Potency and selectivity of inhibition of cathepsin K, L and S by their respective propeptides

Senescence-initiated Reversal of Drug Resistance

The present study was undertaken to verify whether induction of senescence could be sufficient to reverse drug resistance and, if so, to determine the underlying mechanism(s). Our findings indicated that cotreatment of drug-resistant neuroblastoma cells with doxorubicin, at sublethal concentrations, in combination with the pan-caspase inhibitor, Q-VD-OPH, elicited a strong reduction of cell viability that occurred in a caspase-independent manner. This was accompanied by the appearance of a senescence phenotype, as evidenced by increased p21/WAF1 expression and senescence-associated beta-galactosidase activity. Experiments using specific inhibitors of major cellular proteases other than caspases have shown that inhibition of cathepsin L, but not proteasome or cathepsin B, was responsible for the senescence-initiated reversal of drug resistance. This phenomenon appeared to be general because it was valid for other drugs and drug-resistant cell lines. A nonchemical approach, through cell transfection with cathepsin L small interfering RNA, also strongly reversed drug resistance. Further investigation of the underlying mechanism revealed that cathepsin L inhibition resulted in the alteration of intracellular drug distribution. In addition, in vitro experiments have demonstrated that p21/WAF1 is a substrate for cathepsin L, suggesting that inhibition of this enzyme may result in p21/WAF1 stabilization and its increased accumulation. All together, these findings suggest that cathepsin L inhibition in drug-resistant cells facilitates induction of senescence and reversal of drug resistance. This may represent the basis for a novel function of cathepsin L as a cell survival molecule responsible for initiation of resistance to chemotherapy.

Independent Intramolecular Mediators of Folding, Activity, and Inhibition for the Plasmodium falciparum Cysteine Protease Falcipain-2

The Plasmodium falciparum cysteine protease falcipain-2 is a trophozoite hemoglobinase and potential antimalarial drug target. Unlike other studied papain family proteases, falcipain-2 does not require its prodomain for folding to active enzyme. Rather, folding is mediated by an amino-terminal extension of the mature protease. As in related enzymes, the prodomain is a potent inhibitor of falcipain-2. We now report further functional evaluation of the domains of falcipain-2 and related plasmodial proteases. The minimum requirement for folding of falcipain-2 and four related plasmodial cysteine proteases was inclusion of a 14-15-residue amino-terminal folding domain, beginning with a conserved Tyr. Chimeras of the falcipain-2 catalytic domain with extensions from six other plasmodial proteases folded normally and had kinetic parameters (k(cat)/K(m) 124,000-195,000 M(-1) s(-1)) similar to those of recombinant falcipain-2 (k(cat)/K(m) 120,000 M(-1) s(-1)), indicating that the folding domain is functionally conserved across the falcipain-2 subfamily. Correct folding also occurred when the catalytic domain was refolded with a separate prodomain-folding domain construct but not with an isolated folding domain peptide. Thus, the prodomain mediated interaction between the other two domains when they were not covalently bound. The prodomain-catalytic domain interaction was independent of the active site, because it was blocked by free inactive catalytic domain but not by the active site-binding peptide leupeptin. The folded catalytic domain retained activity after purification from the prodomain-folding domain construct (k(cat)/K(m) 168,000 M(-1) s(-1)), indicating that the folding domain is not required for activity once folding has been achieved. Activity was lost after nonreducing gelatin SDS-PAGE but not native gelatin PAGE, indicating that correct disulfide bonds are insufficient to direct appropriate folding. Our results identify unique features of the falcipain-2 subfamily with independent mediation of activity, folding, and inhibition.

Protein inhibitors form complexes with procathepsin L and augment cleavage of the propeptide

The proregion fits tightly into the active site in the tertiary structure of procathepsin L and prevents its activity. We show that complexes between enzyme precursor and its endogenous protein inhibitors-the cystatins-can be formed without prior proteolytic removal of the propeptide. Complexes between cystatins and procathepsin L are formed at acidic pH and their formation is facilitated by acidic oligosaccharides. Binding of the inhibitor to the proenzyme is reversible and the slow dissociation of complex around neutral pH may serve as a pool for the sustained release of the enzyme. Formation of the complex between cystatin and procathepsin L increases the susceptibility of the proregion to proteolytic cleavage. This process may constitute an alternative mechanism of formation of the complex between enzyme and inhibitor without prior activation of the proenzyme.

