Bruce W. Steinert

αIIbβ3 Integrin expression and function in subpopulations of murine tumors

Abstract Subpopulations of B16 amelanotic melanoma (B16a) cells, isolated by centrifugal elutriation from enzymatically dispersed solid tumors, demonstrated different abilities to form lung colonies when injected intravenously. In contrast, no differences in experimental metastasis were observed among subpopulations obtained from Lewis lung (3LL) tumors. Lung colonization by B16a and 3LL subpopulations correlated positively with observed differences (B16a) or lack of differences (3LL) in tumor cell ability to induce aggregation of homologous platelets, to adhere to subendothelial matrix or fibronectin, and with the percentage of cells in the G 2 M phase of the cell cycle. Both B16a and 3LL cells express αIIbβ3 integrin receptors; however, differences in the receptor expression level were found only among B16a subpopulations. Comparison of the amount of αIIbβ3 receptor expressed on cell surface with tumor cell ability to induce platelet aggregation (TCIPA) and to adhere to fibronectin or subendothelial matrix revealed a positive correlation. Pretreatment of tumor cells with αIIbβ3 -specific antibodies inhibited tumor cell matrix adhesion, TCIPA, and lung colony formation. We propose that αIIbβ3 integrin receptor expression, tumor cell matrix adhesion, and tumor cell-induced platelet aggregation can be important parameters to indicate the metastatic potential of some tumor cells and that the αIIbβ3 is a multifunctional receptor involved in both tumor cell-matrix and tumor cell-platelet interactions. Further, the correlation among cell cycle phase, metastatic ability, and receptor expression suggests that metastatic propensity may be transiently expressed and/or increased in some tumor cell subpopulations.

The role of platelet cyclooxygenase and lipoxygenase pathways in tumor cell induced platelet aggregation

Walker 256 carcinosarcoma cells induce the aggregation of rat platelets and concomitant production of eicosanoid metabolites (e.g., 12-hydroxyeicosatetraenoic acid, thromboxane A2). Cyclooxygenase inhibitors, but not lipoxygenase inhibitors, were able to inhibit platelet aggregation induced in vitro by low concentrations of agonists. At high agonist concentrations, neither cyclooxygenase nor lipoxygenase inhibitors affected platelet aggregation; however the combination of both inhibitors resulted in inhibition of aggregation. Also, a low concentration of agonist induced minimal eicosanoid metabolism, whereas a high concentration resulted in increased eicosanoid metabolism. These inhibitors, at the doses tested, did not inhibit protein kinase C activity.

Role of Tumour Cell Eicosanoids and Membrane Glycoproteins IRGPIb and IRGPIIb/IIIa in Metastasis

Eicosanoids are a group of oxygenated arachidonic acid metabolites which include Prostaglandins, thromboxanes, leukotrienes, lipoxins, and various hydroperoxy and hydroxy fatty acids. Eicosanoids are implicated in diverse cellular functions such as Chemotaxis, proliferation, cell-cell signaling etc. The first committed step in the biosynthesis of each of the eicosanoids, the incorporation of molecular oxygen into polyunsaturated fatty acids, is catalyzed by one of a group of enzymes calles fatty acid oxygenases. This group includes the cyclooxygenase (COX) of Prostaglandin endoperoxide synthase (PGH) and various lipoxygenases (LOX).

Role of Eicosanoids in Tumor Cell-Platelet-Endothelial Cell Interactions

Metastases represent perhaps the single most important hindrance to improved cancer patient survival. The formation of metastases is the culmination of a complex series of tumor cell-host interactions called the metastatic cascade (Figure 1; 1,2). During tumor progression, tumor cells become separated from the primary tumor mass and invade into the surrounding tissue. Occasionally, tumor cells will invade blood vessels or lymphatics (intravasation) and are readily disseminated throughout the host. A relatively small percentage of the circulating tumor cells will arrest at the vessel wall and invade into the surrounding tissue (extravasation) to establish a secondary tumor site (3–5). The interaction of the circulating tumor cells with host immune cells may result in destruction of the tumor cells, whereas interaction with host platelets and/or blood coagulation components may enhance the metastatic process. Evidence for the involvement of platelets in metastasis has been demonstrated by several investigators (for review see 6,7). Enhancement of tumor cell arrest and adhesion to the vessel wall by platelets has been postulated as the mechanism for this phenomenon, but its exact nature is undetermined. Because of the potent effects of arachidonic acid metabolites (i.e., prostacyclin and thromboxane A2) on platelet function, we have hypothesized that tumor cells shift the balance between these metabolites in favor of platelet aggregation (8). We have previously proposed the use of modifiers of arachidonic acid metabolism as antimetastatic agents, although the efficacy of antimetastatic therapy with these modifiers has been inconsistent. Prostacyclin (PGI2) has been recently shown to reduce the incidence of pulmonary.