Exploring the role of 5' alternative splicing and of the 3'-untranslated region of cathepsin B mRNA

Beyond basic research: the contribution of cathepsin B to cancer development, diagnosis and therapy

INTRODUCTION

In view of other candidate proteins from the cathepsin family of proteases holding great potential in being targeted during cancer therapy, the importance of Cathepsin B (CtsB) stands out as being truly exceptional. Based on its contribution to oncogenesis, its intimate connection with regulating apoptosis and modulating extracellular and intracellular functions through its secretion or compartmentalized subcellular localization, collectively highlight its complex molecular involvement with a myriad of normal and pathological regulatory processes. Despite its complex functional nature, CtsB is emerging as one of the few cathepsin proteases that has been extensively researched to yield tangible outcomes for cancer therapy.

AREAS COVERED

In this article, we review the scientific literature that has justified or shaped the importance of CtsB expression in cancer progression, from the perspective of highlighting a paradigm that is rapidly changing from basic research toward a broader clinical and translational context.

EXPERT OPINION

In doing so, we detail its maturation as a diagnostic marker through describing the development of CtsB-specific Activity-Based Probes, the rapid evolution of these toward a new generation of Prodrugs, and the evaluation of these in model systems for their therapeutic potential as anti-cancer agents in the clinic.

Extralysosomal cathepsin B in central nervous system: Mechanisms and therapeutic implications

Cathepsin B (CatB) is a typical cysteine lysosomal protease involved in a variety of physiologic and pathological processes. It is expressed in most cell types and is primarily localized within subcellular endosomal and lysosomal compartments. Emerging scientific evidence indicates that lysosomal leaked CatB is involved in mitochondrial stress, inflammasome activation, and nuclear senescence, but without the acidic environment. CatB is also secreted as a myokine, which is involved in muscle‐brain cross talk and neuronal dendritic remodeling. Lysosomal‐leaked and cellular‐secreted CatB functions are dependent on its enzymatic activity at a neutral pH. In the present review, we summarize the available experimental evidence that mechanistically links extralysosomal CatB to physiological and pathological functions in central nervous system, and their potential for use in therapeutic approaches.

Secretory autophagy-induced bladder tumour-derived extracellular vesicle secretion promotes angiogenesis by activating the TPX2-mediated phosphorylation of the AURKA-PI3K-AKT axis

Tumour angiogenesis is an independent risk factor for bladder cancer (BCa) progression, but viable and promising antiangiogenic targets are understudied. Secretory autophagy has received increasing interest recently, while the roles and executing mechanisms in the tumour microenvironment (TME) remain unclear. Herein, we found that active cathepsin B (CTSB) was upregulated in tumour tissues and serum EVs of 241 BCa patients from four cohorts and was significantly associated with poor prognosis. Starving TME (STME)-induced conventional autophagy in BCa cells elevated active CTSB levels by facilitating the expression and nuclear translocation of NFATC2. In addition, STME-induced secretory autophagy simultaneously led to markedly increased secretion of LC3-conjugated EVs loaded with active CTSB (EV-CTSB) into the TME. The increased exogenous active CTSB in endothelial cells by directly ingesting EV-CTSB prominently activated the TPX2-mediated phosphorylation of the AURKA-PI3K-AKT axis, increased VEGFA expression, and promoted angiogenesis. Our findings not only verify that EV-CTSB can be a promising target for antiangiogenic strategies in bladder cancer, but also reveal a novel action pattern based on secretory autophagy-induced EV secretion which is enlightening to explore crosstalk in the TME from various perspectives.