Staphostatins: an expanding new group of proteinase inhibitors with a unique specificity for the regulation of staphopains, Staphylococcus spp. cysteine proteinases

A novel type of cysteine proteinase inhibitor (SspC) has been recently recognized in Staphylococcus aureus (Massimi, I., Park, E., Rice, K., Muller-Esterl, W., Sauder, D.N., and McGavin, M.J. (2002) J Biol Chem 277: 41770-41777). In this paper we have identified homologous proteins encoded in the genome of S. aureus and other coagulase-negative Staphylococci. Collectively we refer to these proteins as staphostatins as they specifically inhibit cysteine proteinases (staphopains) from Staphylococcus spp. The primary structure of staphostatins seems to be unique, although they resemble cystatins in size (105-108 residues). Recombinant staphostatin A, a product of the scpB gene and staphostatin B (SspC) from S. aureus have been characterized in details. Similar to the cystatins, the staphostatins interact specifically with their target proteinases forming tight and stable non-covalent complexes, staphostatin A with staphopain A and staphostatin B with staphopain B. However, in contrast to the cystatins, each of which inhibits broad range of cathepsins, complex formation between staphostatin and staphopain appears to be exclusive, with no cross interaction observed. In addition, the activities of several tested cysteine proteinases of prokaryotic- and eukaryotic-origin were not affected by staphostatins. Such narrow specificity limited to staphopains is presumed to be required to protect staphylococcal cytoplasmic proteins from being degraded by prematurely activated/folded prostaphopains. This function is guaranteed through the unique co-expression of the secreted proteinase and the intracellular inhibitor from the same operon, and represents a unique mechanism of regulation of proteolytic activity in Gram-positive bacteria.

Differential regulation of cathepsin S and cathepsin L in interferon gamma-treated macrophages

Cathepsin S (catS) and cathepsin L (catL) mediate late stages of invariant chain (Ii) degradation in discrete antigen-presenting cell types. Macrophages (Mphis) are unique in that they express both proteases and here we sought to determine the relative contribution of each enzyme. We observe that catL plays no significant role in Ii cleavage in interferon (IFN)-gamma-stimulated Mphis. In addition, our studies show that the level of catL activity is significantly decreased in Mphis cultured in the presence of IFN-gamma whereas catS activity increases. The decrease in catL activity upon cytokine treatment occurs despite the persistence of high levels of mature catL protein, suggesting that a specific inhibitor of the enzyme is up-regulated in IFN-gamma-stimulated peritoneal Mphis. Similar inhibition of activity is observed in dendritic cells engineered to overexpress catL. Such enzymatic inhibition in Mphis exhibits only partial dependence upon Ii and therefore, other mechanisms of catL inhibition are regulated by IFN-gamma. Thus, during a T helper cell type 1 immune response catL inhibition in Mphis results in preferential usage of catS, such that major histocompatibility complex class II presentation by all bone marrow-derived antigen-presenting cell is regulated by catS.

Design of noncovalent inhibitors of human cathepsin L. From the 96-residue proregion to optimized tripeptides

A novel series of noncovalent inhibitors of cathepsin L have been designed to mimic the mode of autoinhibition of procathepsin L. Just like the propeptide, these peptide-based inhibitors have a reverse-binding mode relative to a substrate and span both the S' and S subsites of the enzyme active site. In contrast to previous studies in which even moderate truncation of the full-length propeptide led to rapid reduction in potency, these blocked tripeptide-sized inhibitors maintain nanomolar potency. Moreover, these short peptides show higher selectivity (up to 310-fold) for inhibiting cathepsin L over K versus only 2-fold selectivity of the 96-residue propeptide of cathepsin L. A 1.9 A X-ray crystallographic structure of the complex of cathepsin L with one of the inhibitors confirms the designed reverse-binding mode of the inhibitor as well as its noncovalent nature. Enzymatic analysis also shows the inhibitors to be resistant to hydrolysis at elevated concentrations of the enzyme. The mode of inhibition of these molecules provides a general strategy for inhibiting other cathepsins as well as other proteases.

