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Cysteine cathepsins: their role in tumor progression and recent trends in the development of imaging probes.

Löser R, Pietzsch J - Front Chem (2015)

Bottom Line: The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging.After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response.In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiopharmaceutical and Chemical Biology, Institute of Radiopharmaceutical Cancer Research, Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany ; Department of Chemistry and Food Chemistry, Technische Universität Dresden Dresden, Germany.

ABSTRACT
Papain-like cysteine proteases bear an enormous potential as drug discovery targets for both infectious and systemic human diseases. The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging. After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response. In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

No MeSH data available.


Related in: MedlinePlus

Richard Willstätter (left) and Eugen Bamann (right), the coiners of term cathepsin. © Archive of Deutsche Akademie der Naturforscher Leopoldina, M1 3416 and M1 4898.
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Figure 1: Richard Willstätter (left) and Eugen Bamann (right), the coiners of term cathepsin. © Archive of Deutsche Akademie der Naturforscher Leopoldina, M1 3416 and M1 4898.

Mentions: The term cathepsins was introduced by the famous chemist Richard Willstätter (1872–1942) and his PhD student Eugen Bamann (1900–1981) for the entirety of the intracellular proteases referring to their protein-degrading activity (greek καθεψειν = to digest, to boil) in the first half of the last century (Figure 1) (Willstätter and Bamann, 1929). In this sense, their physiological functions were for long time considered to be restricted to cellular protein catabolism. The class of proteases referred to as cathepsins represents a structurally heterogeneous group of enzymes. However, in humans the majority of them belong to the mechanistic class of the cysteine proteases as they contain a highly conserved cysteine residue in their active sites. Because they share a high degree of homology to the plant enzyme papain, the cysteine cathepsins are included in the C1 family of clan CA according to the MEROPS protease classification system (Rawlings et al., 2014)1. In addition to the 11 papain-like cathepsins B, C, F, H, K, L, O, S, V, W, and X, four non-cysteine cathepsins are present in humans, i.e., the serine proteases cathepsins A (also referred to as serine carboxypeptidase A) and G and the aspartic proteases cathepsins D and E (Kirschke, 2008). With the exception of cathepsin C, which is present as a homotetramer, all cysteine cathepsins are monomeric single domain enzymes with molar masses ranging between 24 and 28 kDa for the mature enzymes. As characteristic for all papain-like enzymes, their structure is composed of two subdomains referred to as the L (left)- and the R (right)-domains. The N-terminal L-domain is dominated by α-helical structures among which a long N-terminal α-helix that harbors the active-site cysteine residue at its beginning is the most striking element. The R-domain is located toward the C-terminus and is characterized by a β-barrel motif. The V-shaped active-site cleft is situated between the two subdomains with the residues cysteine 25, histidine 159, asparagine 175, and glutamine 19 (papain numbering) constituting the catalytic center. The binding sites that recognize the amino acid side chains on the peptidic substrates are located in alternating sequence on the L- and R-subdomains which requires the substrate to be present in the extended conformation for productive binding (McGrath, 1999; Brömme, 2001; Grzonka et al., 2001; Turk et al., 2001b, 2003, 2012; Lecaille et al., 2002; Stoka et al., 2005). Figure 2 explains the structure of papain-like cysteine proteases exemplarily for human cathepsin B. Within the S2 to S1′ binding sites (see Figure 3 for explanation) the contacts to the substrate comprise non-covalent interactions to the side-chain entities and hydrogen bonds to the peptidic backbone. The enzyme-substrate contacts beyond these sites, i.e., S3 and S2′ in the N- and C-terminal direction, respectively, are devoid of hydrogen bonds and less well-defined (Turk et al., 1997; Turk and Gunčar, 2003). Regarding the mechanism of catalysis, instructive insights have been gained from investigations on papain, which in principle can be translated to the other members of the clan CA proteases including the cysteine cathepsins. A catalytic triad is formed by cysteine 25, histidine 159, and asparagine 175. Due to the formation of a thiolate-imidazolium ion pair, the active-site cysteine is present as a permanent negatively charged nucleophile. The ion pair is stabilized by the asparagine 175 whose side chain carbonyl oxygen acts as an H-bond acceptor toward the imidazole NH of histidine 159, which results in an increased basicity of this imidazole moiety (Vernet et al., 1995). Further, stabilization of the thiolate arises by the fact that the active-site cysteine is located at the beginning of the N-terminal α-helix mentioned before, which enables the negative charge to be stabilized by the helix macrodipole (Rullmann et al., 1989). In addition, the thiolate-imidazolium ion pair is shielded from solvent by the side chain of tryptophan 177 (Gul et al., 2008). The stabilized negative charge renders the active-site cysteine capable of nucleophilic attack toward the peptide bond in the substrate resulting in the formation of a transient thiol ester. During this step, a tetrahedral intermediate is passed, whose oxygen-centered negative charge is stabilized by H-bond contacts to one of the side chain amide proton of glutamine 19 and the backbone NH of cysteine 25, the so called oxyanion hole. Upon collapse of this tetrahedral intermediate, a proton is transferred from the imidazolium moiety of histidine 159 to the nitrogen of the cleaved peptide bond which results in the release of the C-terminal cleavage product from the enzyme. In the following step, the neutral imidazole ring of the histidine acts as general base upon nucleophilic attack of the thiol ester by water under the formation of a second tetrahedral intermediate. The collapse of this intermediate releases the N-terminal cleavage product and restores the thiolate-imidazolium ion pair ready for another cycle of catalysis.