Genome-wide comparative analyses of GATA transcription factors among seven Populus genomes

GATA transcription factors (TFs) are widespread eukaryotic regulators whose DNA-binding domain is a class IV zinc finger motif (CX2CX17-20CX2C) followed by a basic region. We identified 262 GATA genes (389 GATA TFs) from seven Populus genomes using the pipeline of GATA-TFDB. Alternative splicing forms of Populus GATA genes exhibit dynamics of GATA gene structures including partial or full loss of GATA domain and additional domains. Subfamily III of Populus GATA genes display lack CCT and/or TIFY domains. 21 Populus GATA gene clusters (PCs) were defined in the phylogenetic tree of GATA domains, suggesting the possibility of subfunctionalization and neofunctionalization. Expression analysis of Populus GATA genes identified the five PCs displaying tissue-specific expression, providing the clues of their biological functions. Amino acid patterns of Populus GATA motifs display well conserved manner of Populus GATA genes. The five Populus GATA genes were predicted as membrane-bound GATA TFs. Biased chromosomal distributions of GATA genes of three Populus species. Our comparative analysis approaches of the Populus GATA genes will be a cornerstone to understand various plant TF characteristics including evolutionary insights.

Proteomic Analysis of Synovial Fibroblasts and Articular Chondrocytes Co-Cultures Reveals Valuable VIP-Modulated Inflammatory and Degradative Proteins in Osteoarthritis

Osteoarthritis (OA) is the most common musculoskeletal disorder causing a great disability and a reduction in the quality of life. In OA, articular chondrocytes (AC) and synovial fibroblasts (SF) release innate-derived immune mediators that initiate and perpetuate inflammation, inducing cartilage extracellular matrix (ECM) degradation. Given the lack of therapies for the treatment of OA, in this study, we explore biomarkers that enable the development of new therapeutical approaches. We analyze the set of secreted proteins in AC and SF co-cultures by stable isotope labeling with amino acids (SILAC). We describe, for the first time, 115 proteins detected in SF-AC co-cultures stimulated by fibronectin fragments (Fn-fs). We also study the role of the vasoactive intestinal peptide (VIP) in this secretome, providing new proteins involved in the main events of OA, confirmed by ELISA and multiplex analyses. VIP decreases proteins involved in the inflammatory process (CHI3L1, PTX3), complement activation (C1r, C3), and cartilage ECM degradation (DCN, CTSB and MMP2), key events in the initiation and progression of OA. Our results support the anti-inflammatory and anti-catabolic properties of VIP in rheumatic diseases and provide potential new targets for OA treatment.

Lost or Forgotten: The nuclear cathepsin protein isoforms in cancer

While research into the role of cathepsins has been progressing at an exponential pace over the years, research into their respective isoform proteins has been less frenetic. In view of the functional and biological potential of such protein isoforms in model systems for cancer during their initial discovery, much later they have offered a new direction in the field of cathepsin basic and applied research. Consequently, the analysis of such isoforms has laid strong foundations in revealing other important regulatory aspects of the cathepsin proteins in general. In this review article, we address these key aspects of cathepsin isoform proteins, with particular emphasis on how they have shaped what is now known in the context of nuclear cathepsin localization and what potential these hold as nuclear-based therapeutic targets in cancer.

Cathepsin B: A sellsword of cancer progression

Clinical, biochemical and molecular biology studies have identified lysosome-encapsulated cellular proteases as critical risk factors for cancer progression. Cathepsins represent a group of such proteases aimed at maintenance of cellular homeostasis. Nevertheless, recent reports suggest that Cathepsin B executes other cellular programs such as controlling tumor growth, migration, invasion, angiogenesis, and metastases development. In fact, elevated levels of Cathepsins are found under different pathological conditions including inflammation, infection, neurodegenerative disease, and cancer. Furthermore, the discovery of Cathepsin B secretion and function as an extracellular matrix protein has broadened our appreciation for the impact of Cathepsin B on cancer progression. Underneath a façade of an intracellular protease with limited therapeutic potential hides a central role of cathepsins in extracellular functions. Moreover, this role is incredibly diverse from one condition to the next - from driving caspase-dependent apoptosis to facilitating tumor neovascularization and metastasis. Here we discuss the role of Cathepsin B in the oncogenic process and perspective the use of Cathepsin B for diagnostic and therapeutic applications.

