Myoepithelial-specific CD44 shedding contributes to the anti-invasive and antiangiogenic phenotype of myoepithelial cells

Proteases and sFlt-1 release in the human placenta

Increased placental release of soluble VEGF receptor-1 (sFlt-1) is believed to play an important role in the pathogenesis of preeclampsia (PE). Although the reason for increased placental sFlt-1 release in PE is unknown, proteolytic effect has been proposed as one of the mechanisms that mediate sFlt-1 release in the placenta. In this study, using various protease inhibitors, we tested the possible role of proteases in sFlt-1 release by human placenta. Villous explants from normal term placentas were incubated with various protease inhibitors including serine protease inhibitors (PMSF, aprotini, and specific chymotrypsin inhibitor (CI)), cysteine protease inhibitor E-64, metalloendopeptidase inhibitor PAD, and universal metalloprotease (ADAM) inhibitor PTM. Culture medium was collected and measured for sFlt-1 by ELISA. Our results showed that villous tissue treated with CI and PTM produced significantly less sFlt-1 than those of controls. PMSF, aprotini, E-64, and PAD had no effect on sFlt-1 release. We further examined chymotrypsin-like protease/chymase and ADAM10 expressions in tissue sections from normal and PE placentas by immunohistochemistry. We found that immunostaining for chymase and ADAM10 was significantly increased in the layer of syncytiotrophoblasts in PE placentas compared to normal placentas. These results suggest chymotrypsin-like serine protease and ADAM10, but not cysteine protease and metalloendopeptidase, may play a role in inducing sFlt-1 release in PE placentas.

Role of myoepithelial cells in breast tumor progression

Myoepithelial cells form a semi-continuous protective sheet separating the human breast epithelium and the surrounding stroma. They suppress stromal invasion of tumor cells by the secretion of various anti-angiogenic and anti-invasive factors. The disruption of this cell layer results in the release of the growth factors, angiogenic factors, and reactive oxygen species causing an alteration in the microenvironment. This helps in the proliferation of surrounding cells and increases the invasiveness of tumor cells. Two theories are proposed for the mechanism of tumor epithelial cells progression from in situ to invasive stage. According to the first theory, tumor cell invasion is triggered by the overproduction of proteolytic enzymes by myoepithelial cells and surrounding tumor cells. The second theory states that tumor invasion is a multistep process, the interactions between damaged myoepithelial cells and the immunoreactive cells trigger the release of basement membrane degrading enzymes causing tumor progression. Further studies in understanding of molecular mechanism of myoepithelial cell functions in tumor suppression may lead to the identification of novel therapeutic targets for breast cancer.

Glycosylated VCAM-1 isoforms revealed in 2D western blots of HUVECs treated with tumoral soluble factors of breast cancer cells

Background

Several common aspects of endothelial phenotype, such as the expression of cell adhesion molecules, are shared between metastasis and inflammation. Here, we analyzed VCAM-1 variants as biological markers of these two types of endothelial cell activation. With the combination of 2-DE and western blot techniques and the aid of tunicamycin, we analyzed N-glycosylation variants of VCAM-1 in primary human endothelial cells stimulated with either TNF or tumoral soluble factors (TSF's) derived from the human breast cancer cell line ZR75.30.

Results

Treatments induced a pro-adhesive endothelial phenotype. 2D western blots analysis of cells subjected to both treatments revealed the expression of the two known VCAM-1 isoforms and of previously unknown isoforms. In particular TSFZR75.30 induced an isoform with a relative molecular mass (Mr) and isoelectric point (pI) of 75-77 kDa and 5.0, respectively.

Conclusion

The unknown isoforms of VCAM-1 that were found to be overexpressed after treatment with TSF's compared with TNF, could serve as biomarkers to discriminate between inflammation and metastasis. 2D western blots revealed three new VCAM-1 isoforms expressed in primary human endothelial cells in response to TSF stimulation. Each of these isoforms varies in Mr and pI and could be the result of differential glycosylation states.

