Regulation of collagenolytic protease secretion through c-Src in osteoclasts

Evolving cancer-niche interactions and therapeutic targets during bone metastasis

Many cancer types metastasize to bone. This propensity may be a product of genetic traits of the primary tumour in some cancers. Upon arrival, cancer cells establish interactions with various bone-resident cells during the process of colonization. These interactions, to a large degree, dictate cancer cell fates at multiple steps of the metastatic cascade, from single cells to overt metastases. The bone microenvironment may even influence cancer cells to subsequently spread to multiple other organs. Therefore, it is imperative to spatiotemporally delineate the evolving cancer-bone crosstalk during bone colonization. In this Review, we provide a summary of the bone microenvironment and its impact on bone metastasis. On the basis of the microscopic anatomy, we tentatively define a roadmap of the journey of cancer cells through bone relative to various microenvironment components, including the potential of bone to function as a launch pad for secondary metastasis. Finally, we examine common and distinct features of bone metastasis from various cancer types. Our goal is to stimulate future studies leading to the development of a broader scope of potent therapies.

The nonreceptor protein tyrosine kinase Src participates in every step of cancer-induced bone pain

Cancer-induced bone pain (CIBP) is a refractory form of pain that has a high incidence in advanced tumors. Src protein tyrosine kinase is mainly composed of six domains, with two states of automatic inhibition and activation. The modular domain allows Src to conveniently regulate by and communicate with a variety of proteins, directly or indirectly participate in each step of the CIBP process. Src is beneficial to the growth and proliferation of tumor cells, and it can promote the metastases of primary tumors to bone. In the microenvironment of bone metastasis, it mainly mediates bone resorption, activates related peripheral receptors to participate in the formation of pain signals, and may promote the generation of pathological sensory nerve fibers. In the process of pain signal transmission, it mainly mediates NMDAR and central glial cells to regulate pain signal intensity and central sensitization, but it is not limited to these two aspects. Both basic experimentation and clinical research have shown encouraging potential, providing new ideas and inspiration for the prevention and treatment of CIBP.

Osteoclast Signal Transduction During Bone Metastasis Formation

Osteoclasts are myeloid lineage-derived bone-resorbing cells of hematopoietic origin. They differentiate from myeloid precursors through a complex regulation process where the differentiation of preosteoclasts is followed by intercellular fusion to generate large multinucleated cells. Under physiological conditions, osteoclastogenesis is primarily directed by interactions between CSF-1R and macrophage colony-stimulating factor (M-CSF, CSF-1), receptor activator of nuclear factor NF-κB (RANK) and RANK ligand (RANKL), as well as adhesion receptors (e.g., integrins) and their ligands. Osteoclasts play a central role in physiological and pathological bone resorption and are also required for excessive bone loss during osteoporosis, inflammatory bone and joint diseases (such as rheumatoid arthritis) and cancer cell-induced osteolysis. Due to the major role of osteoclasts in these diseases the better understanding of their intracellular signaling pathways can lead to the identification of potential novel therapeutic targets. Non-receptor tyrosine kinases and lipid kinases play major roles in osteoclasts and small-molecule kinase inhibitors are emerging new therapeutics in diseases with pathological bone loss. During the last few years, we and others have shown that certain lipid (such as phosphoinositide 3-kinases PI3Kβ and PI3Kδ) and tyrosine (Src−family and Syk) kinases play a critical role in osteoclast differentiation and function in humans and mice. Some of these signaling pathways shows similarity to immunoreceptor-like receptor signaling and involves important other enzymes (e.g., PLCγ2) and adapter proteins (such as the ITAM−bearing adapters DAP12 and the Fc-receptor γ-chain). Here, we review recently identified osteoclast signaling pathways and their role in osteoclast differentiation and function as well as pathological bone loss associated with osteolytic tumors of the bone. A better understanding of osteoclast signaling may facilitate the design of novel and more efficient therapies for pathological bone resorption and osteolytic skeletal metastasis formation.

