenting a potential alternative to high doses of rHuEPO. Here we describe the discovery and characterization of GSK1278863 [2-(1,3-dicyclohexyl-6-hydroxy-2,4-dioxo-1,2,3,4-tetrahydropyrimidine-5-carboxamido) acetic acid], a pyrimidinetrione-glycinamide low nanomolar inhibitor of PHDs 1-3 that stabilizes HIF in cell lines, resulting in the production of increased levels of EPO. In normal mice, a single dose of GSK1278863 induced significant increases in circulating plasma EPO but only minimal increases in plasma vascular endothelial growth factor (VEGF-A) concentrations. GSK1278863 significantly increased reticulocytes and red cell mass parameters in preclinical species after once-daily oral administration and has demonstrated an acceptable nonclinical toxicity profile, supporting continued clinical development. GSK1278863 is currently in phase 3 clinical trials for treatment of anemia in patients with chronic kidney disease.