The kinetics of drug-receptor interactions can profoundly influence in vivo and in vitro pharmacology. In vitro, the potencies of slowly associating agonists may be underestimated in assays capturing transient signaling events. When divergent receptormediated signaling pathways are evaluated using combinations of equilibrium and transient assays, potency differences driven by kinetics may be erroneously interpreted as biased signaling. In vivo, drugs with slow dissociation rates may display prolonged physiologic effects inconsistent with their pharmacokinetic profiles. We evaluated a panel of 5-hydroxytryptamine 2B (5-HT 2B ) receptor agonists in kinetic radioligand binding assays and in transient, calcium flux assays, and inositol phosphate accumulation assays; two functional readouts emanating from Ga qmediated activation of phospholipase C. In binding studies, ergot derivatives demonstrated slow receptor association and dissociation rates, resulting in significantly reduced potency in calcium assays relative to inositol phosphate accumulation assays. Ergot potencies for activation of extracellular signalregulated kinases 1 and 2 were also highly time-dependent. A number of ergots produced wash-resistant 5-HT 2B signaling that persisted for many hours without appreciable loss of potency, which was not explained simply by slow receptordissociation kinetics. Mechanistic studies indicated that persistent signaling originated from internalized or sequestered receptors. This study provides a mechanistic basis for the long durations of action in vivo and wash-resistant effects in ex vivo tissue models often observed for ergots. The 5-HT 2B agonist activity of a number of ergot-derived therapeutics has been implicated in development of cardiac valvulopathy in man. The novel, sustained nature of ergot signaling reported here may represent an additional mechanism contributing to the valvulopathic potential of these compounds.
Bad, an inducer of programmed cell death, was recently isolated from a mouse cDNA library by its ability to bind to the anti-apoptotic protein BCL-2. Sequence analysis suggested that Bad was a member of the BCL-2 gene family that encodes both inducers and inhibitors of programmed cell death. To further analyze the role of BAD in the network of homo-and heterodimers formed by the BCL-2 family, we have cloned the human homologue of BAD and assessed its biological activity and its interactions with wild type and mutant BCL-2 family proteins. Our results indicate that the human BAD protein, like its mouse homologue, is able to induce apoptosis when transfected into mammalian cells. Furthermore, in yeast two-hybrid assays as well as quantitative in vitro interaction assays, human Bad interacted with BCL-2 and BCL-X L . Sequence alignments of human BAD revealed the presence of a BH-3 homology domain as seen in other BCL-2 family proteins. Peptides derived from this domain were able to completely inhibit the dimerization of BAD with BCL-X L . Thus, as previously shown for BAX, BAK, BCL-2, and BCL-X L , the BH3 domain of BAD is required for its dimerization with other BCL-2 family proteins. BAD was further analyzed for its ability to bind to various mutants of BCL-2 and BCL-X L that have lost the ability to bind BAX and BAK, some of which retain biological activity and some of which do not. Surprisingly, all of the mutated BCL-2 and BCL-X L proteins analyzed strongly interacted with human BAD. Our data thus indicate that mutations in BCL-2 and BCL-X L can differentially affect the heterodimeric binding of different death-promoting proteins and have implications concerning the relationship between heterodimerization and biological activity.The BCL-2 family of proteins consists of inhibitors and inducers of programmed cell death or apoptosis (1-3). Inhibitors include the BCL-2 (4, 5) and BCL-X L proteins (6) and inducers include BAX, BAK, and BCL-X S (6 -10). These proteins have been shown to form a network of homo-and heterodimers (11). A number of studies suggest that dimer formation is essential for the biological activity of these molecules. For example, mutagenesis data have demonstrated a correlation between BCL-2 activity and the ability to form heterodimers with BAX (12). However, other experiments with BCL-X L suggested that dimerization with Bax was not necessary for biological activity (13).Bad was originally cloned from mouse cDNA by its ability to bind to BCL-2, both in yeast two-hybrid interactions and by direct biochemical interaction (14). It was subsequently shown to interact more strongly with BCL-X L than with BCL-2, and in functional studies it antagonized the protective effect of BCL-X L . To date, the human homologue has not been reported. The sequences of the BCL-2 family proteins show several regions of clustered conserved residues, termed by some investigators BH-1 to BH-4 domains (12,15,16). The crystal and NMR structures of BCL-X L show a potential binding pocket on the surface of the molecule f...
