Breast cancer-associated mutations in metalloprotease disintegrin ADAM12 interfere with the intracellular trafficking and processing of the protein

ADAM12 abrogation alters immune cell infiltration and improves response to checkpoint blockade therapy in the T11 murine model of triple-negative breast cancer

Immunosuppressive tumor microenvironment (TME) impedes anti-tumor immune responses and contributes to immunotherapy resistance in triple-negative breast cancer (TNBC). ADAM12, a member of cell surface metalloproteases, is selectively upregulated in mesenchymal/claudin-low TNBCs, where its expression is largely restricted to tumor cells. The role of cancer cell-expressed ADAM12 in modulating the immune TME is not known. We show that Adam12 knockout in the T11 mouse syngeneic transplantation model of claudin-low TNBC leads to decreased numbers of tumor-infiltrating neutrophils (TINs)/polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) and increased numbers of tumor-infiltrating B cells and T cells. ADAM12 loss in cancer cells increases chemotaxis of B cells in vitro and this effect is eliminated by inhibition of CXCR4, a receptor for CXCL12, or anti-CXCL12 blocking antibody. Importantly, ADAM12 loss in T11 cancer cells sensitizes tumors to anti-PD1/anti-CTLA4 combination therapy, although the initial responsiveness is followed by acquired therapy resistance. Depletion of B cells in mice eliminates the improved response to immune checkpoint blockade of Adam12 knockout T11 tumors. Analysis of gene expression data for claudin-low TNBCs from the METABRIC patient cohort shows significant inverse correlations between ADAM12 and gene expression signatures of several anti-tumor immune cell populations, as well as a significant positive correlation between ADAM12 and gene expression signature of TINs/PMN-MDSCs. Collectively, these results implicate ADAM12 in immunosuppression within the TME in TNBC.

Mutational drivers of cancer cell migration and invasion

Genomic instability and mutations underlie the hallmarks of cancer-genetic alterations determine cancer cell fate by affecting cell proliferation, apoptosis and immune response, and increasing data show that mutations are involved in metastasis, a crucial event in cancer progression and a life-threatening problem in cancer patients. Invasion is the first step in the metastatic cascade, when tumour cells acquire the ability to move, penetrate into the surrounding tissue and enter lymphatic and blood vessels in order to disseminate. A role for genetic alterations in invasion is not universally accepted, with sceptics arguing that cellular motility is related only to external factors such as hypoxia, chemoattractants and the rigidity of the extracellular matrix. However, increasing evidence shows that mutations might trigger and accelerate the migration and invasion of different types of cancer cells. In this review, we summarise data from published literature on the effect of chromosomal instability and genetic mutations on cancer cell migration and invasion.

ADAM12-L is a direct target of the miR-29 and miR-200 families in breast cancer

Background

ADAM12-L and ADAM12-S represent two major splice variants of human metalloproteinase-disintegrin 12 mRNA, which differ in their 3′-untranslated regions (3′UTRs). ADAM12-L, but not ADAM12-S, has prognostic and chemopredictive values in breast cancer. Expression levels of the two ADAM12 splice variants in clinical samples are highly discordant, suggesting post-transcriptional regulation of the ADAM12 gene. The miR-29, miR-30, and miR-200 families have potential target sites in the ADAM12-L 3′UTR and they may negatively regulate ADAM12-L expression.

Methods

miR-29b/c, miR-30b/d, miR-200b/c, or control miRNA mimics were transfected into SUM159PT, BT549, SUM1315MO2, or Hs578T breast cancer cells. ADAM12-L and ADAM12-S mRNA levels were measured by qRT-PCR, and ADAM12-L protein was detected by Western blotting. Direct targeting of the ADAM12-L 3′UTR by miRNAs was tested using an ADAM12-L 3′UTR luciferase reporter. The rate of ADAM12-L translation was evaluated by metabolic labeling of cells with ³⁵S cysteine/methionine. The roles of endogenous miR-29b and miR-200c were tested by transfecting cells with miRNA hairpin inhibitors.