Inhibition of cathepsin K with lysosomotropic macromolecular inhibitors

Cathepsin K is the major enzyme responsible for the degradation of the protein matrix of bone and probably for the destruction of articular cartilage in rheumatoid arthritis joints. These processes occur mainly in the resorption lacuna and within the lysosomal compartment. Here, we have designed, synthesized, and evaluated new lysosomotropic (water-soluble) polymer-cathepsin K inhibitor conjugates. In particular, we characterized the relationship between conjugate structures and their activity to inhibit cathepsins K, B, L, and papain. A potent selective cathepsin K inhibitor, 1,5-bis(N-benzyloxycarbonylleucyl)carbohydrazide, was modified to 1-(N-benzyloxycarbonylleucyl)-5-(phenylalanylleucyl)carbohydrazide (I) to facilitate polymer conjugation. It was conjugated to the polymer chain termini of two water-soluble polymers [alpha-methoxy poly(ethylene glycol), abbreviated as mPEG-I; semitelechelic poly[N-(2-hydroxypropyl)methacrylamide], abbreviated as ST-PHPMA-I]. The conjugation of inhibitor I to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer side chains was accomplished via either a Gly-Gly spacer (PHPMA-GG-I) or with no spacer between I and the copolymer backbone (PHPMA-I). Kinetic analysis revealed that free inhibitor I possessed an apparent second-order rate constant against cathepsin K (k(obs)/[I] = 1.3 x 10(6) M(-1) s(-1)) similar to that of unmodified 1,5-bis(Cbz-Leu) carbohydrazide, while I conjugated to the chain termini of mPEG and ST-PHPMA-COOH had slightly lower values (about 5 x 10(5) M(-1) s(-1)). The k(obs)/[I] values for I attached to the side chains of HPMA copolymers (PHPMA-GG-I and PHPMA-I) were about 3 x 10(4) M(-1) s(-1). When tested against cathepsin L, inhibitor I and all its polymer conjugates produced k(obs)/[I] values 1-2 orders of magnitude less than those determined for cathepsin K, while for cathepsin B and papain, the values were 2-4 orders of magnitude lower. The ability of mPEG-I and ST-PHPMA-I to inhibit cathepsin K activity in synovial fibroblasts was also evaluated. Both polymer-bound inhibitors were internalized by endocytosis and were ultimately trafficked to the lysosomal compartment. ST-PHPMA-I was internalized faster than mPEG-I. The inhibitory activity in the synovial fibroblast assay correlated with the rate of internalization.

Evolution of placentally expressed cathepsins

Species and strain variants of a family of placentally expressed cathepsins (PECs) were cloned and sequenced in order to identify evolutionary conserved structural characteristics of this large family of cysteine proteases. Cathepsins M, P, Q, and R, are conserved in mice and rats but homologs of these genes are not found in human or rabbit placenta, showing that this family of proteases are probably restricted to rodents. Species-specific gene duplications have given rise to variants of cathepsin M in mice, and cathepsin Q in rats. Although the PECs have diverged at a greater rate than the other lysosomal cathepsins, residues around the specificity sub-sites of the individual enzymes are conserved. Strain-specific polymorphisms show that the evolutionary rate of divergence of cathepsins M and 3, the most recently duplicated pair of mouse genes, is even higher than the other PECs. In human placenta, critical functions of the PECs are probably performed by broader specificity proteases such as cathepsins B and L.

Folding of the Plasmodium falciparum cysteine protease falcipain-2 is mediated by a chaperone-like peptide and not the prodomain

Papain-family cysteine proteases of the malaria parasite Plasmodium falciparum, known as falcipains, are hemoglobinases and potential drug targets. Available data suggest that papain-family proteases require prodomains for correct folding into functional conformations. However, in prior studies of falcipain-2, an Escherichia coli-expressed construct containing only a small portion of the prodomain refolded efficiently, suggesting that this enzyme differs in this regard from other papain-family enzymes. To better characterize the determinants of folding for falcipain-2, we expressed multiple pro- and mature constructs of the enzyme in E. coli and assessed their abilities to refold. Mature falcipain-2 refolded into active protease with very similar properties to those of proteins resulting from the refolding of proenzyme constructs. Deletion of a 17-amino acid amino-terminal segment of the mature protease yielded a construct incapable of correct folding, but inclusion of this segment in trans allowed folding to active falcipain-2. The prodomain was a potent, competitive, and reversible inhibitor of mature falcipain-2 (K(i) 10(-10) m). Our results identify a chaperone-like function of an amino-terminal segment of mature falcipain-2 and suggest that protease inhibition, but not the mediation of folding, is a principal function of the falcipain-2 prodomain.

Cathepsin S regulates the expression of cathepsin L and the turnover of gamma-interferon-inducible lysosomal thiol reductase in B lymphocytes

Loading of antigenic peptide fragments on major histocompatibility complex class II molecules is essential for generation of CD4(+) T cell responses and occurs after cathepsin-mediated degradation of the invariant chain chaperone molecule. Cathepsins are expressed differentially in antigen presenting cells, and mice deficient in cathepsin S or cathepsin L exhibit severely impaired antigen presentation in peripheral lymphoid organs and the thymus, respectively. To determine whether these defects are due solely to the block in invariant chain cleavage, we used cathepsin-deficient B cells to examine the role of cathepsins S and B in the degradation of other molecules important in the class II presentation pathway. Our data indicate that neither cathepsin S nor B is critical for H-2M degradation or processing of precursor gamma-interferon-inducible lysosomal thiol reductase (GILT) to a mature thiol reductase, but suggest a role for cathepsin S in the turnover of mature GILT and in regulating levels of mature cathepsin L protein in B cells. Despite the presence of mature cathepsin L protein, no enzyme activity could be detected in B cells or dendritic cells. These experiments suggest a novel mechanism by which these functionally important enzymes may be regulated.