Abstract
The basement membrane (BM) proteins laminins, which consist of α, β, and γ chains, support tissue structures and cellular functions. To date only α4 and α5 types of laminins have been identified in the BMs of blood vessels. Our recent study suggested the presence of novel α3B-containing laminins in vascular BMs. Here we identified and characterized the third member of vascular laminins, laminin-3B11 (Lm3B11). RT-PCR analysis showed that microvascular endothelial (MVE) cells and umbilical vein endothelial cells expressed the messages for the α3B, β1, β2, and γ1 chains. In the culture of MVE cells, α3B was associated with β1 and γ1, producing Lm3B11. Recombinant Lm3B11 was overexpressed by introducing the cDNAs of the three chains into HEK-293 cells and purified to homogeneity. Purified Lm3B11 exhibited relatively weak cell adhesion activity through both α3β1 and α6β1 integrins. Most characteristically, Lm3B11 strongly stimulated MVE cells to extend many lamellipodial protrusions. This pseudopodial branching was blocked by an inhibitor for Src or phosphatidylinositol 3-kinase. Consistently, Lm3B11 stimulated the phosphorylation of Src and Akt more strongly than other laminins, suggesting that the integrin-derived signaling is mediated by these factors. The unique activity of Lm3B11 appears to be favorable to the branching of capillaries and venules.