Proteomics Demonstration That Normal Breast Epithelial Cells Can Induce Apoptosis of Breast Cancer Cells through Insulin-like Growth Factor-binding Protein-3 and Maspin

Normal breast epithelial cells are known to exert an apoptotic effect on breast cancer cells, resulting in a potential paracrine inhibition of breast tumor development. In this study we purified and characterized the apoptosis-inducing factors secreted by normal breast epithelial cells. Conditioned medium was concentrated by ultrafiltration and separated on reverse phase Sep-Pak C18 and HPLC. The proapoptotic activity of eluted fractions was tested on MCF-7 breast cancer cells, and nano-LC-nano-ESI-MS/MS allowed the identification of insulin-like growth factor-binding protein-3 (IGFBP-3) and maspin as the proapoptotic factors produced by normal breast epithelial cells. Western blot analysis of conditioned media confirmed the specific secretion of IGFBP-3 and maspin by normal cells but not by breast cancer cells. Immunodepletion of IGFBP-3 and maspin completely abolished the normal cell-induced apoptosis of cancer cells, and recombinant proteins reproduced the effect of normal cell-conditioned medium on apoptosis of breast cancer cells. Together our results indicated that normal breast epithelial cells can induce apoptosis of breast cancer cells through IGFBP-3 and maspin. These findings provide a molecular hypothesis for the long observed inhibitory effect of normal surrounding cells on breast cancer development.

UDP-N-acetyl-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase-6 as a new immunohistochemical breast cancer marker

Mucin O-glycosylation is characterized in cancer by aberrant expression of immature carbohydrate structures (Tn, T, and sialyl-Tn antigens). The UDP-N-acetyl-D-galactosamine: polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-T) family enzymes regulate the initial steps of mucin O-glycosylation and could be responsible for the altered glycosylation observed in cancer. Considering that we recently found the ppGalNAc-T6 mRNA expressed in breast carcinomas, we produced a highly specific monoclonal antibody (MAb T6.3) to assess the expression profile of ppGalNAc-T6 protein product in breast tissues. The expression of ppGalNAc-T6 by breast carcinoma cells was confirmed on MCF-7 and T47D cell lines. In formalin-fixed tissues, ppGalNAc-T6 expression was observed in 60/74 (81%) breast cancers, 21/23 (91.3%) adjacent ductal carcinoma in situ (DCIS), 4/20 benign breast lesions (2/2 sclerosing adenosis and 2/13 fibroadenoma), and in 0/5 normal breast samples. We observed a statistically significant association of ppGalNAc-T6 expression with T1 tumor stage. This fact, as well as the observation that ppGalNAc-T6 was strongly expressed in sclerosing adenosis and in most DCIS, suggests that ppGalNAc-T6 expression could be an early event during human breast carcinogenesis. Considering that an abnormal O-glycosylation greatly contributes to the phenotype and biology of breast cancer cells, ppGalNAc-T6 expression could provide new insights about breast cancer glycobiology.

Expression of CD44s, CD44v3 and CD44v6 in benign and malignant breast lesions: correlation and colocalization with hyaluronan

AIMS

To examine the expression of CD 44 s, CD 44 v 3 and CD 44 v 6 in breast lesions, and to correlate it with the expression of hyaluronan (HA).

METHODS AND RESULTS

CD 44 expression was studied in 75 breast tissue samples, consisting of benign, premalignant and malignant breast lesions, using immunohistochemistry. CD 44 s, but not CD 44 v 3 or CD 44 v 6, was found in the stromal cells, and it was similar in benign and malignant tumours. In benign lesions CD 4 v 6 was detected in 20-30% of the ductal epithelial cells, while C 44 v 3 and CD 44 s were not expressed. CD 44 s, CD 44 v 3 and CD 44 v 6 were all up-regulated in the in situ carcinomas and invasive carcinomas. The level of CD 44 expression in carcinoma cells did not correlate with the type or differentiation of the tumours. CD 44 and HA expression levels were not closely linked in the benign or malignant breast lesions, because HA was overexpressed later in breast cancer progression than CD 44. However, in breast carcinomas CD 44 and HA positivity was often found in the same areas of the sections, and the dual staining confirmed actual colocalization of CD 44 s and HA in the same cells.

CONCLUSIONS

CD 44 s, CD 44 v 3 and CD 44 v 6 are up-regulated earlier than HA in breast carcinoma progression, and in later stages they often colocalize with cell surface HA.