Host Cell Proteases: Cathepsins

Cathepsins are proteolytic enzymes with a broad spectrum of substrates. They are known to reside within endo-lysosomes where they acquire optimal conditions for proteolytic activity and substrate cleavage. However, cathepsins have been detected in locations other than the canonical compartments of the endocytotic pathway. They are often secreted from cells in either proteolytically inactive proform or as mature and active enzyme; this may happen in both physiological and pathological conditions. Moreover, cytosolic and nuclear forms of cathepsins have been described and are currently an emerging field of research aiming at understanding their functions in such unexpected cellular locations. This chapter summarizes the canonical pathways of biosynthesis and transport of cathepsins in healthy cells. We further describe how cathepsins can reach unexpected locations such as the extracellular space or the cytosol and the nuclear matrix. No matter where viruses and cathepsins encounter, several outcomes can be perceived. Thus, scenarios are discussed on how cathepsins may support virus entry into host cells, involve in viral fusion factor and polyprotein processing in different host cell compartments, or help in packaging of viral particles during maturation. It is of note to mention that this review is not meant to comprehensively cover the present literature on viruses encountering cathepsins but rather illustrates, on some representative examples, the possible roles of cathepsins in replication of viruses and in the course of disease.

Propeptides as modulators of functional activity of proteases

Most proteases are synthesized in the cell as precursor-containing propeptides. These structural elements can determine the folding of the cognate protein, function as an inhibitor/activator peptide, mediate enzyme sorting, and mediate the protease interaction with other molecules and supramolecular structures. The data presented in this review demonstrate modulatory activity of propeptides irrespective of the specific mechanism of action. Changes in propeptide structure, sometimes minor, can crucially alter protein function in the living organism. Modulatory activity coupled with high variation allows us to consider propeptides as specific evolutionary modules that can transform biological properties of proteases without significant changes in the highly conserved catalytic domains. As the considered properties of propeptides are not unique to proteases, propeptide-mediated evolution seems to be a universal biological mechanism.

Proteolysis mediated by cysteine cathepsins and legumain-recent advances and cell biological challenges

Proteases play essential roles in protein degradation, protein processing, and extracellular matrix remodeling in all cell types and tissues. They are also involved in protein turnover for maintenance of homeostasis and protein activation or inactivation for cell signaling. Proteases range in function and specificity, with some performing distinct substrate cleavages, while others accomplish proteolysis of a wide range of substrates. As such, different cell types use specialized molecular mechanisms to regulate the localization of proteases and their function within the compartments to which they are destined. Here, we focus on the cysteine family of cathepsin proteases and legumain, which act predominately within the endo-lysosomal pathway. In particular, recent knowledge on cysteine cathepsins and their primary regulator legumain is scrutinized in terms of their trafficking to endo-lysosomal compartments and other less recognized cellular locations. We further explore the mechanisms that regulate these processes and point to pathological cases which arise from detours taken by these proteases. Moreover, the emerging biological roles of specific forms and variants of cysteine cathepsins and legumain are discussed. These may be decisive, pathogenic, or even deadly when localizing to unusual cellular compartments in their enzymatically active form, because they may exert unexpected effects by alternative substrate cleavage. Hence, we propose future perspectives for addressing the actions of cysteine cathepsins and legumain as well as their specific forms and variants. The increasing knowledge in non-canonical aspects of cysteine cathepsin- and legumain-mediated proteolysis may prove valuable for developing new strategies to utilize these versatile proteases in therapeutic approaches.