Results

Transfection of miR-29b/c mimics strongly decreased ADAM12-L mRNA levels in SUM159PT and BT549 cells, whereas ADAM12-S levels were not changed. ADAM12-L, but not ADAM12-S, levels were also significantly diminished by miR-200b/c in SUM1315MO2 cells. In Hs578T cells, miR-200b/c mimics impeded translation of ADAM12-L mRNA. Importantly, both miR-29b/c and miR-200b/c strongly decreased steady state levels of ADAM12-L protein in all breast cancer cell lines tested. miR-29b/c and miR-200b/c also significantly decreased the activity of an ADAM12-L 3′UTR reporter, and this effect was abolished when miR-29b/c and miR-200b/c target sequences were mutated. In contrast, miR-30b/d did not elicit consistent and significant effects on ADAM12-L expression. Analysis of a publicly available gene expression dataset for 100 breast tumors revealed a statistically significant negative correlation between ADAM12-L and both miR-29b and miR-200c. Inhibition of endogenous miR-29b and miR-200c in SUM149PT and SUM102PT cells led to increased ADAM12-L expression.

Conclusions

The ADAM12-L 3′UTR is a direct target of miR-29 and miR-200 family members. Since the miR-29 and miR-200 families play important roles in breast cancer progression, these results may help explain the different prognostic and chemopredictive values of ADAM12-L and ADAM12-S in breast cancer.

Pericellular proteolysis in cancer

Pericellular proteases have long been associated with cancer invasion and metastasis due to their ability to degrade extracellular matrix components. Recent studies demonstrate that proteases also modulate tumor progression and metastasis through highly regulated and complex processes involving cleavage, processing, or shedding of cell adhesion molecules, growth factors, cytokines, and kinases. In this review, we address how cancer cells, together with their surrounding microenvironment, regulate pericellular proteolysis. We dissect the multitude of mechanisms by which pericellular proteases contribute to cancer progression and discuss how this knowledge can be integrated into therapeutic opportunities.

Phenotypic diversity of breast cancer-related mutations in metalloproteinase-disintegrin ADAM12

Six different somatic missense mutations in the human ADAM12 gene have been identified so far in breast cancer. Five of these mutations involve highly conserved residues in the extracellular domain of the transmembrane ADAM12-L protein. Two of these extracellular mutations, D301H and G479E, have been previously characterized in the context of mouse ADAM12. Three other mutations, T596A, R612Q, and G668A, have been reported more recently, and their effects on ADAM12-L protein structure/function are not known. Here, we show that ADAM12-L bearing the G668A mutation is largely retained in the endoplasmic reticulum in its nascent, full-length form, with an intact N-terminal pro-domain. The T596A and R612Q mutants are efficiently trafficked to the cell surface and proteolytically processed to remove their pro-domains. However, the T596A mutant shows decreased catalytic activity at the cell surface, while the R612Q mutant is fully active and comparable to the wild-type ADAM12-L. The D301H and G479E mutants, consistent with the corresponding D299H and G477E mutants of mouse ADAM12 described earlier, are not proteolytically processed and do not exhibit catalytic activity at the cell surface. Among all six breast cancer-associated mutations in ADAM12-L, mutations that preserve the activity - R612Q and L792F - occur in triple-negative breast cancers, while loss-of-function mutations - D301H, G479E, T596A, and G668A - are found in non-triple negative cancers. This apparent association between the catalytic activity of the mutants and the type of breast cancer supports a previously postulated role of an active ADAM12-L in the triple negative breast cancer disease.

The B7-1 cytoplasmic tail enhances intracellular transport and mammalian cell surface display of chimeric proteins in the absence of a linear ER export motif

Membrane-tethered proteins (mammalian surface display) are increasingly being used for novel therapeutic and biotechnology applications. Maximizing surface expression of chimeric proteins on mammalian cells is important for these applications. We show that the cytoplasmic domain from the B7-1 antigen, a commonly used element for mammalian surface display, can enhance the intracellular transport and surface display of chimeric proteins in a Sar1 and Rab1 dependent fashion. However, mutational, alanine scanning and deletion analysis demonstrate the absence of linear ER export motifs in the B7 cytoplasmic domain. Rather, efficient intracellular transport correlated with the presence of predicted secondary structure in the cytoplasmic tail. Examination of the cytoplasmic domains of 984 human and 782 mouse type I transmembrane proteins revealed that many previously identified ER export motifs are rarely found in the cytoplasmic tail of type I transmembrane proteins. Our results suggest that efficient intracellular transport of B7 chimeric proteins is associated with the structure rather than to the presence of a linear ER export motif in the cytoplasmic tail, and indicate that short (less than ~ 10-20 amino acids) and unstructured cytoplasmic tails should be avoided to express high levels of chimeric proteins on mammalian cells.