A proteomic approach to characterize protein shedding

Shedding (i.e. proteolysis of ectodomains of membrane proteins) plays an important pathophysiological role. In order to study the feasibility of identifying shed proteins, we analyzed serum-free media of human mammary epithelial cells by mass spectrometry following induction of shedding by the phorbol ester, 4 beta-phorbol 12-myristate 13-acetate (PMA). Different means of sample preparation, including biotinylation of cell surface proteins, isolation of glycosylated proteins, and preparation of crude protein fractions, were carried out to develop the optimal method of sample processing. The collected proteins were digested with trypsin and analyzed by reversed-phase capillary liquid chromatography interfaced to an ion-trap mass spectrometer. The resulting peptide spectra were interpreted using the program SEQUEST. Analyzing the sample containing the crude protein mixture without chemical modification or separation resulted in the greatest number of identifications, including putatively shed proteins. Overall, 45 membrane-associated proteins were identified including 22 that contain at least one transmembrane domain and 23 that indirectly associate with the extracellular surface of the plasma membrane. Of the 22 transmembrane proteins, 18 were identified by extracellular peptides providing strong evidence they originate from regulated proteolysis or shedding processes. We combined results from the different experiments and used a peptide count method to estimate changes in protein abundance. Using this approach, we identified two proteins, syndecan-4 and hepatoma-derived growth factor, whose abundances increased in media of cells treated with PMA. We also detected proteins whose abundances decreased after PMA treatment such as 78 kDa glucose-regulated protein and lactate dehydrogenase A. Further analysis using immunoblotting validated the abundance changes for syndecan-4 and 78 kDa glucose-regulated protein as a result of PMA treatment. These results demonstrate that tandem mass spectrometry can be used to identify shed proteins and to estimate changes in protein abundance.

Enhanced expression of CD44 variants in human atheroma and abdominal aortic aneurysm: possible role for a feedback loop in endothelial cells

CD44, a polymorphic hyaluronate receptor, may participate in chronic inflammation. We hypothesized that CD44 variants contribute to the development of arterial diseases. CD44 levels vary in normal and diseased arterial tissues in the following order: unaffected arteries < fibrous plaques < or = abdominal aortic aneurysm < atheromatous plaques; and correlate with macrophage content. Furthermore, plaque microvessels express CD44, and anti-CD44v3 or anti-CD44v6 treatment reduces endothelial cell proliferation but not apoptosis in vitro, suggesting functionality of these receptors. Endothelial cells express CD44H and CD44v6 after exposure to interleukin-1beta and tumor necrosis factor-alpha. Macrophages, a major source of abundant CD44 in vitro, express not only CD44H but also variants CD44v4/5, CD44v6, and CD44v7/8, isoforms distinctively regulated by proinflammatory cytokines. Several proinflammatory cytokines induce shedding of CD44 from the surface of macrophages and endothelial cells. Soluble CD44 stimulates the expression and release of interleukin-1beta from endothelial cells, suggesting a positive feedback loop of this cytokine. By demonstrating augmented expression of CD44 and variants within human atheroma and in abdominal aortic aneurysm as well as the vascular cell release of sCD44, a process regulated by proinflammatory cytokines, this study provides new insights on the functions of CD44 in arterial diseases.

Molecular characterization of the tumor microenvironment in breast cancer

Here we describe the comprehensive gene expression profiles of each cell type composing normal breast tissue and in situ and invasive breast carcinomas using serial analysis of gene expression. Based on these data, we determined that extensive gene expression changes occur in all cell types during cancer progression and that a significant fraction of altered genes encode secreted proteins and receptors. Despite the dramatic gene expression changes in all cell types, genetic alterations were detected only in cancer epithelial cells. The CXCL14 and CXCL12 chemokines overexpressed in tumor myoepithelial cells and myofibroblasts, respectively, bind to receptors on epithelial cells and enhance their proliferation, migration, and invasion. Thus, chemokines may play a role in breast tumorigenesis by acting as paracrine factors.

