The functional interactions between CD98, beta1-integrins, and CD147 in the induction of U937 homotypic aggregation

Cross-linking CD98 promotes integrin-like signaling and anchorage-independent growth

CD98, an early marker of T-cell activation, is an important regulator of integrin-mediated adhesion events. Previous studies suggest that CD98 is coupled to both cellular activation and transformation and is involved in the pathogenesis of viral infection, inflammatory disease, and cancer. Understanding of the molecular mechanisms underlying CD98 activity may have far-reaching practical applications in the development of novel therapeutic strategies in these disease states. Using small cell lung cancer cell lines, which are nonadherent, nonpolarized, and highly express CD98, we show that, in vitro, under physiological conditions, CD98 is constitutively associated with beta1 integrins regardless of activation status. Cross-linking CD98 with the monoclonal antibody 4F2 stimulated phosphatidylinositol (PI) 3-kinase, PI(3,4,5)P(3), and protein kinase B in the absence of integrin ligation or extracellular matrix engagement. Furthermore, cross-linking CD98 promoted anchorage-independent growth. Using fibroblasts derived from beta1 integrin null stem cells (GD25), wild-type GD25beta1, or GD25 cells expressing a mutation preventing beta1 integrin-dependent FAK phosphorylation, we demonstrate that a functional beta1 integrin is required for CD98 signaling. We propose that by cross-linking CD98, it acts as a "molecular facilitator" in the plasma membrane, clustering beta1 integrins to form high-density complexes. This results in integrin activation, integrin-like signaling, and anchorage-independent growth. Activation of PI 3-kinase may, in part, explain cellular transformation seen on overexpressing CD98. These results may provide a paradigm for events involved in such diverse processes as inflammation and viral-induced cell fusion.

Extracellular matrix metalloproteinase inducer (EMMPRIN) is induced upon monocyte differentiation and is expressed in human atheroma

OBJECTIVE

Because extracellular matrix metalloproteinase inducer (EMMPRIN), a tumor cell-derived protein, induces matrix metalloproteinases (MMPs) in fibroblasts and because MMPs are important in atheroma formation, we investigated if EMMPRIN was expressed in granulocyte/macrophage-colony stimulating factor (GM-CSF)-differentiated human peripheral blood monocytes (HPBM) and macrophage foam cells. In addition, EMMPRIN was studied for its expression in human atheroma.

METHODS AND RESULTS

After 10 days of GM-CSF-induced monocyte differentiation, EMMPRIN mRNA increased 5- to 8-fold relative to undifferentiated monocytes. GM-CSF treatment of HPBM revealed that both EMMPRIN mRNA and protein were upregulated by day 2 over undifferentiated monocytes. GM-CSF-differentiated HPBM showed characteristic macrophage phenotype by showing increases in pancake-like morphology and increases in biochemical markers such as apolipoprotein E, MMP-9, and cholesterol ester (CE). While acetylated LDL treatment of the 10-day GM-CSF-differentiated HPBM increased CE mass 13- to 321-fold, EMMPRIN expression was unchanged relative to nonlipid-loaded macrophages. In human coronary atherosclerotic samples, EMMPRIN was observed in CD68(+) macrophage-rich areas as well as areas of MMP-9 expressions.

CONCLUSIONS

Based on these data, we conclude that monocyte differentiation induces EMMPRIN expression, CE enrichment of foam cells has no further effect on EMMPRIN expression, and EMMPRIN is present in human atheroma. Therefore, EMMPRIN may play a role in atherosclerosis development.

Interaction with glycosaminoglycans is required for cyclophilin B to trigger integrin-mediated adhesion of peripheral blood T lymphocytes to extracellular matrix

Cyclophilins A and B (CyPA and CyPB) are cyclosporin A-binding proteins that are involved in inflammatory events. We have reported that CyPB interacts with two types of cell-surface-binding sites. The first site corresponds to a functional receptor and requires interaction with the central core of CyPB. This region is highly conserved in cyclophilins, suggesting that CyPA and CyPB might share biological activities mediated by interaction with this receptor. The second site is identified with glycosaminoglycans (GAGs), the binding region located in the N terminus of CyPB. The difference in the N-terminal extensions of CyPA and CyPB suggests that a unique interaction with GAGs might account for selective activity of CyPB. To explore this hypothesis, we analyzed the lymphocyte responses triggered by CyPA, CyPB, and CyPB(KKK-), a mutant unable to interact with GAGs. The three ligands seemed capable enough to elicit calcium signal and chemotaxis by binding to the same signaling receptor. In contrast, only CyPB enhanced firm adhesion of T cells to the extracellular matrix. This activity depended on the interactions with GAGs and signaling receptor. CyPB-mediated adhesion required CD147 presumably because it was a costimulatory molecule and was related to an activation of alpha4beta1 and alpha4beta7 integrins. Finally, we showed that CyPB was capable mainly to enhance T cell adhesion of the CD4+CD45RO+ subset. The present data indicate that CyPB rather than CyPA is a proinflammatory factor for T lymphocytes and highlight the crucial role of CyPB-GAG interaction in the chemokine-like activity of this protein.