Biomedical subjects
Publications and source records attributed to Fatih Ozdener.
Lilly is developing fasidotril, a diester prodrug of the active metabolite fasidotrilat, for the potential treatment of hypertension and congestive heart failure (CHF). Phase II trials to investigate the potential of fasidotril for the treatment of hypertension and CHF had commenced by the late 1990s, and were ongoing in July 2003 in the US and Europe.
Phospholipase C-gamma-2 (PLCgamma2) activation is a key signaling event for many cell functions. In order to delineate the pathways that lead to PLCgamma2 activation, we devised a quick method for obtaining sufficient PLCgamma2. We obtained the full-length cDNA for human PLCgamma2 and expressed it in E. coli using the expression vector pT5T. To enhance the protein expression, tandem AGG-AGG arginine codons at the amino acid positions 1204-1205 were replaced by CGG-CGG arginine codons. The protein expression was detected in a Western blot analysis by both anti-PLCgamma2 antibodies and the antibodies that are raised against the tripeptide epitope (Glu-Glu-Phe) tag that are genetically-engineered to its carboxyl terminal. Crude lysates that were prepared from bacteria that express PLCgamma2 were found to catalyze the hydrolysis of phosphatidylinositol 4,5 bisphosphate. Similar to previous reports on PLCgamma2 that is isolated from mammalian tissue, the recombinant enzyme was Ca2+ dependent with optimal activity at 1-10 microM Ca2+.
Collagen activates platelets by transducing signals through glycoprotein VI (GPVI). It is not clear whether collagen can directly activate fibrinogen receptors on the adherent platelets without a role for positive feedback agonists. We investigated the contribution of secondary G protein signaling to the mechanism of GPVI-stimulated platelet aggregation using the GPVI-selective agonists, convulxin and collagen-related peptide (CRP) as well as collagen. Adenosine diphosphate (ADP) scavengers or ADP receptor antagonists shifted the concentration-response curve slightly to the right at low concentrations of convulxin, whereas platelet aggregation at higher concentrations of convulxin was unaffected by these agents. ADP receptor antagonists shifted the concentration-response curve of collagen- or CRP-induced platelet aggregation to the right at all the concentrations. Protein kinase C inhibitor, Ro 31-8220, or a calcium chelator 5,5'-dimethyl-BAPTA shifted the concentration-response curve of convulxin-induced platelet aggregation to the right. In addition, pretreatment with both Ro 31-8220 and dimethyl-BAPTA resulted in total inhibition of convulxin-mediated aggregation. Blockade of either the calcium- or protein kinase C-regulated pathway leads to inhibition of fibrinogen receptor activation on platelets adherent to collagen, but inhibition of both pathways leads to abolished fibrinogen receptor activation. We conclude that collagen-induced activation of fibrinogen receptor on adherent platelets through GPVI signaling occurs without any significant role for secreted ADP or thromboxane A(2). Furthermore, protein kinase C- and calcium-regulated pathways independently contribute to GPVI-mediated platelet aggregation.
Phospholipase Cgamma2 (PLCgamma2) has been implicated in collagen-induced signal transduction in platelets and antigen-dependent signaling in B-lymphocytes. It has been suggested that tyrosine kinases activate PLCgamma2. We expressed the full-length cDNA for human PLCgamma2 in bacteria and purified the recombinant enzyme. The recombinant enzyme was Ca(2+)-dependent with optimal activity in the range of 1 to 10 microM Ca(2+). In vitro phosphorylation experiments with recombinant PLCgamma2 and recombinant Lck, Fyn, and Lyn tyrosine kinases showed that phosphorylation of PLCgamma2 led to activation of the recombinant enzyme. Using site-directed mutagenesis, we investigated the role of specific tyrosine residues in activation of PLCgamma2. A mutant form of PLCgamma2, in which all three tyrosines at positions 743, 753, and 759 in the SH2-SH3 linker region were replaced by phenylalanines, exhibited decreased Lck-induced phosphorylation and completely abolished the Lck-dependent activation of PLCgamma2. Individual mutations of these tyrosine residues demonstrated that tyrosines 753 and 759, but not 743, were responsible for Lck-induced activation of PLCgamma2. To confirm these results, we procured a phosphospecific antibody to a peptide containing phosphorylated tyrosines corresponding to residues 753 and 759. This antibody recognized phosphorylated wild-type PLCgamma2 on Western blots but did not interact with unphosphorylated PLCgamma2 or with PLCgamma2 containing mutated tyrosine residues at 753 and 759. Using this antibody, we showed in intact platelets that collagen, a PLCgamma2-dependent agonist, induces phosphorylation of PLCgamma2 at Y753 and Y759. These studies demonstrate the importance of these two tyrosine residues in regulating the activity of PLCgamma2.
Otsuka Pharmaceuticals in collaboration with Bristol-Myers Squibb is developing aripiprazole, a dual dopamine autoreceptor agonist and postsynaptic D2 receptor antagonist, for the potential treatment of psychoses including schizophrenia [281327], [340364]. A regulatory filing for schizophrenia in the US was submitted at the end of 2001 [340364]. The compound entered phase III trials in Japan in 1995 [192966]. Although presynaptic dopamine autoreceptor agonists may be efficacious in the treatment of schizophrenia, they may also potentially increase the risk for exacerbation of psychosis through stimulation of postsynaptic dopaminergic receptors [245791], [350478], [350479]. However, earlier neuropharmacology studies have shown that aripiprazole can act as a presynaptic D2 agonist while displaying an antagonistic effect at the postsynaptic D2 receptors [281327], [337126], [350479], [424587], [424588]. In animal models, aripiprazole inhibits the apomorphine-induced stereotypy, without causing catalepsy [281327], [337126]. Moreover, in contrast to classical antipsychotics that produce disabling movement disorders, aripiprazole does not cause an upregulation of D2 receptors or an increase in expression of the c-fos mRNA in the striatum, in agreement with the low risk for extrapyramidal side effects (EPS) during aripiprazole treatment [245781], [262096], [350481], [350483]. Collectively, aripiprazole is an important atypical antipsychotic candidate with a favorable safety profile. Moreover, the mechanism of action of aripiprazole differentiates it from both typical and atypical antipsychotics and hence, may provide important leads for pharmacotherapy of schizophrenia and other psychotic disorders. In January 2000, Lehman Brothers predicted peak sales of aripiprazole could reach US $500 million [357788]. In February 2001, Credit Suisse First Boston predicted sales of US $403 million in 2005 [399484].