Ligand activation of peroxisome proliferator-activated receptor delta suppresses cathepsin B expression in human endothelial cells in a posttranslational manner

Peroxisome proliferator-activated receptor (PPAR) delta agonists are known to have distinct anti-inflammatory and antitumor effects; though, the knowledge regarding their mode of action has thus far been limited. Different cathepsins have been shown to be upregulated in a broad range of pathological events, such as rheumatoid arthritis, psoriasis, atherosclerosis and diverse tumor entities, for example melanoma. Recent work demonstrated that cathepsin B in particular is an important pro-angiogenic protease in various pathological conditions. We therefore analysed whether cathepsins are a valid target for PPARδ agonists. This study reveals an inhibitory effect of two commonly used PPARδ agonists, GW501516 and L-165,041, on the protein expression and enzyme activity of cathepsin B in human endothelial cells. In contrast, no inhibitory effects were observed on cathepsin L and cathepsin D protein expression after treatment with PPARδ agonists. Furthermore, the results substantiate that PPARδ activators mediate their inhibitory action in a PPARδ-dependent manner and that the underlying regulatory mechanism is not based on a transcriptional but rather on a posttranslational mode of action, via the reduction in the cathepsin B protein half-life. Mechanisms conveying the suppressive effect by 5'-alternative splicing, a 3'-UTR-dependent way or by miRNA could be excluded. The data of this study explore cathepsin B as a new valid target for PPARδ agonists in endothelial cells. The results bolster other studies demonstrating PPARδ agonists as anti-inflammatory and anticarcinogenic agents and thus might have the potential to help to develop new pharmaceutical drugs.

Alternative Pre-mRNA Splicing, Cell Death, and Cancer

Alternative splicing is one of the most powerful mechanisms for generating functionally distinct products from a single genetic loci and for fine-tuning gene activities at the post-transcriptional level. Alternative splicing plays important roles in regulating genes critical for cell death. These cell death genes encode death ligands, cell surface death receptors, intracellular death regulators, signal transduction molecules, and death executor enzymes such as caspases and nucleases. Alternative splicing of these genes often leads to the formation of functionally different products, some of which have antagonistic effects that are either cell death-promoting or cell death-preventing. Differential alternative splicing can affect expression, subcellular distribution, and functional activities of the gene products. Molecular defects in splicing regulation of cell death genes have been associated with cancer development and resistance to treatment. Studies using molecular, biochemical, and systems-based approaches have begun to reveal mechanisms underlying the regulation of alternative splicing of cell death genes. Systematic studies have begun to uncover the multi-level interconnected networks that regulate alternative splicing. A global picture of the complex mechanisms that regulate cell death genes at the pre-mRNA splicing level has thus begun to emerge.

A double-headed cathepsin B inhibitor devoid of warhead

Most synthetic inhibitors of peptidases have been targeted to the active site for inhibiting catalysis through reversible competition with the substrate or by covalent modification of catalytic groups. Cathepsin B is unique among the cysteine peptidase for the presence of a flexible segment, known as the occluding loop, which can block the primed subsites of the substrate binding cleft. With the occluding loop in the open conformation cathepsin B acts as an endopeptidase, and it acts as an exopeptidase when the loop is closed. We have targeted the occluding loop of human cathepsin B at its surface, outside the catalytic center, using a high-throughput docking procedure. The aim was to identify inhibitors that would interact with the occluding loop thereby modulating enzyme activity without the help of chemical warheads against catalytic residues. From a large library of compounds, the in silico approach identified [2-[2-(2,4-dioxo-1,3-thiazolidin-3-yl)ethylamino]-2-oxoethyl] 2-(furan-2-carbonylamino) acetate, which fulfills the working hypothesis. This molecule possesses two distinct binding moieties and behaves as a reversible, double-headed competitive inhibitor of cathepsin B by excluding synthetic and protein substrates from the active center. The kinetic mechanism of inhibition suggests that the occluding loop is stabilized in its closed conformation, mainly by hydrogen bonds with the inhibitor, thus decreasing endoproteolytic activity of the enzyme. Furthermore, the dioxothiazolidine head of the compound sterically hinders binding of the C-terminal residue of substrates resulting in inhibition of the exopeptidase activity of cathepsin B in a physiopathologically relevant pH range.