Primary breast myoepithelial cells exert an invasion-suppressor effect on breast cancer cells via paracrine down-regulation of MMP expression in fibroblasts and tumour cells

In normal breast and ductal carcinoma in situ, myoepithelial cells form an incomplete layer separating the epithelial compartment from the stromal environment. Transition to invasive disease is marked by penetration of the myoepithelial-basement membrane (BM) interface. One mechanism involved in tumour invasion is breakdown of extracellular matrices by matrix metalloproteinases (MMPs). It was hypothesized that myoepithelial cells may modulate tumour invasion by controlling MMP gene expression, both in tumour cells and in peri-ductal fibroblasts. To investigate this, myoepithelial cells from normal breast were purified and characterized and their effect on tumour cell invasive potential was assessed. The effect on MMP gene expression of breast cancer cells cultured alone or in combination with primary normal breast fibroblasts was also analysed using RT-PCR with ELISA quantitation, with zymographic analysis to measure enzyme activity. Normal breast myoepithelial cells significantly reduced invasion by the breast cancer cell lines MCF-7, T47D, MDA-MB 231, and MDA-MB 468 when they were cultured alone or in the presence of a fibroblast population. Reduced invasion was associated with changes in MMP gene expression. In those tumour cells expressing MMP, there was a significant down-regulation of MMP-2 (MDA-MB 468, p<0.001), MMP-9 (MDA-MB 231, p=0.05; MDA-MB 468, p<0.001), and MT1-MMP (p<0.001 for both MDA-MB 231 and MDA-MB 468). Myoepithelial cells also caused a significant decrease in MMP gene expression in co-cultured fibroblasts. Furthermore, this was associated with reduced gelatinolytic activity as identified by zymography. This study demonstrates for the first time that primary myoepithelial cells from normal breast reduce breast cancer cell invasion and that this is mediated via modulation of both tumour cell and fibroblast function. This emphasizes the importance of the myoepithelial cell in controlling the breast microenvironment and focuses on the potential significance of the loss of this population with disease progression.

CD44 in cancer

CD44 is a multistructural and multifunctional cell surface molecule involved in cell proliferation, cell differentiation, cell migration, angiogenesis, presentation of cytokines, chemokines, and growth factors to the corresponding receptors, and docking of proteases at the cell membrane, as well as in signaling for cell survival. All these biological properties are essential to the physiological activities of normal cells, but they are also associated with the pathologic activities of cancer cells. Experiments in animals have shown that targeting of CD44 by antibodies, antisense,and CD44-soluble proteins markedly reduces the malignant activities of various neoplasms, stressing the therapeutic potential of anti-CD44 agents. Furthermore, because alternative splicing and posttranslational modifications generate many different CD44 sequences, including, perhaps, tumor-specific sequences, the production of anti-CD44 tumor-specific agents may be a realistic therapeutic approach. However, in many cancers (renal cancer and non-Hodgkin's lymphomas are exceptions), a high level of CD44 expression is not always associated with an unfavorable outcome. On the contrary, in some neoplams CD44 upregulation is associated with a favorable outcome. Even worse, in many cases different research grows analyzing the same neoplastic disease reached contradictory conclusions regarding the correlation between CD44 expression and disease prognosis, possibly due to differences in methodology. These problems must be resolved before applying anti-CD44 therapy to human cancers.

CD44 directs membrane-type 1 matrix metalloproteinase to lamellipodia by associating with its hemopexin-like domain

Membrane-type 1 matrix metalloproteinase (MT1- MMP) localizes at the front of migrating cells and degrades the extracellular matrix barrier during cancer invasion. However, it is poorly understood how the polarized distribution of MT1-MMP at the migration front is regulated. Here, we demonstrate that MT1-MMP forms a complex with CD44H via the hemopexin-like (PEX) domain. A mutant MT1-MMP lacking the PEX domain failed to bind CD44H and did not localize at the lamellipodia. The cytoplasmic tail of CD44H, which comprises interfaces that associate with the actin cytoskeleton, was important for its localization at lamellipodia. Overexpression of a CD44H mutant lacking the cytoplasmic tail also prevented MT1-MMP from localizing at the lamellipodia. Modulation of F-actin with cytochalasin D revealed that both CD44H and MT1-MMP co-localize closely with the actin cytoskeleton, dependent on the cytoplasmic tail of CD44H. Thus, CD44H appears to act as a linker that connects MT1-MMP to the actin cytoskeleton and to play a role in directing MT1-MMP to the migration front. The PEX domain of MT1-MMP was indispensable in promoting cell migration and CD44H shedding.