Src siRNA prevents corticosteroid-associated osteoporosis in a rabbit model

In an established steroid-associated osteonecrosis (SAON) rabbit model we found recently that blockage Src by siRNA could improve reconstructive repair of osteonecrosis via enhancing osteogenesis and inhibiting bone resorption. The current study investigated if blocking Src was able to prevent steroid-associated osteoporosis (SAOP) in the same SAON animal model. Rabbits were treated with pulsed lipopolysaccharide (LPS) and corticosteroid methylprednisolone (MPS). At 2, 4, and 6weeks after induction, Src siRNA, control siRNA and saline were intramedullary injected into proximal femur, respectively. Two fluorescent dyes xylenol orange and calcein green were injected before sacrificing the animals for in vivo labeling of the newly formed bone. At week 6 after induction, proximal femora of rabbits were dissected for micro-CT and histological analysis. Results showed significant bone loss in the metaphysis of femoral head in the control rabbits after SAON induction. Src siRNA treatment was able to prevent steroid-associate bone loss in trabecular bone and increase cortical bone thickness at femoral neck. Histomorphometry showed that Src siRNA increased the osteoblastic bone formation and decreased the eroded bone surfaces suggesting decreased osteoclastic bone resorption. This was the first study to report bone loss after SAON induction in rabbit model that could be prevented by knocking down Src by siRNA.

Cathepsin L targeting in cancer treatment

Proteolytic enzymes may serve as promising targets for novel therapeutic treatment strategies seeking to impede cancer progression and metastasis. One such enzyme is cathepsin L (CTSL), a lysosomal cysteine protease. CTSL upregulation, a common occurrence in a variety of human cancers, has been widely correlated with metastatic aggressiveness and poor patient prognosis. In addition, CTSL has been implicated to contribute to cancer-associated osteolysis, a debilitating morbidity affecting both life expectancy and the quality of life. In this review, we highlight the mechanisms by which CTSL contributes to tumor progression and dissemination and discuss the therapeutic utility of CTSL intervention strategies aimed at impeding metastatic progression and bone resorption.

Regulation of osteoclasts by membrane-derived lipid mediators

Osteoclasts are bone-resorbing cells of monocytic origin. An imbalance between bone formation and resorption can lead to osteoporosis or osteopetrosis. Osteoclastogenesis is triggered by RANKL- and IP3-induced Ca²⁺ influx followed by activation of NFATc1, a master transcription factor for osteoclastogenic gene regulation. During differentiation, osteoclasts undergo cytoskeletal remodeling to migrate and attach to the bone surface. Simultaneously, they fuse with each other to form multinucleated cells. These processes require PI3-kinase-dependent cytoskeletal protein activation to initiate cytoskeletal remodeling, resulting in the formation of circumferential podosomes and fusion-competent protrusions. In multinucleated osteoclasts, circumferential podosomes mature into stabilized actin rings, which enables the formation of a ruffled border where intensive membrane trafficking is executed. Membrane lipids, especially phosphoinositides, are key signaling molecules that regulate osteoclast morphology and act as second messengers and docking sites for multiple important effectors. We examine the critical roles of phosphoinositides in the signaling cascades that regulate osteoclast functions.

Membrane trafficking in osteoblasts and osteoclasts: new avenues for understanding and treating skeletal diseases

The endocytic and exocytic/secretory pathways are two major intracellular membrane trafficking routes that regulate numerous cellular functions in a variety of cell types. Osteoblasts and osteoclasts, two major bone cells responsible for bone remodeling and homeostasis, are no exceptions. During the past few years, emerging evidence has pinpointed a critical role for endocytic and secretory pathways in osteoblast and osteoclast differentiation and function. The endosomal membrane provides a platform to integrate bone tropic signals of hormones and growth factors in osteoblasts. In osteoclasts, endocytosis, followed by transcytosis, of degraded bone matrix promotes bone resorption. Secretory pathways, especially lysosome secretion, not only participate in bone matrix deposition by osteoblasts and degradation of mineralized bone matrix by osteoclasts; they may also be involved in the coupling of bone resorption and bone formation during bone remodeling. More importantly, mutations in genes encoding regulatory factors within the endocytic and secretory pathways have been identified as causes for bone diseases. Identification of the molecular mechanisms of these genes in bone cells may provide new therapeutic targets for skeletal disorders.