The cathepsin family and their role in colorectal cancer

Cathepsins are a class of globular proteases, initially described as intracellular peptide hydrolases, although several cathepsins also have extracellular functions. Cathepsins B, C, F, H, L, K, O, S, V, W, and X are cysteine proteases of the papain family, and represent the largest and best-known class of the cathepsins. Cathepsin G is a serine carboxypeptidases, and cathepsins D and E are aspartic proteases. Cathepsins are synthesized as inactive proenzymes and processed to become mature and active enzymes. Endogenous protein inhibitors, such as cystatins and some serpins, inhibit active enzymes. As primarily lysosomal proteases, cathepsins play important roles in proteolysis during physiological processes, as well as in several diseases. On the basis of their ability to degrade extracellular matrix proteins, cathepsins have been implicated to play a role in invasion and metastasis of colorectal cancer. In the present review, the role of cathepsins in the disease process of colorectal cancers and the correlation of cathepsin expression and activity with clinicopathological features is discussed. Furthermore, we give an overview of the recent developments of cathepsins in animal models and in in vitro experiments of colorectal disease, and provide information on inhibitors of cathepsins as possible therapeutics.

Diversity in tissue expression, substrate binding, and SCF complex formation for a lectin family of ubiquitin ligases

Post-translational modification of proteins regulates many cellular processes. Some modifications, including N-linked glycosylation, serve multiple functions. For example, the attachment of N-linked glycans to nascent proteins in the endoplasmic reticulum facilitates proper folding, whereas retention of high mannose glycans on misfolded glycoproteins serves as a signal for retrotranslocation and ubiquitin-mediated proteasomal degradation. Here we examine the substrate specificity of the only family of ubiquitin ligase subunits thought to target glycoproteins through their attached glycans. The five proteins comprising this FBA family (FBXO2, FBXO6, FBXO17, FBXO27, and FBXO44) contain a conserved G domain that mediates substrate binding. Using a variety of complementary approaches, including glycan arrays, we show that each family member has differing specificity for glycosylated substrates. Collectively, the F-box proteins in the FBA family bind high mannose and sulfated glycoproteins, with one FBA protein, FBX044, failing to bind any glycans on the tested arrays. Site-directed mutagenesis of two aromatic amino acids in the G domain demonstrated that the hydrophobic pocket created by these amino acids is necessary for high affinity glycan binding. All FBA proteins co-precipitated components of the canonical SCF complex (Skp1, Cullin1, and Rbx1), yet FBXO2 bound very little Cullin1, suggesting that FBXO2 may exist primarily as a heterodimer with Skp1. Using subunit-specific antibodies, we further demonstrate marked divergence in tissue distribution and developmental expression. These differences in substrate recognition, SCF complex formation, and tissue distribution suggest that FBA proteins play diverse roles in glycoprotein quality control.

Cysteine cathepsins: cellular roadmap to different functions

Cysteine cathepsins belong to the papain-like family C1 of clan CA cysteine peptidases. These enzymes are ubiquitously expressed and exert their proteolytic activity mainly, but not exclusively within the compartments along the endocytic pathway. Moreover, cysteine cathepsins are active in pericellular environments as soluble enzymes or bound to cell surface receptors at the plasma membrane, and possibly even within secretory vesicles, the cytosol, mitochondria, and within the nuclei of eukaryotic cells. Proteolytic actions performed by cysteine cathepsins are essential in the maintenance of homeostasis and depend heavily upon their correct sorting and trafficking within cells. As a consequence, the numerous and diverse approaches to identification, qualitative and quantitative determination, and visualization of cysteine cathepsin functions in vitro, in situ, and in vivo cover the entire spectrum of biochemistry, molecular and cell biology. This review focuses upon the transport pathways directing cysteine cathepsins to their points of action and thus emphasizes the broader role and functionality of cysteine cathepsins in a number of specific cellular locales. Such understanding will provide a foundation for future research investigating the involvement of these peptidases with their substrates, inhibitors, and the intertwined proteolytic networks at the hubs of complex biological systems.