Metalloproteinase-disintegrin ADAM12 is associated with a breast tumor-initiating cell phenotype

Members of the ADAM family of proteases have been associated with mammary tumorigenesis. Gene profiling of human breast tumors identified several intrinsic subtypes of breast cancer, which differ in terms of their basic biology, response to chemotherapy/radiation, preferential sites of metastasis, and overall patient survival. Whether or not the expression of individual ADAM proteases is linked to a particular subtype of breast cancer and whether the functions of these ADAMs are relevant to the cancer subtype have not been investigated. We analyzed several transcriptomic datasets and found that ADAM12L is specifically up-regulated in claudin-low tumors. These tumors are poorly differentiated, exhibit aggressive characteristics, have molecular signatures of epithelial-to-mesenchymal transition (EMT), and are rich in markers of breast tumor-initiating cells (BTICs). Consistently, we find that ADAM12L, but not the alternative splice variant ADAM12S, is a part of stromal, mammosphere, and EMT gene signatures, which are all associated with BTICs. In patients with estrogen receptor-negative tumors, high expression of ADAM12L, but not ADAM12S, is predictive of resistance to neoadjuvant chemotherapy. Using MCF10DCIS.com breast cancer cells, which express the endogenous ADAM12L and efficiently form mammospheres when plated at the density of single cell per well, we show that ADAM12L plays an important role in supporting mammosphere growth. We postulate that ADAM12L may serve as a novel marker and/or a novel therapeutic target in BTICs.

Epigenetic regulation by Z-DNA silencer function controls cancer-associated ADAM-12 expression in breast cancer: cross-talk between MeCP2 and NF1 transcription factor family

A disintegrin and metalloprotease domain-containing protein 12 (ADAM-12) is upregulated in many human cancers and promotes cancer metastasis. Increased urinary level of ADAM-12 in breast and bladder cancers correlates with disease progression. However, the mechanism of its induction in cancer remains less understood. Previously, we reported a Z-DNA-forming negative regulatory element (NRE) in ADAM-12 that functions as a transcriptional suppressor to maintain a low-level expression of ADAM-12 in most normal cells. We now report here that overexpression of ADAM-12 in triple-negative MDA-MB-231 breast cancer cells and breast cancer tumors is likely due to a marked loss of this Z-DNA-mediated transcriptional suppression function. We show that Z-DNA suppressor operates by interaction with methyl-CpG-binding protein, MeCP2, a prominent epigenetic regulator, and two members of the nuclear factor 1 family of transcription factors, NF1C and NF1X. While this tripartite interaction is highly prevalent in normal breast epithelial cells, both in vitro and in vivo, it is significantly lower in breast cancer cells. Western blot analysis has revealed significant differences in the levels of these 3 proteins between normal mammary epithelial and breast cancer cells. Furthermore, we show, by NRE mutation analysis, that interaction of these proteins with the NRE is necessary for effective suppressor function. Our findings unveil a new epigenetic regulatory process in which Z-DNA/MeCP2/NF1 interaction leads to transcriptional suppression, loss of which results in ADAM-12 overexpression in breast cancer cells.

Cell-surface metalloprotease ADAM12 is internalized by a clathrin- and Grb2-dependent mechanism

ADAM12 (A Disintegrin And Metalloprotease 12), a member of the ADAMs family of transmembrane proteins, is involved in ectodomain shedding, cell-adhesion and signaling, with important implications in cancer. Therefore, mechanisms that regulate the levels and activity of ADAM12 at the cell-surface are possibly crucial in these contexts. We here investigated internalization and subsequent recycling or degradation of ADAM12 as a potentially important regulatory mechanism. Our results show that ADAM12 is constitutively internalized primarily via the clathrin-dependent pathway and is subsequently detected in both early and recycling endosomes. The protease activity of ADAM12 does not influence this internalization mechanism. Analysis of essential elements for internalization established that proline-rich regions in the cytoplasmic domain of ADAM12, previously shown to interact with Src-homology 3 domains, were necessary for proper internalization. These sites in the ADAM12 cytoplasmic domain interacted with the adaptor protein growth factor receptor-bound protein 2 (Grb2) and knockdown of Grb2 markedly reduced ADAM12 internalization. These studies establish that internalization is indeed a mechanism that regulates ADAM cell surface levels and show that ADAM12 internalization involves the clathrin-dependent pathway and Grb2.

