Endorepellin, a novel inhibitor of angiogenesis derived from the C terminus of perlecan

Perlecan and its immunoglobulin like domain IV are abundant in vitreous and serum of the chick embryo

Perlecan is a highly conserved heparan sulfate proteoglycan in cartilage and basement membranes. We identified chick perlecan and a 90 KD perlecan fragment in vivo using a newly generated monoclonal antibody. Chick perlecan is, like its human and mouse homologue, a hybrid heparan sulfate/chondroitin sulfate proteoglycan with a core protein of 400 KD. Analysis of the 90 KD fragment by Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) and Capillary LC nano Electrospray Ionization tandem MS (LC nano ESI MS/MS) showed that it belonged to domain IV of the perlecan core protein. We found that full-length perlecan and its domain IV fragment are abundant in embryonic vitreous body and serum. Their expression in vitreous and serum is greatly down-regulated shortly after hatching of the chick. We speculate that the abundance of perlecan in the embryonic circulation and vitreous reflects the ongoing formation of new BMs in the expanding vascular system and the growing retina. In addition, we found that perlecan as a substrate does not support, rather inhibits neurite outgrowth.

Matrix metalloproteinases and matrikines in angiogenesis

Neoangiogenesis, the formation of new blood capillaries from pre-existing vessels, plays an important role in a number of physiological and pathological processes, particularly in tumor growth and metastasis. Extracellular proteolysis by matrix metalloproteinases or other neutral proteinases is an absolute requirement for initiating tumor invasion and angiogenesis. Cryptic segments or pre-existing domains within larger proteins, most of them belonging to the extracellular matrix, can be exposed by conformational changes and/or generated by partial enzymatic hydrolysis. They can positively or negatively regulate important functions of endothelial cells including adhesion, migration, proliferation, cell survival and cell-cell interactions. Such regulations by cryptic segments and proteolytic fragments led to the concept of matricryptins and matrikines, respectively. Matrix metalloproteinases and matrikines in conjunction with other pro- or anti-angiogenic factors might act in concert at any step of the angiogenesis process. A number of matrikines have been identified as potent anti-angiogenic factors, which could provide a new alternative to anti-proteolytic strategies for the development of anti-angiogenic therapeutic molecules aimed at inhibiting tumor growth and metastasis. Some of them are currently being investigated in clinical trials.

Apoptosis of endothelial cells triggers a caspase‐dependent anti‐apoptotic paracrine loop active on vascular smooth muscle cells

Increased endothelial apoptosis and decreased apoptosis of vascular smooth muscle cells (VSMC) are central to initiation of myo-intimal thickening. We hypothesized that apoptosis of endothelial cells (EC) induces the release of anti-apoptotic mediator(s) active on VSMC. We found that serum-free medium conditioned by apoptotic EC decreases apoptosis of VSMC compared with fresh serum-free medium. Inhibition of endothelial apoptosis during conditioning with a pan-caspase inhibitor ZVAD-FMK blocked the release of the anti-apoptotic factor(s) active on VSMC. VSMC exposed to serum-free medium conditioned by apoptotic EC showed increased ERK 1/2 phosphorylation, enhanced Bcl-xl expression, and inhibition of p53 expression. Fractionation of the conditioned medium followed by mass spectral analysis identified one bioactive component as a C-terminal fragment of the domain V of perlecan. Serum-free medium supplemented with either a synthetic peptide containing the EGF motif of the domain V of perlecan or chondroitin 4-sulfate, a glycosaminoglycan anchored on the domain V of perlecan, increased ERK 1/2 phosphorylation and Bcl-xl protein levels while inhibiting apoptosis of VSMC. These results suggest that a proteolytic activity developing downstream of activated caspases in apoptotic EC initiates degradation of pericellular proteoglycans and liberation of bioactive fragments with a robust impact on inhibition of VSMC apoptosis.

