Expression of VLA-2, VLA-3, and alpha(v) integrin receptors in uveal melanoma: association with microvascular architecture of the tumour and prognostic value

Targeted therapy for uveal melanoma

Uveal melanoma is the most common primary intra-ocular malignancy in adults. Overall mortality rate remains high because of the development of metastatic disease, which is highly resistant to systemic therapy. Improved understanding of the molecular pathogenesis of cancers has led to a new generation of therapeutic agents that interfere with a specific pathway critical in tumor development or progression. Although no specific genes have been linked to the pathogenesis of uveal melanoma, which differs from that of cutaneous melanoma, progress has been made in identifying potential targets involved in uveal melanoma apoptosis, proliferation, invasion, metastasis, and angiogenesis. This review focuses on the prospects for improving the systemic therapy of uveal melanoma using molecularly targeted agents that are currently in clinical use as well as agents being tested in clinical trials. Preclinical studies suggest potential benefit of inhibitors of Bcl-2, ubiquitin-proteasome, histone deactylase, mitogen-activated protein kinase and phosphatidylinositol-3-kinase-AKT pathways, and receptor tyrosine kinases. Modifiers of adhesion molecules, matrix metalloproteinase, and angiogenic factors also have demonstrated potential benefit. Clinical trials of some of these approaches have been initiated in patients with metastatic uveal melanoma as well as in the adjuvant setting after primary therapy.

Increased p21-activated kinase-1 expression is associated with invasive potential in uveal melanoma

Prognosis in patients with uveal melanoma is poor as approximately half of all tumors metastasize and currently there are no effective treatments for disseminated disease. Differences in invasiveness between uveal melanomas could therefore be of major significance regarding clinical outcome. To identify genes associated with invasive potential, we have used microarray expression profiling combined with phenotypic characterization of uveal melanoma and melanocyte cell lines to define a gene signature associated with cellular invasion. A panel of 14 uveal cell cultures was assessed using three assays of invasiveness: matrigel invasion chamber system, scratch wound closure and cell motility. We identified a set of 853 differentially expressed transcripts (Wilcoxon-Mann-Whitney test, P<0.01) that discriminated between samples with high or low invasive capacity based on a composite phenotype that takes into account behavior across all three assays. Of particular interest, expression of two members of the p21-activated kinase (PAK) family, PAK1 and PAK7, was elevated in the more invasive cultures. PAK1 has previously been shown to play a central role in regulating cell motility and invasiveness in other cell types, and increased expression has been observed in breast and colorectal carcinomas. Using small interfering RNA-mediated PAK1 knockdown, we showed up to a five-fold decrease in invasion through matrigel, indicating that elevated levels of PAK1 are associated with invasive potential in uveal melanoma. These data implicate PAK1 as a potential new target for therapy of these tumors.

Integrin α2β1 mediates the anti-angiogenic and anti-tumor activities of angiocidin, a novel tumor-associated protein

We recently characterized an anti-tumor protein termed angiocidin. Here, we report that angiocidin may inhibit angiogenesis by binding collagen and its receptors. Angiocidin bound purified type I collagen and alpha2beta1 with high affinity. K562 cells expressing alpha2beta1 bound and adhered to angiocidin while K562 cells which only expressed alpha5beta1 integrin showed no binding and adhesion. Binding was specific since a neutralizing antibody against alpha2beta1 inhibited binding but antibodies against alpha5beta1 had no effect. Additionally, angiocidin co-localized with alpha2beta1 on K562 alpha2beta1 transfected cells, pancreatic cancer colo 357 cells, breast cancer MB-231 cells and human umbilical endothelial vein (HUVE) cells. In an alpha2beta1-dependent collagen gel angiogenesis assay, angiocidin showed potent inhibitory activity. We identified a 20-amino-acid amino terminal peptide of angiocidin that bound both alpha2beta1 and type I collagen. This peptide promoted alpha2beta1-dependent cell adhesion and inhibited tumor growth and angiogenesis. Taken together, these results are consistent with the conclusion that the anti-tumor activity of angiocidin arises from its ability to ligate collagen and alpha2beta1 on endothelial cells and tumor cells. Our results provide support for the concept that targeting matrix-cell interactions is a viable strategy for the development of anti-cancer therapeutics.

Invasive and noninvasive uveal melanomas have different adhesive properties

AIMS

To establish if invasive and noninvasive uveal melanomas have differences in expression of adhesion molecules, and whether their adhesive interactions with the extracellular matrix (ECM) and endothelium vary.

METHODS

Cells from an invasive and noninvasive uveal melanoma cell line and hepatic and dermal microvascular endothelial cells were assessed by flow cytometry for adhesion molecule expression. Tumour cell adhesion to ECM substrates (collagens I and IV, fibronectin, laminin, and vitronectin) and endothelial cells was also investigated using a commercially available assay or a fluorescence-based in vitro assay, respectively. The significance of results comparing cell lines was determined using a Student's t-test, whereby P-values of less than 0.05 were taken as significant.

RESULTS

alpha1- and alpha4-integrins were not expressed by noninvasive cells, but were detected on invasive cells. The invasive cell line also expressed higher levels of other integrins than the noninvasive line. Correspondingly, invasive cells adhered in higher numbers to ECM substrates and endothelial cells, and for the latter, the difference was highly significant (P<0.001). No preference in adhesion of invasive cells for the hepatic endothelium was observed.

CONCLUSIONS

Successful attachment to and migration through the ECM, basement membrane, and endothelium are vital processes involved in malignant progression. Differential expression of alpha1- and alpha4-integrins by invasive and noninvasive cells infers a role for these receptors in invasion, while the ability of invasive cells to adhere more efficiently to the endothelium suggests that this is a critical factor in uveal melanoma invasion.

Microvascular loops and networks in uveal melanoma

Microvascular patterns--three-dimensional architectural arrangements of microvessels and extravascular matrix in uveal melanoma--were discovered when investigators were looking for histopathological features of sufficient size to be imaged clinically. Evidence that these patterns may be formed by tumour cells and that they may be able to conduct plasma and blood as well as discovery of similar elements in other cancers make them of general importance. Of nine different patterns described, closed microvascular loops and networks have been studied most extensively. When cell type, microvascular density and nucleolar size are controlled for, these two patterns independently predict time to metastasis. In addition to visualization in tumour specimens stained with periodic acid-Schiff reagent, they can often be visualized clinically on confocal indocyanine green angiography. The presence of networks is clinically associated with probability of growth of small uveal melanocytic tumours and with the rate of regression of uveal melanoma after brachytherapy. Networks are also associated with development of exudative retinal detachment from uveal melanoma. Histopathological studies show that loops and networks are less common in tumours enucleated after irradiation and that they are frequently repeated in metastases of uveal melanoma. Avenues for immediate future research include detailed elucidation of the histogenesis of microvascular patterns and determination of these patterns in metastatic melanoma to identify new histopathological characteristics for prognostication when clinical metastases have developed.