Integrin-alpha3 mediates binding of Chordin to the cell surface and promotes its endocytosis

Aptamer Targets Triple-Negative Breast Cancer through Specific Binding to Surface CD49c

Simple Summary

Targeted therapy directed against many biomarkers has not shown significant improvement in outcome in TNBC, and therefore it is urgent to discover more biomarker candidates. Here, we found a DNA aptamer that bound to TNBC cells and identified CD49c as a specific surface marker for TNBC cells using the aptamer-facilitated biomarker discovery technology. The findings suggest that this DNA aptamer can be a drug delivery vehicle and CD49c is a potential target of targeted therapy for TNBC.

Abstract

Although targeted cancer therapy can induce higher therapeutic efficacy and cause fewer side effects in patients, the lack of targetable biomarkers on triple-negative breast cancer (TNBC) cells limits the development of targeted therapies by antibody technology. Therefore, we investigated an alternative approach to target TNBC by using the PDGC21T aptamer, which selectively binds to poorly differentiated carcinoma cells and tumor tissues, although the cellular target is still unknown. We found that synthetic aptamer probes specifically bound cultured TNBC cells in vitro and selectively targeted TNBC xenografts in vivo. Subsequently, to identify the target molecule on TNBC cells, we performed aptamer-mediated immunoprecipitation in lysed cell membranes followed by liquid chromatography tandem mass spectrometry (LC-MS/MS). Sequencing analysis revealed a highly conserved peptide sequence consistent with the cell surface protein CD49c (integrin α3). For target validation, we stained cultured TNBC and non-TNBC cells with an aptamer probe or a CD49c antibody and found similar cell staining patterns. Finally, competition cell-binding assays using both aptamer and anti-CD49c antibody revealed that CD49c is the biomarker targeted by the PDGC21T aptamer on TNBC cells. Our findings provide a molecular foundation for the development of targeted TNBC therapy using the PDGC21T aptamer as a targeting ligand.

BMP antagonists in tissue development and disease

Bone morphogenic proteins (BMPs) are important growth regulators in embryogenesis and postnatal homeostasis. Their tight regulation is crucial for successful embryonic development as well as tissue homeostasis in the adult organism. BMP inhibition by natural extracellular biologic antagonists represents the most intensively studied mechanistic concept of BMP growth factor regulation. It was shown to be critical for numerous developmental programs, including germ layer specification and spatiotemporal gradients required for the establishment of the dorsal-ventral axis and organ formation. The importance of BMP antagonists for extracellular matrix homeostasis is illustrated by the numerous human connective tissue disorders caused by their mutational inactivation. Here, we will focus on the known functional interactions targeting BMP antagonists to the ECM and discuss how these interactions influence BMP antagonist activity. Moreover, we will provide an overview about the current concepts and investigated molecular mechanisms modulating BMP inhibitor function in the context of development and disease.

The role of chordin fragments generated by partial tolloid cleavage in regulating BMP activity

Chordin-mediated regulation of bone morphogenetic protein (BMP) family growth factors is essential in early embryogenesis and adult homoeostasis. Chordin binds to BMPs through cysteine-rich von Willebrand factor type C (vWC) homology domains and blocks them from interacting with their cell surface receptors. These domains also self-associate and enable chordin to target related proteins to fine-tune BMP regulation. The chordin–BMP inhibitory complex is strengthened by the secreted glycoprotein twisted gastrulation (Tsg); however, inhibition is relieved by cleavage of chordin at two specific sites by tolloid family metalloproteases. As Tsg enhances this cleavage process, it serves a dual role as both promoter and inhibitor of BMP signalling. Recent developments in chordin research suggest that rather than simply being by-products, the cleavage fragments of chordin continue to play a role in BMP regulation. In particular, chordin cleavage at the C-terminus potentiates its anti-BMP activity in a type-specific manner.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Position matters: variability in the spatial pattern of BMP modulators generates functional diversity