An essential role of metalloprotease-disintegrin ADAM12 in triple-negative breast cancer

In the absence of HER2 overexpression, triple-negative breast cancers (TNBCs) rely on signaling by epidermal growth factor receptor (EGFR/ErbB1/HER1) to convey growth signals and stimulate cell proliferation. Soluble EGF-like ligands are derived from their transmembrane precursors by ADAM proteases, but the identity of the ADAM that is primarily responsible for ligand release and activation of EGFR in TNBCs is not clear. Using publicly available gene expression data for patients with lymph node-negative breast tumors who did not receive systemic treatment, we show that ADAM12L is the only ADAM with an expression level significantly associated with decreased distant metastasis-free survival times. Similar effect was not observed for patients with ER-negative non-TNBCs. There was a positive correlation between ADAM12L and HB-EGF and EGFR in TNBCs, but not in ER-negative non-TNBCs. We further demonstrate that ectopic expression of ADAM12L increased EGFR phosphorylation in a mouse intraductal xenograft model of early breast cancer. Finally, we detect strong correlation between the level of anti-ADAM12L and anti-phospho-EGFR immunostaining in human breast tumors using tissue microarrays. These studies suggest that ADAM12L is the primary protease responsible for the activation of EGFR in early stage, lymph node-negative TNBCs. Thus, our results may provide novel insight into the biology of TNBC.

Functional analysis of a breast cancer-associated mutation in the intracellular domain of the metalloprotease ADAM12

A recently identified breast cancer-associated mutation in the metalloprotease ADAM12 alters a potential dileucine trafficking signal, which could affect protein processing and cellular localization. ADAM12 belongs to the group of A Disintegrin And Metalloproteases (ADAMs), which are typically membrane-associated proteins involved in ectodomain shedding, cell-adhesion, and signaling. ADAM12 as well as several members of the ADAM family are over-expressed in various cancers, correlating with disease stage. Three breast cancer-associated somatic mutations were previously identified in ADAM12, and two of these, one in the metalloprotease domain and another in the disintegrin domain, were investigated and found to result in protein misfolding, retention in the secretory pathway, and failure of zymogen maturation. The third mutation, p.L792F in the ADAM12 cytoplasmic tail, was not investigated, but is potentially significant given its location within a di-leucine motif, which is recognized as a potential cellular trafficking signal. The present study was motivated both by the potential relevance of this documented mutation to cancer, as well as for determining the role of the di-leucine motif in ADAM12 trafficking. Expression of ADAM12 p.L792F in mammalian cells demonstrated quantitatively similar expression levels and zymogen maturation as wild-type (WT) ADAM12, as well as comparable cellular localizations. A cell surface biotinylation assay demonstrated that cell surface levels of ADAM12 WT and ADAM12 p.L792F were similar and that internalization of the mutant occurred at the same rate and extent as for ADAM12 WT. Moreover, functional analysis revealed no differences in cell proliferation or ectodomain shedding of epidermal growth factor (EGF), a known ADAM12 substrate between WT and mutant ADAM12. These data suggest that the ADAM12 p.L792F mutation is unlikely to be a driver (cancer causing)-mutation in breast cancer.

Somatic mutations in the Notch, NF-KB, PIK3CA, and Hedgehog pathways in human breast cancers

Exome sequencing of human breast cancers has revealed a substantial number of candidate cancer genes with recurring but infrequent somatic mutations. To determine more accurately their mutation prevalence, we performed a mutation analysis of 36 novel candidate cancer genes in 96 human breast cancers. Somatic mutations with potential impact on protein function were observed in the genes ADAM12, CENTB1, CENTG1, DIP2C, GLI1, GRIN2D, HDLBP, IKBKB, KPNA5, NFKB1, NOTCH1, and OTOF. These findings strengthen the evidence for involvement of the Notch, Hedgehog, NF-KB, and PIK3CA pathways in breast cancer development, and point to novel processes that likely are involved.

