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Elastin microfibril interface-located proteins (EMILINs) constitute a family of extracellular matrix (ECM) glycoproteins characterized by the presence of an EMI domain at the N terminus and a gC1q domain at the C terminus. EMILIN1, the archetype molecule of the family, is involved in elastogenesis and hypertension etiology, whereas the function of EMILIN2 has not been resolved. Here, we provide evidence that the expression of EMILIN2 triggers the apoptosis of different cell lines. Cell death depends on the activation of the extrinsic apoptotic pathway following EMILIN2 binding to the TRAIL receptors DR4 and, to a lesser extent, DR5. Binding is followed by receptor clustering, colocalization with lipid rafts, death-inducing signaling complex assembly, and caspase activation. The direct activation of death receptors by an ECM molecule that mimics the activity of the known death receptor ligands is novel. The knockdown of EMILIN2 increases transformed cell survival, and overexpression impairs clonogenicity in soft agar and three-dimensional growth in natural matrices due to massive apoptosis. These data demonstrate an unexpected direct and functional interaction of an ECM constituent with death receptors and discloses an additional mechanism by which ECM cues can negatively affect cell survival.

MULTIMERIN2 (MMRN2), also known as Endoglyx-1, is an extracellular matrix glycoprotein whose function has so far remained elusive. Given its specific localization in tight association with the endothelium we hypothesized that this protein could modulate neo-angiogenesis. By multiple assays we showed that MMRN2 significantly impaired endothelial cell (EC) migration and organization of a functional vessel network. The interaction of ECs with MMRN2 induced a striking impairment of VEGFR1 and VEGFR2 activation. We focused our attention on VEGFR2, a chief regulator of angiogenesis, and clarified that MMRN2 interfered with the VEGF/VEGFR2 axis through a direct binding with VEGF-A. This novel interaction was assessed in several assays and the affinity was estimated (Kd∼50 nM). We next questioned whether the anti-angiogenic properties of MMRN2 could impair tumor growth. Although overexpression of MMRN2 by HT1080 cells did not affect their growth and apoptotic rate in vitro, it remarkably affected their growth in vivo. In fact, MMRN2-positive cells failed to efficiently grow and form well-vascularized tumors; a similar outcome was observed following treatment of established tumors with a MMRN2 adenoviral construct. Tumor-section immunostaining revealed a strong co-localization of VEGF-A with the ectopically expressed MMRN2. These novel findings suggest that VEGF may be sequestered by MMRN2 and be less available for the engagement to the receptors. Taken together these results highlight MMRN2 as a crucial player in the regulation of EC function, neo-angiogenesis and hence tumor growth. We hypothesize that secreted and deposited MMRN2 may function as a homeostatic barrier halting the sprouting of novel vessels, and suggest that these studies may embody the potential for the development of novel tools for cancer treatment. [ABSTRACT FROM AUTHOR]

Extracellular matrix (ECM) is an essential component of the stromal microenvironment both from a structural and a functional point of view. The ECM functions as a scaffold for tissue organization and regulates growth factors and chemokines availability thus contributing to maintain tissue homeostasis. Attachment of cells to ECM is essential to support cell survival, growth, and proliferation, and the lack of these interactions can trigger a type of cell death named anoikis. Several studies point out that alterations of ECM composition are often responsible of many pathological conditions such as cancer, of which it has been demonstrated to be occasionally the main promoter. ECM does not always represent a prosurvival stimulus; among the different array of ECM molecules a set of proteins can negatively affect cell viability and are thought to play an important role in tumor progression. For this reason attention has been focused on these molecules as potential tools or targets for therapy. [ABSTRACT FROM AUTHOR]