Identification of Peptide Epoxy-Ketone Proteasome Inhibitors with Differential Active Site Selectivity.

The proteasome is a multi-subunit structure responsible for degrading the majority of proteins in the cell. The three catalytic subunits in the 20S core of the proteasome are defined by their preference for amino acid side chains at the P1 position: the beta5 subunit (chymotrypsin-like), the beta2 subunit (trypsin-like), and the beta1 subunit (caspase-like) hydrolyze substrates after hydrophobic, basic, and acidic residues, respectively. Bortezomib (Velcade®), currently an approved drug for the treatment of multiple myeloma, is a potent inhibitor of the beta5 subunit but also has activity against the beta1 subunit (9-fold less active) and the beta2 subunit (250-fold less active). PR-171, a peptide epoxy-ketone proteasome inhibitor currently in Phase I clinical trials in multiple myeloma and NHL patients, has a distinct profile of inhibition. Similar to bortezomib, PR-171 is most potent against the beta5 subunit, but differs in its activity against the beta2 and beta1 subunits (45-fold less active and 140-fold less active, respectively). Although both bortezomib and PR-171 are cytotoxic to a variety of tumor cell types, the connection between inhibition of specific proteasomal subunits and cellular consequences, including induction of apoptosis, is currently unclear. To investigate this relationship, we synthesized a series of PR-171 analogs and profiled them in our proteasome-specific active site ELISA binding assay to identify compounds with differential binding profiles for the three catalytic subunits. Using this approach, we have identified compounds that bind all three catalytic sites with approximately equivalent activity as well as compounds that have increased specificity for the beta5 subunit. We have extended this approach to search for compounds with differential binding profiles for the immunoproteasome catalytic subunits. The immunoproteasome is the predominant form of the proteasome expressed in cells of hematopoetic origin and in other cell types after interferon-g stimulation. In the 20S core of the immunoproteasome the beta5, beta2 and beta1 subunits of the “constitutive” proteasome are replaced by the homologous subunits LMP7, MECL1, and LMP2. We have successfully identified inhibitors with distinct active site profiles for the immunoproteasome as well as the constitutive proteasome. The impact of differential active site inhibition on tumor cell viability is under investigation in a variety of solid and hematological tumor cell lines expressing the constitutive proteasome or the immunoproteasome.

Stimulation of Na,K-ATPase by hypothyroidism in the thyroid gland

Although studies have documented the regulatory effects of thyroid hormones on the Na,K-ATPase in peripheral tissues, there is little information on the regulation of this transporter in the thyroid gland itself. Accordingly, we investigated the effects of thyroid status on Na,K-ATPase specific activity and the abundance of its constituent subunits in rat thyroid. Exogenous tri-iodothyronine (T3) was administered daily to produce hyperthyroidism. 6n-propyl-2-thiouracil (PTU), an inhibitor of thyroid hormone synthesis, was used to induce hypothyroidism. There was a four-fold increase in Na,K-ATPase specific activity in the follicular membranes from PTU-treated animals after 7 days. Enzymatic activities were not changed in the T3-treated glands. Immunoblotting of membranes from T3-treated rats revealed a 75% reduction in alpha1 subunit abundance and a slight, but nonsignificant reduction in beta1 abundance. On the other hand, the membranes from PTU-treated rats displayed 136 and 567% increases in the abundance of the alpha1 and beta1 subunits respectively. These data demonstrate that thyroid hormone status regulates Na,K-ATPase in the gland, but the effects are in direct contrast to those seen in the periphery.

The ‘ligand-induced conformational change’ of alpha 5 beta 1 integrin. Relocation of alpha 5 subunit to uncover the beta 1 stalk region

Integrin heterodimers undergo a conformational change upon the binding of ligand to their extracellular domains. An anti-beta1 integrin monoclonal antibody AG89 can detect such a conformational change since it recognizes a ligand-inducible epitope in the stalk-like region of beta1 subunits. The binding of a 125I-labeled AG89 Fab fragment to alpha5 beta1 integrins on K562 cells was assessed and analyzed by the Scatchard method. High affinity binding sites for AG89 are present on cells treated with ligand peptide. In addition, results revealed that cells treated with EDTA also express AG89 binding sites with the same affinity although the number of binding sites is 4-fold lower. AG89 immunoprecipitated alpha5 beta1 complexes from surface-labeled K562 cells treated with ligand peptide. By contrast, it immunoprecipitated only beta1 chains when the ligand peptide was absent, suggesting that high affinity binding sites on EDTA-treated cells are associated with non-functional beta1 monomer. Additional studies show that the epitope for AG89 is constitutively exposed on mutant beta1 that cannot complex with alpha5. These data suggest that the AG89 epitope is masked by the alpha5 subunit. Ligand binding and integrin activation may uncover the beta1 stalk region by triggering a conformational shift of alpha5 relative to beta1.