Mast cell chymase modifies cell-matrix interactions and inhibits mitogen-induced proliferation of human airway smooth muscle cells

The hallmarks of chronic, severe asthma include prominent airway inflammation and airway smooth muscle (ASM) hypertrophy and hyperplasia. One of the factors that contribute to the injury and repair process within the airway is activation of proteases and turnover of extracellular matrix components. Mast cells, which are present in increased numbers in the asthmatic airway, are a rich source of the neutral protease chymase, which can degrade several basement membrane components. Recent data suggest that proteases also play a critical role in regulating the expression of CD44, the primary receptor for the matrix glycosaminoglycan hyaluronan. In this study we investigated the effects of chymase treatment on human ASM cell function. We found that chymase degraded the smooth muscle cell pericellular matrix. This was accompanied by an increased release of fibronectin and soluble CD44, but not soluble ICAM-1 or soluble hyaluronan, into the conditioned medium. In addition, chymase inhibited T cell adhesion to ASM and dramatically reduced epidermal growth factor-induced smooth muscle cell proliferation. These data suggest that the local release of mast cell chymase may have profound effects on ASM cell function and airway remodeling.

Normal breast epithelial cells induce p53-dependent apoptosis and p53-independent cell cycle arrest of breast cancer cells

Cancer development depends not only on the nature of cancerous cells themselves, but also on the regulatory effects of various normal cells. The present study was performed to investigate the effect of normal breast epithelial cells (NBEC) on the growth of breast cancer cells under various conditions. We demonstrated that NBEC-conditioned medium (NBEC-CM) inhibited growth of breast cancer cell lines in monolayer culture and three-dimensional collagen gel culture, as well as in soft agar. In MCF-7 and T-47D cells which have a functional p53, NBEC-CM induced apoptosis without modifying cell cycle progression. In MDA-MB-231 and BT-20 cells that have a non-functional p53, NBEC-CM did not induce apoptosis, although a slight G1 blokage was observed in MDA-MB-231 cells. Transient transfections of MCF-7 and T-47D cells demonstrated that NBEC-triggered apoptosis was mediated by endogenous p53. Moreover, pifithrin-alpha which specifically inhibits the transcriptional activity of p53, completely abolished NBEC-induced apoptosis in both MCF-7 and T-47D cells, indicating that p53 mediated apoptosis via its transcriptional activity. Finally, orthovanadate, a protein tyrosine phosphatase inhibitor, completely inhibited NBEC-triggered apoptosis, indicating that NBEC-triggered apoptosis was regulated by tyrosine phosphatases.

The importance of being a myoepithelial cell

The mammary myoepithelial cell was named the 'Cinderella of mammary cell biology' in light of the earlier focus on the luminal cell. Mammary myoepithelial cells have recently been described as 'natural tumour suppressors'. We now need to understand more about their origin and to reconsider their place in the complex process of mammary morphogenesis. In the present review, we discuss the lineage segregation of mammary myoepithelial cells and their functions in mammary gland development. These functions include their effects on luminal cell growth and differentiation, their key role in the establishment of the polarised mammary epithelial bilayer and the control of stromal invasion in breast cancer.

Role of myofibroblasts at the invasion front

Tumor progression occurs within a microecosystem, where cancer cells and myofibroblasts exchange proteinases and cytokines that promote growth directly through stimulation of proliferation and survival, as well as invasion through local proteolysis of the extracellular matrix and stimulation of motility. Myofibroblasts maintain the capacity of fibroblasts to induce differentiation. Fibroblasts are the main source of tumor-associated myofibroblasts. The transition to myofibroblasts also occurs in noncancerous situations. This transition is modulated by mechanical stress and cytokines, amongst which transforming growth factor-beta. The cross-talk between cancer cells and myofibroblasts illustrates the microecosystem of tumor invasion. In order to consider myofibroblasts as a possibly new target for cancer therapy, further characterization of the molecular cross-talk between myofibroblasts and cancer cells is required.

The metastasis gene osteopontin: a candidate target for cancer therapy

Malignant tumors are characterized by dysregulated growth control, overcoming of replicative senescence, and metastasis formation. Current therapeutic regimens mostly exert their effects through inhibition of cell cycle progression, leaving two major components of transformation untouched. The cytokine osteopontin is essential for the dissemination of various cancers. Past research has implied several modes in which osteopontin and its main receptors on tumor cells can be suppressed. Osteopontin expression is inhibitable on the levels of gene transcription and the RNA message, and the osteopontin protein can be blocked with antibodies or synthetic peptides. The osteopontin receptor CD44 has been targeted by diverse therapeutic strategies, including cytotoxic and immunotherapeutic approaches. The receptor integrin alpha(V)beta(3) contributes not only to tumor cell dissemination, but also to angiogenesis and osteolysis in bone metastases. Small molecule inhibitors of this receptor are under study as drug candidates. Because receptors and cytokine ligands that mediate metastasis formation are sparsely expressed in the adult healthy organism and are more readily reached by pharmaceuticals than intracellular drug targets they may represent a particularly suitable focus for therapeutic intervention.