The heparanase system and tumor metastasis: is heparanase the seed and soil?

Tumor metastasis, the leading cause of cancer patients' death, is still insufficiently understood. While concepts and mechanisms of tumor metastasis are evolving, it is widely accepted that cancer metastasis is accompanied by orchestrated proteolytic activity executed by array of proteases. While matrix metalloproteinases (MMPs) attracted much attention, other proteases constitute the tumor milieu, of which a large family consists of cysteine proteases named cathepsins. Like MMPs, some cathepsins are often upregulated in cancer and, once secreted or localized to the cell surface, can degrade components of the extracellular matrix. In addition, cathepsin L is held responsible for processing and activation of heparanase, an endo-β-glucuronidase capable of cleaving heparan sulfate side chains of heparan sulfate proteoglycans, activity that is strongly implicated in cell dissemination associated with tumor metastasis, angiogenesis, and inflammation. In this review, we discuss recent progress in heparanase research focusing on heparanase-related molecules namely, cathepsin L and heparanase 2 (Hpa2), a heparanase homolog.

Screening of protein kinase inhibitors identifies PKC inhibitors as inhibitors of osteoclastic acid secretion and bone resorption

Background

Bone resorption is initiated by osteoclastic acidification of the resorption lacunae. This process is mediated by secretion of protons through the V-ATPase and chloride through the chloride antiporter ClC-7. To shed light on the intracellular signalling controlling extracellular acidification, we screened a protein kinase inhibitor library in human osteoclasts.

Methods

Human osteoclasts were generated from CD14+ monocytes. The effect of different kinase inhibitors on lysosomal acidification in human osteoclasts was investigated using acridine orange for different incubation times (45 minutes, 4 and 24 hours). The inhibitors were tested in an acid influx assay using microsomes isolated from human osteoclasts. Bone resorption by human osteoclasts on bone slices was measured by calcium release. Cell viability was measured using AlamarBlue.

Results

Of the 51 compounds investigated only few inhibitors were positive in both acidification and resorption assays. Rottlerin, GF109203X, Hypericin and Ro31-8220 inhibited acid influx in microsomes and bone resorption, while Sphingosine and Palmitoyl-DL-carnitine-Cl showed low levels of inhibition. Rottlerin inhibited lysosomal acidification in human osteoclasts potently.

Conclusions

In conclusion, a group of inhibitors all indicated to inhibit PKC reduced acidification in human osteoclasts, and thereby bone resorption, indicating that acid secretion by osteoclasts may be specifically regulated by PKC in osteoclasts.

Cathepsin L in metastatic bone disease: therapeutic implications

Cathepsin L is a lysosomal cysteine proteinase primarily devoted to the metabolic turnover of intracellular proteins. However, accumulating evidence suggests that this endopeptidase might also be implicated in the regulation of other important biological functions, including bone resorption in normal and pathological conditions. These findings support the concept that cathepsin L, in concert with other proteolytic enzymes involved in bone remodeling processes, could contribute to facilitate bone metastasis formation. In support of this hypothesis, recent studies indicate that cathepsin L can foster this process by triggering multiple mechanisms which, in part, differ from those of the major cysteine proteinase of osteoclasts, namely cathepsin K. Therefore, cathepsin L can be regarded as an additional target in the treatment of patients with metastatic bone disease. This review discusses the clinical and therapeutic implications related to these findings.

Biochemical markers in preclinical models of osteoporosis

Although several treatments for osteoporosis exist, further understanding of the mode of action of current treatments, as well as development of novel treatments, are of interest. Thus, preclinical models of osteoporosis are very useful, as they provide the possibility for gaining knowledge about the cellular mechanisms underlying the disease and for studying pharmaceutical prevention or intervention of the disease in simple and strictly controlled systems. In this review, we present a comprehensive collection of studies using biochemical markers of bone turnover for investigation of preclinical models of osteoporosis. These range from pure and simple in vitro systems, such as osteoclast cultures, to ex vivo models, such as cultures of embryonic murine tibiae and, finally, to in vivo models, such as ovariectomy and orchidectomy of rats. We discuss the relevance of the markers in the individual models, and compare their responses to those observed using 'golden standard' methods.