Bone morphogenetic proteins (BMPs) perform a variety of functions during development. Considering a single BMP, what enables its multiple roles in tissues of varied sizes and shapes? What regulates the spatial distribution and activity patterns of the BMP in these different developmental contexts? Some BMP functions require controlling spread of the BMP morphogen, while others require formation of localized, high concentration peaks of BMP activity. Here we review work in Drosophila that describes spatial regulation of the BMP encoded by decapentaplegic (dpp) in different developmental contexts. We concentrate on extracellular modulation of BMP function and discuss the mechanisms that generate concentrated peaks of Dpp activity, subdivide territories of different activity levels or regulate spread of the Dpp morphogen from a point source. We compare these findings with data from vertebrates and non-model organisms to discuss how changes in the regulation of Dpp distribution by extracellular modulators may lead to variability in dpp function in different species.

alphaPS1betaPS integrin receptors regulate the differential distribution of Sog fragments in polarized epithelia

Bone morphogenetic proteins (BMPs) have important functions during epithelial development. In Drosophila, extracellular Short gastrulation (Sog) limits the action of the BMP family member Decapentaplegic (Dpp). We have shown that Integrin receptors regulate Sog activity and distribution during pupal wing development to direct placement of wing veins. Here, we show that Integrins perform a similar function in the follicular epithelium, impacting Dpp function during oogenesis and embryonic development. As reported for the wing, this effect is specific to mew, which codes for alphaPS1 integrin. Sog is subject to cleavage by metalloproteases, generating fragments with different properties. We also show that Integrins regulate the distribution of C- and N-terminal Sog fragments in both epithelia, suggesting they may regulate the quality of BMP outputs. Our data indicate that alphaPS1betaPS integrin receptors regulate the amount and type of Sog fragments available for diffusion in the extracellular space during oogenesis and pupal wing development.

Regulated adhesion as a driving force of gastrulation movements

Recent data have reinforced the fundamental role of regulated cell adhesion as a force that drives morphogenesis during gastrulation. As we discuss, cell adhesion is required for all modes of gastrulation movements in all organisms. It can even be instructive in nature, but it must be tightly and dynamically regulated. The picture that emerges from the recent findings that we review here is that different modes of gastrulation movements use the same principles of adhesion regulation, while adhesion molecules themselves coordinate the intra- and extracellular changes required for directed cell locomotion.

von Willebrand Factor Type C Domain-containing Proteins Regulate Bone Morphogenetic Protein Signaling through Different Recognition Mechanisms

Bone morphogenetic protein (BMP) function is regulated in the extracellular space by many modulator proteins, including those containing a von Willebrand factor type C (VWC) domain. The function of the VWC domain-containing proteins in development and diseases has been extensively studied. The structural basis, however, for the mechanism by which BMP is regulated by these proteins is still poorly understood. By analyzing chordin, CHL2 (chordin-like 2), and CV2 (crossveinless 2) as well as their individual VWC domains, we show that the VWC domain is a versatile binding module that in its multiple forms and environments can expose a variety of binding specificities. Three of four, two of three, and one of five VWCs from chordin, CHL2, and CV2, respectively, can bind BMPs. Using an array of BMP-2 mutant proteins, it can be demonstrated that the binding-competent VWC domains all use a specific subset of BMP-2 binding determinants that overlap with the binding site for the type II receptors (knuckle epitope) or for the type I receptors (wrist epitope). This explains the competition between modulator proteins and receptors for BMP binding and therefore the inhibition of BMP signaling. A subset of VWC domains from CHL2 binds to the Tsg (twisted gastrulation) protein similar to chordin. A stable ternary complex consisting of BMP-2, CHL2, and Tsg can be formed, thus making CHL2 a more efficient BMP-2 inhibitor. The VWCs of CV2, however, do not interact with Tsg. The present results show that chordin, CHL2, and CV2 regulate BMP-2 signaling by different recognition mechanisms.

Shaping BMP morphogen gradients in the Drosophila embryo and pupal wing

In the early Drosophila embryo, BMP-type ligands act as morphogens to suppress neural induction and to specify the formation of dorsal ectoderm and amnioserosa. Likewise, during pupal wing development, BMPs help to specify vein versus intervein cell fate. Here, we review recent data suggesting that these two processes use a related set of extracellular factors, positive feedback, and BMP heterodimer formation to achieve peak levels of signaling in spatially restricted patterns. Because these signaling pathway components are all conserved, these observations should shed light on how BMP signaling is modulated in vertebrate development.