Increased expression of ADAM12 and ADAM17 genes in laser-capture microdissected breast cancers and correlations with clinical and pathological characteristics

ADAMs (a desintegrin and metalloprotease) are transmembrane glycoproteins involved in cell growth, differentiation, motility, and respectively, tumor growth and progression. Our aim was to evaluate ADAM12 spliced variants (ADAM12L - long membrane-bound and ADAM12S - secreted-short variant) and ADAM17 genes expression in breast cancers and to correlate their level of expression with clinical and pathological characteristics. Expression of ADAMs was analyzed using quantitative reverse-transcription polymerase chain reaction in laser-capture microdissected specimens of breast cancers and corresponding non-neoplastic breast tissues from 92 patients. The proteins' expression was confirmed by immunohistochemistry. Significantly elevated amounts of ADAM12L, ADAM12S and ADAM17 transcripts were found in malignant breast cells compared with normal breast tissue and both ADAMs proteins showed moderate to strong immunoexpression in tumor cells and peritumoral fibroblasts. ADAM12L and ADAM12S expressions were correlated with age, younger patients having higher expression of ADAM12L and ADAM12S; ductal cancers had higher expression of ADAM12L compared with lobular types, whereas ADAM12S was higher expressed in lobular cancers; higher expressions were found for both ADAM12 and ADAM17 in HER2/neu positive and highly proliferative cancers. High-grade cancers showed significantly increased expression of ADAM17. Our study on laser-capture microdissected specimens confers motivation for future work on development of ADAM-selective inhibitors for treatment of breast cancers.

Structural and functional impact of cancer-related missense somatic mutations

A number of large-scale cancer somatic genome sequencing projects are now identifying genetic alterations in cancers. Evaluation of the effects of these mutations is essential for understanding their contribution to tumorigenesis. We have used SNPs3D, a software suite originally developed for analyzing nonsynonymous germ-line variants, to identify single-base mutations with a high impact on protein structure and function. Two machine learning methods are used: one identifying mutations that destabilize protein three-dimensional structure and the other utilizing sequence conservation and detecting all types of effects on in vivo protein function. Incorporation of detailed structure information into the analysis allows detailed interpretation of the functional effects of mutations in specific cases.  Data from a set of breast and colorectal tumors were analyzed. In known cancer genes, mutations approaching 100% of mutations are found to impact protein function, supporting the view that these methods are appropriate for identifying driver mutations. Overall, 50-60% of all somatic missense mutations are predicted to have a high impact on structural stability or to more generally affect the function of the corresponding proteins. This value is similar to the fraction of all possible missense mutations that have a high impact and is much higher than the corresponding one for human population single-nucleotide polymorphisms, at about 30%. The majority of mutations in tumor suppressors destabilize protein structure, while mutations in oncogenes operate in more varied ways, including destabilization of less active conformational states. The set of high-impact mutations encompasses the possible drivers.

Metalloprotease-disintegrin ADAM12 expression is regulated by Notch signaling via microRNA-29

Metalloprotease-disintegrin ADAM12 is overexpressed and frequently mutated in breast cancer. We report here that ADAM12 expression in cultured mammalian cells is up-regulated by Notch signals. Expression of a constitutively active form of Notch1 in murine fibroblasts, myoblasts, or mammary epithelial cells or activation of the endogenous Notch signaling by co-culture with ligand-expressing cells increases ADAM12 protein and mRNA levels. Up-regulation of ADAM12 expression by Notch requires new transcription, is activated in a CSL-dependent manner, and is abolished upon inhibition of IκB kinase. Expression of a constitutively active Notch1 in NIH3T3 cells increases the stability of Adam12 mRNA. We further show that the microRNA-29 family, which has a predicted conserved site in the 3'-untranslated region of mouse Adam12, plays a critical role in mediating the stimulatory effect of Notch on ADAM12 expression. In human cells, Notch up-regulates the expression of the long form, but not the short form, of ADAM12 containing a divergent 3'-untranslated mRNA region. These studies uncover a novel paradigm in Notch signaling and establish Adam12 as a Notch-related gene.