On the birth of breast cancer

Breast carcinoma is one of the most common neoplasms in women and is a leading cause of cancer related deaths worldwide. In recent years improved diagnostic tools have made it possible to detect breast cancers at early, even pre-invasive stages leading to a significant decrease in breast cancer mortality rates over the past decades. The increased number of patients diagnosed with pre-invasive breast tumors opened up new avenues in research and new dilemmas in clinical practice, since our understanding of the pathophysiology of such lesions is just beginning to emerge. Part of the delay and difficulty with analyzing pre-invasive tumors including ductal carcinoma in situ has been due to the lack of appropriate techniques suitable for studies of small, frequently microscopic size tumors. Recently developed technologies such as DNA microarrays and SAGE (serial analysis of gene expression) have made it possible to obtain comprehensive gene expression profiles of breast carcinomas of all stages. The application of these genomics approaches in combination with the complete sequence of the human genome and extensive molecular epidemiological studies is likely to further our understanding of the molecular basis of mammary tumorigenesis and will identify targets for risk prediction, cancer prevention and treatment.

The plasticity of human breast carcinoma cells is more than epithelial to mesenchymal conversion

The human breast comprises three lineages: the luminal epithelial lineage, the myoepithelial lineage, and the mesenchymal lineage. It has been widely accepted that human breast neoplasia pertains only to the luminal epithelial lineage. In recent years, however, evidence has accumulated that neoplastic breast epithelial cells may be substantially more plastic in their differentiation repertoire than previously anticipated. Thus, along with an increasing availability of markers for the myoepithelial lineage, at least a partial differentiation towards this lineage is being revealed frequently. It has also become clear that conversions towards the mesenchymal lineage actually occur, referred to as epithelial to mesenchymal transitions. Indeed, some of the so-called myofibroblasts surrounding the tumor may have an epithelial origin rather than a mesenchymal origin. Because myoepithelial cells, epithelial to mesenchymal transition-derived cells, genuine stromal cells and myofibroblasts share common markers, we now need to define a more ambitious set of markers to distinguish these cell types in the microenvironment of the tumors. This is necessary because the different microenvironments may confer different clinical outcomes. The aim of this commentary is to describe some of the inherent complexities in defining cellular phenotypes in the microenvironment of breast cancer and to expand wherever possible on the implications for tumor suppression and progression.

Myoepithelial-specific CD44 shedding is mediated by a putative chymotrypsin-like sheddase

Our previous studies have demonstrated that myoepithelial cells, which surround incipient carcinomas in situ of the breast and other organs, exert antiinvasive and antiangiogenic effects in vitro through the elaboration of a number of different suppressor molecules among which include the shed membrane CD44. The present study addresses the mechanism of this myoepithelial CD44 shedding. This CD44 shedding is enhanced by PMA pretreatment, is specific for myoepithelial CD44, and inhibited by chymotrypsin-like inhibitors (chymostatin, alpha(1)-antichymotrypsin, TPCK, and SCCA-2) but not by trypsin-like inhibitors (TLCK), nor papain-like inhibitors (SCCA-1) nor hydroxamate-based or general metalloproteinase inhibitors (BB2516 (marimastat), 1,10-phenanthroline, and TIMP-1). The effect of PMA can be mimicked by exogenous chymotrypsin but not by other proteases. The CD44 shedding activity cannot be transferred by conditioned media, cell-cell contact, peripheral membrane, or integral membrane fractions. However, cell-free purified integral plasma membrane fractions obtained from myoepithelial cells pretreated with PMA also exhibit CD44 shedding which is inhibited by chymotrypsin-like inhibitors. These findings support the presence and activation of a putative chymotrypsin-like sheddase as the mechanism of CD44 shedding in myoepithelial cells.