Role of endocrine-immune dysregulation in osteoporosis, sarcopenia, frailty and fracture risk

Osteoporosis, a key predictor of hip fractures can be treated using a variety of safe and effective interventions. Nevertheless, optimally effective strategies for the prevention of hip fractures must also incorporate efforts to address a broad range of other potentially reversible factors. Hyperthyroidism, anticonvulsants, caffeine and smoking may decrease bone mass and increase fracture risk at any age. In older individuals it is important to also consider additional risk factors, including long-acting benzodiazepines, poor vision and sarcopenia. The presence of sarcopenia, an age-related decline in muscle bulk and quality enhances the risk of frailty and possibly also hip fracture, particularly if associated with diminished functional mobility, lower quadriceps strength and poor balance or body sway. In this review we examine evidence which indicates the presence of endocrine-immune dysregulation in both osteoporosis and sarcopenia. Post-menopausal declines in serum estrogen and androgen levels contribute to increases in local bone levels of cytoclastic cytokines, followed by increased osteoclastogenesis and bone loss. Similarly, the presence of decreased gonadal hormones and IGF-1, combined with unusually high peripheral levels of cytokines, inflammatory mediators and coagulation markers all enhance the risk of sarcopenia and frailty. We propose that a translational research approach which emphasizes common pathophysiologic mechanisms in osteoporosis and sarcopenia could accelerate the speed of discovery of effective strategies for both frailty and hip fracture prevention.

Proteolytic Excision of a Repressive Loop Domain in Tartrate-resistant Acid Phosphatase by Cathepsin K in Osteoclasts

Tartrate-resistant acid phosphatase (TRAP) is a metallophosphoesterase participating in osteoclast-mediated bone turnover. Activation of TRAP is associated with the redox state of the di-iron metal center as well as with limited proteolytic cleavage in an exposed loop domain. The cysteine proteinases cathepsin B, L, K, and S as well as the matrix metalloproteinase-2, -9, -13, and -14 are expressed by osteoclasts and/or other bone cells and have been implicated in the turnover of bone and cartilage. To identify proteases that could act as activators of TRAP in bone, we report here that cathepsins K and L, in contrast to the matrix metalloproteinases, efficiently cleaved and activated recombinant TRAP in vitro. Activation of TRAP by cathepsin K/L was because of increases in catalytic activity, substrate affinity, and sensitivity to reductants. Processing by cathepsin K occurred sequentially by an initial excision of the loop peptide Gly(143)-Gly(160) followed by the removal of a Val(161)-Ala(162) dipeptide at the N terminus of the C-terminal 16-kDa TRAP subunit. Cathepsin L initially released a shorter Gln(151)-Gly(160) peptide and completed processing at Ser(145) or Gly(143) at the C terminus of the N-terminal 23-kDa TRAP subunit and at Arg(163) at the N terminus of the C-terminal 16-kDa TRAP subunit. Mutation of Ser(145) to Ala partly mimicked the effect of proteolysis on catalytic activity, identifying Ser(145) as well as Asp(146) (Funhoff, E. G., Ljusberg, J., Wang, Y., Andersson, G., and Averill, B. A. (2001) Biochemistry 40, 11614-11622) as repressive amino acids of the loop region to maintain the TRAP enzyme in a catalytically latent state. The C-terminal sequence of TRAP isolated from rat bone was consistent with cathepsin K-mediated processing in vivo. Moreover, cathepsin K, but not cathepsin L, co-localized with TRAP in osteoclast-resorptive compartments, supporting a role for cathepsin K in the extracellular processing of monomeric TRAP in the resorption lacuna.