A novel interaction between perlecan protein core and progranulin: potential effects on tumor growth

In an in vivo search of novel partners for perlecan, a major heparan sulfate proteoglycan of basement membranes and cell surfaces, we identified progranulin, a secreted growth factor, as a strong interacting protein. Unambiguous interaction, first observed with the yeast two-hybrid system, was corroborated by co-immunoprecipitation studies using cell-free transcription/translation and transient cell transfection assays. The interaction of progranulin with perlecan domain V involved the first two laminin- and epidermal growth factor-like repeats. Within progranulin, the subdomains interacting most with perlecan harbored granulins F and B. Kinetics analysis of the interaction using surface plasmon resonance showed a saturable binding of relative low affinity (KD approximately 1 microM). These results were supported by significant expression overlap of these two proteins in a series of ovarian tumor tissue microarrays. Progranulin was present within proliferating blood vessels of ovarian carcinomas and perivascular matrices, with a distribution similar to perlecan. Notably, both progranulin and domain V stimulated the growth of adrenal carcinoma cells. However, when used together in equimolar amounts, the two proteins counteracted each other's activity. Thus, progranulin/perlecan interaction could contribute to a fine regulation of tumor angiogenesis and could ultimately affect cancer growth.

The basement membrane matrix in malignancy

Cancer is the second most common cause of death among Americans, although for several age groups it ranks first. Most of these deaths are not due to the primary tumour but rather to tumour cell metastases to distant organs. There are many steps that lead to metastasis, all of which are being studied with the goal of preventing these fatalities. Normally, cells attach to the extracellular matrix to maintain tissue integrity. During cancer progression, cells become more motile and acquire invasive qualities. Tumour cells move along blood and lymph vessels or invade into them to travel to distant sites. Then, the tumour cells must attach to the vessel wall, extravasate from the vessel, invade the new tissue, proliferate, and form a secondary tumour. Angiogenesis, the formation of new blood vessels, is critical to survival of these cells at the new site and is also important for primary tumour growth and spread. Tumour cell metastasis is a complex cascade of sequential steps, each of which is not yet fully understood. Progress has been made in identifying several key activators, one of which is the extracellular matrix. A major tumour promoter is the glycoprotein laminin, which is predominantly found in the extracellular matrix produced by endothelial and epithelial cells. This review will follow the metastatic process with particular attention to the effect of laminin on tumour cells.

Basement membranes: structure, assembly and role in tumour angiogenesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Perlecan protein core interacts with extracellular matrix protein 1 (ECM1), a glycoprotein involved in bone formation and angiogenesis

The goal of this study was to discover novel partners for perlecan, a major heparan sulfate proteoglycan of basement membranes, and to examine new interactions through which perlecan may influence cell behavior. We employed the yeast two-hybrid system and used perlecan domain V as bait to screen a human keratinocyte cDNA library. Among the strongest interacting clones, we isolated a approximately 1.6-kb cDNA insert that encoded extracellular matrix protein 1 (ECM1), a secreted glycoprotein involved in bone formation and angiogenesis. The sequencing of the clone revealed the existence of a novel splice variant that we name ECM1c. The interaction was validated by co-immunoprecipitation studies, using both cell-free systems and mammalian cells, and the specific binding site within each molecule was identified employing various deletion mutants. The C terminus of ECM1 interacted specifically with the epidermal growth factor-like modules flanking the LG2 subdomain of perlecan domain V. Perlecan and ECM1 were also co-expressed by a variety of normal and transformed cells, and immunohistochemical studies showed a partial expression overlap, particularly around dermal blood vessels and adnexal epithelia. ECM1 has been shown to regulate endochondral bone formation, stimulate the proliferation of endothelial cells, and induce angiogenesis. Similarly, perlecan plays an important role in chondrogenesis and skeletal development, as well as harboring pro- and anti-angiogenic activities. Thus, a physiological interaction could also occur in vivo during development and in pathological events, including tissue remodeling and tumor progression.