Apoptosis is an intrinsic and fundamental biologic process that plays a critical role in the normal development of multicellular organisms and in the maintainance of tissue homeostasis. Some of the well known regulators of apoptosis are cytokines of the tumor necrosis factor (TNF) ligand family, such as Fas ligand (Fas L) and TNF, which induce apoptosis by activation of their corresponding receptors, Fas and TNFR-1. Recently, a new member of the TNF family known as TRAIL (TNF-related apoptosis-inducing ligand) was identified and shown to induce p53-independent apoptosis in a variety of tumor cell lines but not in normal cells. Four human receptors for TRAIL were also recently identified and designated TRAIL-R1, -R2, -R3, and -R4. The aim of this study is to examine whether TRAIL and TRAIL receptors (-R1, -R2, -R3) are expressed in uterine cervical cancer and whether it is correlated with apoptosis, TRAIL, and TRAIL receptors. The subjects were 20 patients who were diagnosed with cervical cancer. Western blotting was performed in nine cases and immunohistochemical staining for TRAIL and TRAIL receptors (-R1, -R2, -R3) and TUNEL method for detection of apoptosis was performed in 11 cases. There were proteins for TRAIL, TRAIL-R1, -R2, and -R3 in tissues from cervical cancer. All TRAIL receptors were expressed in both normal cervical epithelium and tumor cells, and TRAIL-R1 and -R2 were more strongly expressed in tumor cells than normal epithelium (P < 0.05). Apoptosis correlated with expression of TRAIL-R1 and -R2 (P < 0.05). This study suggests that TRAIL induces apoptosis in cervical cancer through its receptors.
The design and synthesis of a new series of potent non-prostanoid IP receptor agonists that showed oral efficacy in the rat monocrotaline model of pulmonary arterial hypertension (PAH) are described. Detailed profiling of a number of analogues resulted in the identification of 5c (ralinepag) that has good selectivity in both binding and functional assays with respect to most members of the prostanoid receptor family and a more modest 30- to 50-fold selectivity over the EP3 receptor. In our hands, its potency and efficacy are comparable or superior to MRE269 (the active metabolite of the clinical compound NS-304) with respect to in vitro IP receptor dependent cAMP accumulation assays. 5c had an excellent PK profile across species. Enterohepatic recirculation most probably contributes to a concentration-time profile after oral administration in the cynomolgus monkey that showed a very low peak-to-trough ratio. Following the identification of an acceptable solid form, 5c was selected for further development for the treatment of PAH.
The extracellular signal-regulated kinases 1/2 (ERK1/2) are serine/threonine-selective protein kinases involved in proliferation and differentiation of cells, including thymocytes. The requirement of ERK1/2 for thymocyte differentiation and maturation has been well established; however, their role in regulating thymocyte survival and apoptosis has not been resolved.Here, we asked whether ERK1/2 affected thymocyte survival in vitro in response to apoptotic stimuli. The results show that phorbol 12-myristate 13-acetate (PMA) treatment (with or without ionomycin) and serum starvation (s/s) induced sustained ERK1/2 activation in murine thymocytes. Importantly, pharmacological treatment of thymocytes with the MEK inhibitor UO126 revealed that PMAinduced ERK1/2 activation was pro-apoptotic, whereas serum starvation-induced ERK1/2 activation inhibited apoptosis and promoted cell survival. While basal MEK activity was required for both s/s and PMA-induced ERK1/2 activation, MEK activity increased only in response to PMA. The results show that the suppression of ERK1/2 phosphatases was responsible for s/s-induced sustained ERK1/2 activation. Unexpectedly, neither s/s-induced pro-apoptotic nor PMA-induced anti-apoptotic functions of ERK1/2 depended on the Bcl-2 family phosphoprotein Bim EL , which was previously implicated in thymocyte apoptosis. Lastly, etoposide treatment of immature thymocytes induced both p53 and ERK1/2 activation, but ERK1/2 activity did not affect the phosphorylation and stabilization of p53. Thus, ERK1/2 has a dual role in promoting cell survival and cell death in thymocytes in the context of different stimuli.
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