Expression and Localization of Cathepsin K In Adult Rat Sertoli Cells1

The cathepsins are a family of cysteine proteases that have been broadly implicated in proteolytic processes during cell growth, cell development, and normal adult cellular function. Cathepsin L is a major secretory product of rat and mouse Sertoli cells, the absence of which in furless mice is associated with atrophy of some seminiferous tubules. However, furless mice produce viable sperm, suggesting the possibility that other members of the cathepsin family of proteases may complement cathepsin L action in the testis. Our objective herein was to begin to test this hypothesis. To this end, we first utilized cDNA microarray technology to identify the members of the cathepsin gene family expressed by freshly isolated adult rat Sertoli cells. This approach, complemented by Northern blot analyses, showed that in addition to cathepsin L, cathepsin K is highly and specifically expressed in Sertoli cells. As is also true of cathepsin L, cathepsin K mRNA was found to be expressed by Sertoli cells at specific stages of the cycle of the seminiferous epithelium, with maximal expression at stages VI-VII. The use of immunocytochemical methods revealed that cathepsin K protein localizes to the cytoplasm of Sertoli cells at stages VI-VIII, to small punctuate lysosomes at stages I-VIII and XIII-XIV, and to early and late residual bodies at stages IX-XII. This localization was found to be similar to that of cathepsin L. The similarity in the expression and localization of cathepsin K and cathepsin L suggest that the two proteases may have similar functions. If true, this might explain the fertility of furless mice. Further, the results suggest that cathepsin K, in both its secreted and lysosomal forms, may play a role in the degradation of Sertoli cell residual bodies.

Pyrazolo Pyrimidine-Type Inhibitors of Src Family Tyrosine Kinases Promote Ovarian Steroid-Induced Differentiation of Human Endometrial Stromal Cells In Vitro1

Reversible protein tyrosine phosphorylation, coordinately controlled by protein tyrosine kinases and phosphatases, is a critical element in signal transduction pathways regulating a wide variety of biological processes, including cell growth, differentiation, and tumorigenesis. We have previously reported that c-Src belonging to the Src family tyrosine kinase (SFK) becomes dephosphorylated at tyrosine 530 (Y530) and thereby activated during progestin-induced differentiation of human endometrial stromal cells (i.e., decidualization). In this study, to elucidate the role of decidual c-Src activation, we examined whether 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1) and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), both potent and selective SFK inhibitors, affected the ovarian steroid-induced decidualization in vitro. Unexpectedly, PP1 paradoxically increased the kinase activity of decidual c-Src together with dephosphorylation of Y530 in the presence of ovarian steroids. Concomitantly, PP1 enhanced morphological and functional decidualization, as determined by induction of decidualization markers, such as insulin-like growth factor binding protein-1 and prolactin. PP2 also advanced decidualization along with up-regulation of the active form of c-Src whose Y-530 was dephosphorylated. In contrast to PP1 and PP2, herbimycin A, a tyrosine kinase inhibitor with less specificity for SFKs, showed little enhancing effect on the expression of both IGFBP-1 and active c-Src. These results suggest that SFKs, including c-Src, may play a significant role in stromal cell differentiation, providing a clue for a possible therapeutic strategy to modulate endometrial function by targeting signaling pathway(s) involving SFKs.

Structure-based design of specific cathepsin inhibitors and their application to protection of bone metastases of cancer cells

We report the antihypercalcemic and antimetastatic effects of CLIK-148 in vivo, which is a specific inhibitor of cathepsin L. The decalcification during bone absorption is followed by the degradation of type-1 collagen by osteoclastic cathepsins. Tumor-bearing osteoclasts or TNF-alpha-activated osteoclasts secrete large amounts of cysteine proteases, especially procathepsin L, which powerfully degrade type-1 collagen leading to tumor-associated bone absorption and release of bone calcium. The bone pit formations in vitro, which are caused by osteoclasts derived from human bone marrow cells activated by RANKL and M-CSF and also by mice osteoclasts activated by TNF-alpha, are significantly prevented by CLIK-148 treatment. We evaluated the in vivo inhibitory effect of malignant hypercalcemia induced by LJC-1 human mandibular cancer inoculation by CLIK-148 treatment, and the CLIK-148 treatment significantly protected against the tumor-induced hypercalcemia. On the protection of bone metastasis of colon 26 PMF-15 implanted to mouse calvaria, CLIK-148 treatment significantly inhibited calvaria bone absorption (direct metastasis). The CLIK-148 treatment also reduced distant bone metastasis to the femur and tibia of melanoma A375 tumors implanted into the left ventricle of the heart.

