Methylthio-DADMe-immucillin-A (MT-DADMeCellular proliferation is associated with increased levels of polyamine biosynthesis and polyamine pools (1). Target validation for the polyamine pathway as an anticancer approach has come from ␣-difluoromethylornithine (DFMO), 3 a suicide inhibitor of ornithine decarboxylase (ODC) and the committed step of polyamine biosynthesis (2, 3). ODC is a difficult cancer target because of its rapid turnover and the dose-limiting toxicity of anti-ODC agents (4). Because of these difficulties, DFMO has not gained wide use. But the polyamine pathway, through its close interaction with S-adenosylmethionine (AdoMet) recycling, remains a target for cancer therapy. We investigated the possibility that feedback inhibition by 5Ј-methylthioadenosine (MTA), induced by a transition state analogue inhibitor of 5Ј-methylthioadenosine phosphorylase (MTAP), could be used to block this pathway and initiate anticancer effects. The results indicate that blocking MTA recycling with transition state analogues of MTAP induces apoptosis through specific epigenetic changes in specific cultured cancer cell lines. Inhibition of MTAP is effective in treating a xenograft model of head and neck cancer in mice.MTA is a product of both spermidine and spermine synthases and provides product inhibition at two sequential sites in the polyamine pathway (Fig. 1A). In humans, MTA is degraded exclusively by MTAP (EC 2.4.2.28), expressed from a single gene locus at 9p21. MTAP produces adenine and 5-methylthio-␣-D-ribose-1-phosphate (Fig. 1B), and these products are recycled to AdoMet. Inhibitors of MTAP are therefore expected to increase intracellular MTA, cause feedback inhibition of polyamine biosynthesis, prevent AdoMet recycling, and disrupt AdoMet-dependent methylation activity. One or more of these activities is expected to be associated with antiproliferative activity (5-8).The transition state structure of human MTAP has been established by kinetic isotope effects and quantum chemical calculations. It is characterized by a late transition state with weak participation of the phosphate nucleophile, similar to that of human purine nucleoside phosphorylase but slightly more advanced (Fig. 1B) (9 -15). Analogues of the human MTAP transition state have been synthesized and are powerful and specific inhibitors (16 -18). Methylthio-DADMe-Immucillin-A (MT-DADMe-ImmA) is a chemically stable transition state analogue of human MTAP and is a slow onset tightly binding inhibitor with a dissociation constant of 86 pM (18). * This work was supported by National Institutes of Health Grants GM41916and CA85953 and a pilot project award from P30 CA013330. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The cells' ability to proliferate in response to growth factor stimulation is significantly altered during cancer progression. To investigate the mechanisms underlying these alterations in prostate cancer, the role and expression of B 1A integrin and type 1 insulin-like growth factor receptor (IGF-IR), known to contribute to cell proliferation and transformation, were analyzed. Using small interfering RNA oligonucleotides to down-regulate B 1A , we show that B 1A expression is required for IGF-IR-mediated prostate cancer cell proliferation and anchorage-independent growth. In vivo, using age-matched transgenic adenocarcinoma of mouse prostate (TRAMP) mice at different stages of prostate cancer [prostatic intraepithelial neoplasia, PIN; well-differentiated adenocarcinoma, WD; and poorly differentiated adenocarcinoma, PD], the expression of B 1A and of IGF-IR was studied. B 1A and IGF-IR expression levels were concurrently up-regulated in high PIN and WD, whereas their expression did not correlate in late-stage PD. In contrast to the up-regulated expression of B 1A , the levels of B 1C , a B 1 cytoplasmic variant that inhibits cell proliferation, were down-regulated in all stages of prostate cancer. A similar expression pattern was observed for a B 1C downstream effector, Grb2-associated binder-1 (Gab1) which is known to inhibit IGF-IR phosphorylation. To analyze in vitro the mechanistic implications of B 1A , B 1C , and Gab1 deregulation in prostate cancer, we investigated whether expression of either B 1 variant in B 1 -null cells affected IGF-IR localization. We found that IGF-IR and B 1A were colocalized in highly specialized integrin signaling compartments, designated focal contacts. However, in the presence of B 1C , IGF-IR remained diffuse on the cell surface and did not localize to focal contacts. The findings that B 1 integrins and IGF-IR are concurrently deregulated and that expression of B 1 integrins is necessary to achieve appropriate IGF-IR intracellular distribution point to the important role that the cross-talk between these receptors may have during prostate cancer progression and will be helpful in formulating new therapeutic strategies. (Cancer Res 2005; 65(15): 6692-700)
Disaggregation and reactivation of aggregated proteins by chaperones is well established. However, little is known regarding such kind of function of single-domain small cyclophilins (CyPs). Here we demonstrate that, with increasing concentrations, fully active adenosine kinase (AdK) of Leishmania donovani tends to form soluble aggregates, resulting in inactivation. Using this inactive enzyme as the substrate, it is shown that a CyP from L. donovani (LdCyP) alone can cause complete disaggregation, leading to reactivation of the enzyme. The reactivating ability of LdCyP remains unaffected even in the presence of cyclosporin A and macromolecular crowding agents. The reactivation occurs noncatalytically and is reversible. A truncated LdCyP, devoid of 88 amino acids from the N terminus, is found to be required in near stoichiometric proportion to reactivate AdK, suggesting essentiality of the C-terminal region. Gel filtration and light-scattering experiments together with protein cross-linking studies revealed that both full-length LdCyP and the truncated form directly interact with AdK and convert oligomeric forms of the enzyme to monomeric state. Homology modeling studies suggest that the exposed hydrophobic residues of LdCyP, by interacting with solvent-accessible hydrophobic surface of AdK, pull apart its aggregated inactive oligomers to functional monomers. Clearly, the results are consistent with the interpretation that the higher efficiency of the truncated LdCyP is most likely due to increased exposure of the hydrophobic residues on its surface. These observations, besides establishing L. donovani AdK as one of the model enzymes to study aggregation-disaggregation of proteins, raise the possibility that single-domain small CyPs, under physiological conditions, may regulate the activity of aggregation-prone proteins by ensuring their disaggregation.
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