Chordin is underexpressed in ovarian tumors and reduces tumor cell motility

Ovarian cancers mostly derive from the monolayer epithelium that covers the ovary. There are currently very few molecular clues to the etiology of this cancer. Bone morphogenetic proteins (BMPs) are required for follicular development and female fertility and are expressed in the ovarian surface epithelium (OSE). We previously reported the expression of human chordin (CHRD), a BMP extracellular regulator, in the ovary. Here we show that CHRD is underexpressed in epithelium ovary cancer and epithelial cancer cell lines as compared with normal tissues and OSE, respectively. Besides, we detected BMP expression in all ovarian cell lines analyzed. To determine the functional relevance of the absence of CHRD mRNA in tumors and cancer cell lines, we studied the effects of CHRD on two cancer cell lines, BG1 and PEO14. Migratory and invasive properties were greatly reduced, whereas cell adhesion to the support was enhanced. In addition, we detected chordin (Chrd) expression in OSE of rat ovaries in a pattern similar to that of BMP4. Altogether, these results suggest that CHRD could participate in regulating BMP activity in normal OSE physiology, and that its mis-expression in OSE may facilitate cancer incidence and/or progression.

Heparan sulfate proteoglycans exert positive and negative effects in Shh activity

Hedgehog (Hh) proteins are morphogens involved in short- and long-range effects during early embryonic development. Genetic analysis in fly and vertebrate embryos showed that heparan sulfate proteoglycans (HSPGs) are required for Hh transport and signaling. To further understand how HSPGs regulate Sonic hedgehog (Shh), we performed experiments using cell culture and biochemical assays. When the synthesis of HSPGs was reduced, a decrease in Shh activity was observed. Contrary to that, addition of a peptide that competes the binding of Shh to HSPGs resulted in augmentation of Shh activity. From these results, we concluded that HSPGs exert positive and negative effects in Shh activity. This dual effect correlates with the finding that Shh interacts preferentially with two HSPGs. The current model for the role of HSPGs in Shh diffusion is discussed in view of our findings.

Cell-surface Heparan Sulfate Proteoglycans Potentiate Chordin Antagonism of Bone Morphogenetic Protein Signaling and Are Necessary for Cellular Uptake of Chordin

Signaling by bone morphogenetic proteins (BMPs) plays a central role in early embryonic patterning, organogenesis, and homeostasis in a broad range of species. Chordin, an extracellular antagonist of BMP signaling, is thought to readily diffuse in tissues, thus forming gradients of BMP inhibition that result in reciprocal gradients of BMP signaling. The latter determine cell fates along the embryonic dorsoventral axis. The secreted protein Twisted Gastrulation (TSG) is thought to help shape BMP signaling gradients by acting as a cofactor that enhances Chordin inhibition of BMP signaling. Here, we demonstrate that mammalian Chordin binds heparin with an affinity similar to that of factors known to functionally interact with heparan sulfate proteoglycans (HSPGs) in tissues. We further demonstrate that Chordin binding in mouse embryonic tissues was dependent upon its interaction with cell-surface HSPGs and that Chordin bound to cell-surface HSPGs (e.g. syndecans), but not to basement membranes containing the HSPG perlecan. Surprisingly, mammalian TSG did not bind heparin unless prebound to Chordin and/or BMP-4, although Drosophila TSG has been reported to bind heparin on its own. Results are also presented that indicate that Chordin-HSPG interactions strongly potentiate the antagonism of BMP signaling by Chordin and are necessary for the retention and uptake of Chordin by cells. These data and others regarding Chordin diffusion have implications for the paradigm of how Chordin is thought to regulate BMP signaling in the extracellular space and how gradients of BMP signaling are formed.

Brain meets pancreas: netrin, an axon guidance molecule, controls epithelial cell migration

A recent publication shows that two integrins, alpha6beta4 and alpha3beta1, interact with netrin-1, a protein best known for its role in axon guidance. Consistent with a role in pancreatic development in vivo, netrin is expressed together with these integrins in fetal pancreatic epithelium. The paper presents strong evidence that pancreatic epithelial cells adhere, spread and migrate on netrin-coated substrata through alpha6beta4. Data also indicate that alpha3beta1 mediates HGF-stimulated cell migration on netrin-coated substrata. These results indicate that netrins might have much broader functions than previously suspected.