The Src signaling pathway regulates osteoclast lysosomal enzyme secretion and is rapidly modulated by estrogen

To investigate the role of the pp60src signaling pathway in osteoclast activity, we have used dominant negative pp60src, c-ras, and c-raf expression vectors to individually disrupt their functions in osteoclasts. Osteoclasts were transiently transfected and secretions of cathepsin B/K and tartrate-resistant acid phosphatase (TRAP) were monitored. Expression of these constructs increased secretion of lysosomal enzymes. In contrast, constitutively active pp60src expression caused decreased lysosomal enzyme secretion. Osteoclasts respond to 17-beta estradiol (17betaE2) treatment with decreased lysosomal enzyme secretion. Therefore, we investigated the effects of E2 on pp60src kinase activity and observed an E2 time- and dose-dependent decrease in cytoskeletal membrane-associated pp60src tyrosine kinase activity. We have shown that estrogen decreases lysosomal enzyme gene expression and secretion; so we have examined the effects of the expression constructs on estrogen regulation of enzyme secretion. Constitutively active pp60src blocked E2 effects on secretion whereas expression of dominant negative pp60src, c-Ras, or c-Raf enhanced E2 effects. These data support that the kinase domain of cytoskeletal-associated pp60src is likely to be involved in the regulation of lysosomal enzyme secretion.

Significance of cathepsin B accumulation in synovial fluid of rheumatoid arthritis

We measured and compared the activities of various kinds of proteinases, such as cysteine, serine, aspartic, and metalloproteinases, in synovial fluids of 16 patients with rheumatoid arthritis (RA) and 18 patients with osteoarthritis (OA). More than 19-fold higher activity of cathepsin B and about 6-fold higher activity of prolylendopeptidase, compared to those of OA, were accumulated in RA fluid. Moreover, levels of cathepsins B and S using the corresponding sandwich enzyme immunoassays were statistically higher in RA fluid than those in OA. Significant amounts of 41-kDa and 35-kDa procathepsin L were detected in RA fluid using gelatin zymography, while 41-kDa enzyme alone was detected in OA. Cathepsin B in RA fluid could degrade collagen, and this degradation was suppressed by the addition of CA-074, a specific inhibitor of cathepsin B. Therefore, cathepsin B may participate in joint destruction of RA, and its inhibitor may be effective for RA care.

Selected glimpses into the activation and function of Src kinase

Since the discovery of the v-src and c-src genes and their products, much progress has been made in the elucidation of the structure, regulation, localization, and function of the Src protein. Src is a non-receptor protein tyrosine kinase that transduces signals that are involved in the control of a variety of cellular processes such as proliferation, differentiation, motility, and adhesion. Src is normally maintained in an inactive state, but can be activated transiently during cellular events such as mitosis, or constitutively by abnormal events such as mutation (i.e. v-Src and some human cancers). Activation of Src occurs as a result of disruption of the negative regulatory processes that normally suppress Src activity, and understanding the various mechanisms behind Src activation has been a target of intense study. Src associates with cellular membranes, in particular the plasma membrane, and endosomal membranes. Studies indicate that the different subcellular localizations of Src could be important for the regulation of specific cellular processes such as mitogenesis, cytoskeletal organization, and/or membrane trafficking. This review will discuss the history behind the discovery and initial characterization of Src and the regulatory mechanisms of Src activation, in particular, regulation by modification of the carboxy-terminal regulatory tyrosine by phosphatases and kinases. Its focus will then turn to the different subcellular localizations of Src and the possible roles of nuclear and perinuclear targets of Src. Finally, a brief section will review some of our present knowledge regarding Src involvement in human cancers.