The role of SnoN in transforming growth factor beta1-induced expression of metalloprotease-disintegrin ADAM12

Increased expression of metalloprotease-disintegrin ADAM12 is a hallmark of several pathological conditions, including cancer, cardiovascular disease, and certain inflammatory diseases of the central nervous system or the muscoskeletal system. We show that transforming growth factor beta1 (TGFbeta1) is a potent inducer of ADAM12 mRNA and protein in mouse fibroblasts and in mouse and human mammary epithelial cells. Induction of ADAM12 is detected within 2 h of treatment with TGFbeta1, is Smad2/Smad3-dependent, and is a result of derepression of the Adam12 gene. SnoN, a negative regulator of the TGFbeta signaling pathway, is a master regulator of ADAM12 expression in response to TGFbeta1 stimulation. Overexpression of SnoN in NIH3T3 cells reduces the magnitude of ADAM12 induction by TGFbeta1 treatment. Down-regulation of SnoN expression by short hairpin RNA enhances TGFbeta1-induced expression of ADAM12. In a panel of TGFbeta1-responsive cancer cell lines with high expression of SnoN, induction of ADAM12 by TGFbeta1 is significantly impaired, suggesting that the endogenous SnoN plays a role in regulating ADAM12 expression in response to TGFbeta1. Identification of SnoN as a repressor of the ADAM12 gene should contribute to advances in the studies on the role of ADAM12 in tumor progression and in the development of other pathologies.

Molecular dissection of abnormal wound healing processes resulting in keloid disease

Keloids are locally aggressive scars that typically invade into healthy surrounding skin and cause both physical and psychosocial distress to the patient. These pathological scars occur following minimal skin trauma after a variety of causes including burns and trauma. Although the pathogenesis of keloid disease is not well understood, it is considered to be the end product of an abnormal healing process. The aim of this review was to investigate the molecular and cellular pathobiology of keloid disease in relation to the normal wound healing process. The molecular aberrances in keloids that correlate with the molecular mechanisms in normal wound healing can be categorized into three groups: (1) extracellular matrix proteins and their degradation, (2) cytokines and growth factors, and (3) apoptotic pathways. With respect to cellular involvements, fibroblasts are the most well-studied cell population. However, it is unclear whether the fibroblast is the causative cell; they are modulated by other cell populations in wound repair, such as keratinocytes and macrophages. This review presents a detailed account of individual phases of the healing process and how they may potentially be implicated in aberrant raised scar formation, which may help in clarifying the mechanisms involved in keloid disease pathogenesis.

Differential gene expression analysis of subcutaneous fat, fascia, and skin overlying a Dupuytren's disease nodule in comparison to control tissue

Dupuytren's disease (DD) is a benign fibroproliferative tumor with an unknown etiology and high recurrence postsurgery. Several observations suggest the possible involvement of skin overlying nodule (SON) and the subcutaneous fat in the pathogenesis of DD. This study aims to (1) compare the gene expression levels of SON and subcutaneous fat in DD and normal subjects and (2) to compare transverse palmar fascia (Skoog's fibers) from DD patients as internal control tissue, with palmar fascia (transverse carpal ligament) from patients undergoing carpal tunnel release as external control. Skin, fat, and fascia were obtained from five DD patients of Caucasian origin (age = 66 +/- 14) and from five control subjects (age = 57 +/- 19) undergoing carpal tunnel release. Total ribonucleic acids was extracted from each sample and used for complementary deoxyribonucleic acid synthesis. Real-time quantitative polymerase chain reaction was used to assess the gene expression levels of six candidate genes: A disintegrin and metalloproteinase domain (ADAM12), aldehyde dehydrogenase 1 family member A1 (ALDH1A1), iroquois homeoboxprotein 6 (IRX6), periostin, osteoblast specific factor, proteoglycan 4, and tenascin C. Using independent t test, ADAM12, ALDH1A1, and IRX6 expression levels in DD fats were significantly (p < 0.05) higher than those in the controls. There is no significant difference in the gene expression levels of all six genes when comparing disease and control fascia and skin. Interestingly, ADAM12 up-regulation has also been observed in several other fibrotic and proliferative disorders. In conclusion, this study demonstrates potential roles for subcutaneous fat in DD pathogenesis as well as supports the use of transverse palmar fascia as appropriate control tissues in DD research.