Chondrocyte hypertrophy and apoptosis induced by GROalpha require three-dimensional interaction with the extracellular matrix and a co-receptor role of chondroitin sulfate and are associated with the mitochondrial splicing variant of cathepsin B

CXCR2 ligands contribute to chondrocyte hypertrophy and apoptosis, important determinants in cartilage pathophysiology. We unraveled the kinetics of signaling, biochemical, transcriptional, and morphological events triggered by GROalpha in human osteoarthritic chondrocytes kept in three-dimensional culture. p38 MAPK activation was assessed with a highly sensitive ELISA. Effector caspase activation was evaluated by cleavage of a fluorogenic substrate. Gene expression of key markers of hypertrophy (MMP-13, Runx-2) and matrix synthesis (aggrecan), and of cathepsin B isoform CB(-2,3) was evaluated by real time PCR. Occurrence of the morphological markers of apoptosis was investigated by transmission electron microscopy (TEM). GROalpha led to p38 MAPK activation in passaged chondrocytes cultured in micromass but not as a high-density monolayer. This caused the downstream triggering of chondrocyte hypertrophy (MMP-13 and Runx-2 upregulation, and calcium deposition) and apoptosis/anoikis following concurrence of matrix degrading activity, and inhibition of matrix synthesis which also involved the induction of CB(-2,3). These phenomena proved to be dependent on the co-receptor role of sulfated glycosaminoglycans (sGAG) and the activation of p38 MAPK, since they were abrogated either by preincubation with soluble chondroitin-4 sulfate or p38 MAPK inhibitors. The co-receptor role of sGAG was further demonstrated by colocalization experiments of these molecules with GROalpha in the stimulated micromasses. These findings suggest that extracellular matrix exerts a regulatory role in chondrocytes differentiation, and that meaningful investigation of the effects of chemokines on chondrocyte biology requires culture conditions respectful of both the differentiated status of the chondrocytes and of their three-dimensional interaction with the extracellular matrix.

Regulation of human cathepsin B by alternative mRNA splicing: homeostasis, fatal errors and cell death

One of the control mechanisms of cathepsin B biosynthesis and trafficking operates through alternative splicing of pre-mRNA. An mRNA lacking exon 2 is more efficiently translated than that containing all exons, and may be responsible for elevated biosynthesis and enzyme routing to the extracellular space, with critical consequences for connective tissue integrity in pathologies such as cancer and arthritis. mRNA missing exons 2 and 3 encodes a truncated procathepsin B form that is targeted to mitochondria. This enzyme variant is catalytically inactive because it cannot properly fold. However, it provokes a cascade of events, which result first in morphological changes in intracellular organelles and the nucleus, finally leading to cell death.

Complex splicing control of the human Thrombopoietin gene by intronic G runs

The human thrombopoietin (THPO) gene displays a series of alternative splicing events that provide valuable models for studying splicing mechanisms. The THPO region spanning exon 1–4 presents both alternative splicing of exon 2 and partial intron 2 (IVS2) retention following the activation of a cryptic 3′ splice site 85 nt upstream of the authentic acceptor site. IVS2 is particularly rich in stretches of 3–5 guanosines (namely, G1–G10) and we have characterized the role of these elements in the processing of this intron. In vivo studies show that runs G7–G10 work in a combinatorial way to control the selection of the proper 3′ splice site. In particular, the G7 element behaves as the splicing hub of intron 2 and its interaction with hnRNP H1 is critical for the splicing process. Removal of hnRNP H1 by RNA interference promoted the usage of the cryptic 3′ splice site so providing functional evidence that this factor is involved in the selection of the authentic 3′ splice site of THPO IVS2.

