Profiling proteasome activity in tissue with fluorescent probes

With proteasome inhibitors in use in the clinic for the treatment of multiple myeloma and with clinical trials in progress investigating the treatment of a variety of hematologic and solid malignancies, accurate methods that allow profiling of proteasome inhibitor specificity and efficacy in patients are in demand. Here, we describe the development, full biochemical validation, and comparison of fluorescent proteasome activity reporters that can be used to profile proteasome activities in living cells with high sensitivity. Seven of the synthesized probes tested label proteasomes in lysates, although the fluorescent dye used affects their specificity. Two differentially labeled probes tested are suitable for studying proteasome activity in living cells by gel-based assays, by confocal laser scanning microscopy, and by flow cytometry. We established methods using these fluorescent reporters to profile proteasome activity in different mouse tissues, carefully avoiding postlysis artifacts, and we show that proteasome subunit activity is regulated in an organ-specific manner. The techniques described here could be used to study in vivo pharmacological properties of proteasome inhibitors.

2,5'-Disubstituted adenosine derivatives: evaluation of selectivity and efficacy for the adenosine A(1), A(2A), and A(3) receptor

Novel 2,5'-disubstituted adenosine derivatives were synthesized in good overall yields starting from commercially available guanosine. Binding affinities were determined for rat adenosine A(1) and A(2A) receptors and human A(3) receptors. E(max) values were determined for the stimulation or inhibition of cAMP production in CHO cells expressing human adenosine A(2A) (EC(50) values as well) or A(3) receptors, respectively. The compounds displayed affinities in the nanomolar range for both the adenosine A(2A) and A(3) receptor, without substantial preference for either receptor. The derivatives with a 2-(1-hexynyl) group had the highest affinities for both receptors; compound 4 (2-(1-hexynyl)adenosine) had the highest affinity for the adenosine A(2A) receptor with a K(i) value of 6 nM (A(3)/A(2A) selectivity ratio of approximately 3), whereas compound 37 (2-(1-hexynyl)-5'-S-methyl-5'-thioadenosine) had the highest affinity for the adenosine A(3) receptor with a K(i) value of 15 nM (A(2A)/A(3) selectivity ratio of 4). In general, compounds with a relatively small 5'-S-alkyl-5'-thio substituent (methyl-5'-thio) displayed the highest affinities for both the adenosine A(2A) and A(3) receptor; the larger ones (n- or i-propyl-5'-thio) increased the selectivity for the adenosine A(3) receptor. The novel compounds were also evaluated in cAMP assays for their (partial) agonistic behavior. Overall, the disubstituted derivatives behaved as partial agonists for both the adenosine A(2A) and A(3) receptor. The compounds showed somewhat higher intrinsic activities on the adenosine A(2A) receptor than on the A(3) receptor. Compounds 37, 40 and 45, 48, with either a 5'-S-methyl-5'-thio or a 5'-S-i-propyl-5'-thio substituent had the lowest intrinsic activities on the adenosine A(2A) receptor. For the A(3) receptor, compounds 34, 35, 38, 39, and 46, 47, with a 5'-S-ethyl-5'-thio or a 5'-S-n-propyl-5'-thio substituent had the lowest intrinsic activities.

Functional role of adenosine receptor subtypes in the regulation of blood-brain barrier permeability: possible implications for the design of synthetic adenosine derivatives

The objective of this investigation was to determine the functional role of adenosine receptor subtypes in the regulation of blood-brain barrier (BBB) permeability. The presence of the equilibrative es and ei nucleoside transporters at the BBB was also determined. Studies were conducted in an experimental in vitro BBB model comprising bovine brain capillary endothelial cells (BCECs) and rat astrocytes (RAs). The presence of the receptors and transporters was investigated by a combination of RT-PCR and radioligand binding assays. Changes in paracellular permeability were investigated on basis of changes in trans-endothelial-electrical-resistance (TEER) and transport of paracellular markers. In BCECs the presence of A(2A) and A(3) receptors and the es nucleoside transporter was demonstrated. The A(1) receptor was absent, while the presence of the A(2B) receptor and the ei nucleoside transporter remained uncertain. In RAs the presence of all four receptor subtypes and the es and ei nucleoside transporters was demonstrated. Upon application of selective agonists no significant changes in TEER or the transport of the paracellular markers were observed. The functional role of adenosine receptor subtypes in regulating the paracellular permeability of the BBB is probably small. It is unlikely therefore that the BBB transport of synthetic adenosine analogues is modified by permeability changes. The es nucleoside transporter might play a role in the BBB transport of synthetic adenosine analogues.

2,8-Disubstituted adenosine derivatives as partial agonists for the adenosine A2A receptor

Novel 2,8-disubstituted adenosine derivatives were synthesized in good overall yields starting from 2-iodoadenosine. Binding affinities were determined for rat adenosine A(1) and A(2A) receptors and human A(3) receptors. Some compounds displayed good adenosine A(2A) receptor affinities, with most of the 2-(1-hexynyl)- and 2-[(E)-1-hexenyl]-substituted derivatives having K(i) values in the nanomolar range. Although the introduction of an 8-alkylamino substituents decreased the affinity for the adenosine A(2A) receptor somewhat, the selectivity for this receptor compared to A(3) was improved significantly. The 8-methylamino (12) and 8-propylamino (14) derivatives of 2-(1-hexynyl)adenosine (3), showed reasonable A(2A) receptor affinities with K(i) values of 115 and 82nM, respectively, and were 49- and 26-fold selective for the adenosine A(2A) receptor compared to the A(3) receptor. The compounds were also evaluated for their ability to stimulate the cAMP production in CHO cells expressing the human adenosine A(2A) receptor. 2-(1-Hexynyl)adenosine (3) and 2-[(E)-1-hexenyl]adenosine (4) both showed submaximal levels of produced cAMP, compared to the reference full agonist CGS 21680, and thus behaved as partial agonists. Most 8-alkylamino-substituted derivatives of 3, displayed similar cAMP production as 3, and behaved as partial agonists as well. Introduction of alkylamino groups at the 8-position of 4, showed a slight reduction of the efficacy compared to 4, and these compounds were partial agonists also.

Development of an activity-based probe for autotaxin

Autotaxin (ATX), or ecto-nucleotide pyrophosphatase/phosphodiesterase 2 (ENPP2), is a secreted lysophospholipase D that hydrolyses lysophosphatidylcholine into the lipid mediator lysophosphatidic acid (LPA), a mitogen and chemoattractant for many cell types. ATX has been implicated in tumour progression and inflammation, and might serve as a biomarker. Here we describe the development of a fluorescent activity-based probe that covalently binds to the active site of ATX. The probe consists of a lysophospholipid-based backbone linked to a trapping moiety that becomes reactive after phosphate ester hydrolysis, and a Cy5 fluorescent dye to allow visualisation of active ATX. The probe reacts specifically with the three known isoforms of ATX, it competes with small-molecule inhibitors for binding to ATX and allows ATX activity in plasma to be determined. Our activity-based reporter will be useful for monitoring ATX activity in biological fluids and for inhibitor screening.