The ADAMs: signalling scissors in the tumour microenvironment

Over the last few years disintegrin metalloproteinases of the Adam (a disintegrin and metalloproteinase) family have been associated with the process of proteolytic 'shedding' of membrane-associated proteins and hence the rapid modulation of key cell signalling pathways in the tumour microenvironment. Furthermore, numerous members of the Adam family have been associated with tumorigenesis and tumour progression. The question now arises of whether pharmacological manipulation of their functions would be a useful adjunct to therapies targeting intercellular communications. To learn from the lessons of matrix metalloproteinase inhibitors as anticancer agents, there are many facets of the biological and clinical relevance of the ADAMs that need to be understood before embarking with confidence on such an approach.

The role of Delta-like 1 shedding in muscle cell self-renewal and differentiation

Myogenic cells have the ability to adopt two divergent fates upon exit from the cell cycle: differentiation or self-renewal. The Notch signaling pathway is a well-known negative regulator of myogenic differentiation. Using mouse primary myoblasts cultured in vitro or C2C12 myogenic cells, we found that Notch activity is essential for maintaining the expression of Pax7, a transcription factor associated with the self-renewal lineage, in quiescent undifferentiated myoblasts after they exit the cell cycle. Stimulation of the Notch pathway by expression of a constitutively active Notch-1, or co-culture of myogenic cells with CHO cells transfected with Delta like-1 (DLL1), increases the level of Pax7. DLL1, a ligand for Notch receptor, is shed by ADAM metalloproteases in a pool of Pax7+ C2C12 reserve cells, but it remains intact in differentiated myotubes. DLL1 shedding changes the receptor/ligand ratio and modulates the level of Notch signaling. Inhibition of DLL1 cleavage by a soluble, dominant-negative mutant form of ADAM12 leads to elevation of Notch signaling, inhibition of differentiation, and expansion of the pool of self-renewing Pax7+/MyoD- cells. These results suggest that ADAM-mediated shedding of DLL1 in a subset of cells during myogenic differentiation in vitro contributes to downregulation of Notch signaling in neighboring cells and facilitates their progression into differentiation. We propose that the proteolytic processing of DLL1 helps achieve an asymmetry in Notch signaling in initially equivalent myogenic cells and helps sustain the balance between differentiation and self-renewal.

RACK1, a new ADAM12 interacting protein. Contribution to liver fibrogenesis

ADAM12 belongs to a disintegrin-like and metalloproteinase-containing protein family that possesses multidomain structures composed of a pro-domain, a metalloprotease, disintegrin-like, cysteine-rich, epidermal growth factor-like, and transmembrane domains, and a cytoplasmic tail. Overexpression of several ADAMs has been reported in human cancer, and we recently described the involvement of ADAM12 in liver injury (Le Pabic, H., Bonnier, D., Wewer, U. M., Coutand, A., Musso, O., Baffet, G., Clement, B., and Theret, N. (2003) Hepatology 37, 1056-1066). In this study, we used a yeast two-hybrid screening of a cDNA library from human hepatocellular carcinoma to analyze binding partners of ADAM12. We identify RACK1, a receptor for activated protein kinase C (PKC), as a new ADAM12 interacting protein. RACK1 is up-regulated in patients with hepatocellular carcinoma and is highly expressed by activated hepatic stellate cells. We demonstrate the involvement of RACK1 in mediating the PKC-dependent translocation of ADAM12 to membranes of activated hepatic stellate cells. In particular, treatment of cells with phorbol esters enhances ADAM12 immunostaining in the membrane fractions and the co-immunoprecipitation of ternary complexes containing RACK1, ADAM12, and PKC. By using RNA interference, we demonstrate that inhibition of RACK1 expression diminishes the phorbol 12-myristate 13-acetate-dependent translocation of ADAM12 to membranes of hepatic stellate cells. Finally, hepatic stellate cells cultured on coated type I collagen induces relocalization of ADAM12 in the membrane, suggesting that this major matrix component in liver cancer and fibrogenesis might stimulate ADAM12 translocation to the cell membrane where its shedding activity takes place.