Cysteine cathepsins: multifunctional enzymes in cancer

Cysteine cathepsins are highly upregulated in a wide variety of cancers by mechanisms ranging from gene amplification to post-transcriptional modification. Their localization within intracellular lysosomes often changes during neoplastic progression, resulting in secretion of both inactive and active forms and association with binding partners on the tumour cell surface. Secreted, cell-surface and intracellular cysteine cathepsins function in proteolytic pathways that increase neoplastic progression. Direct proof for causal roles in tumour growth, migration, invasion, angiogenesis and metastasis has been shown by downregulating or ablating the expression of individual cysteine cathepsins in tumour cells and in transgenic mouse models of human cancer.

Placental cathepsin M is alternatively spliced and exclusively expressed in the spongiotrophoblast layer

Cathepsin M and cathepsin-3 are cysteine peptidases expressed exclusively in the murine placenta. Their expression increases continuously from 11.5 dpc until the end of gestation. The cathepsin M gene consists of 8 exons and 7 introns covering 6 kb of genomic DNA on mouse chromosome 13. Multiple variants of CTSM were identified which display alternative splicing of exon 2 or exon 7. Alternative splicing of exon 2 does not affect the translated region of CTSM whereas aberrant splicing of exon 7 will results in enzymatically inactive versions of CTSM which still might retain inhibitory activity towards cysteine peptidases. Besides two defined major transcription start sites the putative promoter region comprises of a TATA-box and a relatively low (41%) G+C content reflecting its highly specific spatial and temporal expression pattern. Similar features are found within the promoter region of CTS3 which is highly homologous to CTSM. Both cathepsin M and -3 expression are confined to the spongiotrophoblast layer of the mouse placenta an expression pattern which is unique among cysteine peptidases located within the cluster of cathepsin J-like peptidases on mouse chromosome 13.

Cathepsin B promotes both motility and invasiveness of oral carcinoma cells

We previously demonstrated that overexpression of cathepsin B (CB) protease in oral squamous cell carcinomas correlated positively with advanced tumor stage and poor histologic malignancy grade. Here we examined whether CB contributes to the invasiveness of oral carcinoma cells. For RNA-mediated inhibition, two ribozymes that target CB mRNA were designed and stably expressed in the oral squamous cell carcinoma cell line 1386Tu. Both ribozymes diminished expression of CB mRNA, protein, and activity, without affecting cathepsin D or beta-actin, as determined by quantitative real-time PCR, Western blots, and protease activity assays. Matrigel invasion assays showed that the invasiveness of the cells was significantly reduced by the expressed ribozymes and, surprisingly, the motilities of the ribozyme-transfected cells were also diminished. Our results document a direct role for CB in promoting oral cancer spread and invasion, and open the possibility of controlling oral carcinoma malignancy and metastasis by targeting CB with RNA inhibitor strategies.

Exon skipping of cathepsin B: mitochondrial targeting of a lysosomal peptidase provokes cell death

The alternatively spliced messenger RNA of the human cysteine peptidase cathepsin B missing exons 2 and 3 encodes a truncated form of the enzyme lacking the signal peptide and part of the inhibitory propeptide. This deletion results in a new N-terminal leader sequence characteristic of proteins predestined for transport into mitochondria. We determined enzyme targeting to intracellular organelles by transfecting HeLa cells with constructs containing segments of variable length of the N terminus of truncated cathepsin B fused to green fluorescent protein. Co-localization of the constructs with mitochondria and the endoplasmic reticulum was probed with specific markers. None of the chimeric products were found in the endoplasmic reticulum, showing that truncated cathepsin B is misrouted from its regular biosynthetic pathway and forced to enter the mitochondria instead of lysosomes as its final destination. The first 20 amino acids of the new N terminus were necessary and sufficient for mitochondrial targeting, but only cells expressing the complete truncated cathepsin B sequence died by nuclear fragmentation. This new and unexpected behavior draws attention to an additional extralysosomal role for a cysteine peptidase with several recognized important pathophysiological functions. Mitochondrial targeting of cathepsin B may have significant consequences on cell life in pathological or physiological situations characterized by excessive transcription of the cathepsin B message lacking exons 2 and 3, as observed for instance in osteoarthritic cartilage.