Cysteine cathepsins: their role in tumor progression and recent trends in the development of imaging probes.

Löser R, Pietzsch J - Front Chem (2015)

Richard Willstätter (left) and Eugen Bamann (right), the coiners of term cathepsin. © Archive of Deutsche Akademie der Naturforscher Leopoldina, M1 3416 and M1 4898.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4477214&req=5

Figure 1: Richard Willstätter (left) and Eugen Bamann (right), the coiners of term cathepsin. © Archive of Deutsche Akademie der Naturforscher Leopoldina, M1 3416 and M1 4898.
Mentions: The term cathepsins was introduced by the famous chemist Richard Willstätter (1872–1942) and his PhD student Eugen Bamann (1900–1981) for the entirety of the intracellular proteases referring to their protein-degrading activity (greek καθεψειν = to digest, to boil) in the first half of the last century (Figure 1) (Willstätter and Bamann, 1929). In this sense, their physiological functions were for long time considered to be restricted to cellular protein catabolism. The class of proteases referred to as cathepsins represents a structurally heterogeneous group of enzymes. However, in humans the majority of them belong to the mechanistic class of the cysteine proteases as they contain a highly conserved cysteine residue in their active sites. Because they share a high degree of homology to the plant enzyme papain, the cysteine cathepsins are included in the C1 family of clan CA according to the MEROPS protease classification system (Rawlings et al., 2014)1. In addition to the 11 papain-like cathepsins B, C, F, H, K, L, O, S, V, W, and X, four non-cysteine cathepsins are present in humans, i.e., the serine proteases cathepsins A (also referred to as serine carboxypeptidase A) and G and the aspartic proteases cathepsins D and E (Kirschke, 2008). With the exception of cathepsin C, which is present as a homotetramer, all cysteine cathepsins are monomeric single domain enzymes with molar masses ranging between 24 and 28 kDa for the mature enzymes. As characteristic for all papain-like enzymes, their structure is composed of two subdomains referred to as the L (left)- and the R (right)-domains. The N-terminal L-domain is dominated by α-helical structures among which a long N-terminal α-helix that harbors the active-site cysteine residue at its beginning is the most striking element. The R-domain is located toward the C-terminus and is characterized by a β-barrel motif. The V-shaped active-site cleft is situated between the two subdomains with the residues cysteine 25, histidine 159, asparagine 175, and glutamine 19 (papain numbering) constituting the catalytic center. The binding sites that recognize the amino acid side chains on the peptidic substrates are located in alternating sequence on the L- and R-subdomains which requires the substrate to be present in the extended conformation for productive binding (McGrath, 1999; Brömme, 2001; Grzonka et al., 2001; Turk et al., 2001b, 2003, 2012; Lecaille et al., 2002; Stoka et al., 2005). Figure 2 explains the structure of papain-like cysteine proteases exemplarily for human cathepsin B. Within the S2 to S1′ binding sites (see Figure 3 for explanation) the contacts to the substrate comprise non-covalent interactions to the side-chain entities and hydrogen bonds to the peptidic backbone. The enzyme-substrate contacts beyond these sites, i.e., S3 and S2′ in the N- and C-terminal direction, respectively, are devoid of hydrogen bonds and less well-defined (Turk et al., 1997; Turk and Gunčar, 2003). Regarding the mechanism of catalysis, instructive insights have been gained from investigations on papain, which in principle can be translated to the other members of the clan CA proteases including the cysteine cathepsins. A catalytic triad is formed by cysteine 25, histidine 159, and asparagine 175. Due to the formation of a thiolate-imidazolium ion pair, the active-site cysteine is present as a permanent negatively charged nucleophile. The ion pair is stabilized by the asparagine 175 whose side chain carbonyl oxygen acts as an H-bond acceptor toward the imidazole NH of histidine 159, which results in an increased basicity of this imidazole moiety (Vernet et al., 1995). Further, stabilization of the thiolate arises by the fact that the active-site cysteine is located at the beginning of the N-terminal α-helix mentioned before, which enables the negative charge to be stabilized by the helix macrodipole (Rullmann et al., 1989). In addition, the thiolate-imidazolium ion pair is shielded from solvent by the side chain of tryptophan 177 (Gul et al., 2008). The stabilized negative charge renders the active-site cysteine capable of nucleophilic attack toward the peptide bond in the substrate resulting in the formation of a transient thiol ester. During this step, a tetrahedral intermediate is passed, whose oxygen-centered negative charge is stabilized by H-bond contacts to one of the side chain amide proton of glutamine 19 and the backbone NH of cysteine 25, the so called oxyanion hole. Upon collapse of this tetrahedral intermediate, a proton is transferred from the imidazolium moiety of histidine 159 to the nitrogen of the cleaved peptide bond which results in the release of the C-terminal cleavage product from the enzyme. In the following step, the neutral imidazole ring of the histidine acts as general base upon nucleophilic attack of the thiol ester by water under the formation of a second tetrahedral intermediate. The collapse of this intermediate releases the N-terminal cleavage product and restores the thiolate-imidazolium ion pair ready for another cycle of catalysis.

Bottom Line: The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging.After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response.In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiopharmaceutical and Chemical Biology, Institute of Radiopharmaceutical Cancer Research, Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany ; Department of Chemistry and Food Chemistry, Technische Universität Dresden Dresden, Germany.

ABSTRACT
Papain-like cysteine proteases bear an enormous potential as drug discovery targets for both infectious and systemic human diseases. The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging. After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response. In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

No MeSH data available.


Related in: MedlinePlus