Laminin-3B11, a Novel Vascular-type Laminin Capable of Inducing Prominent Lamellipodial Protrusions in Microvascular Endothelial Cells

Abstract

The basement membrane (BM) proteins laminins, which consist of ␣, ␤, and ␥ chains, support tissue structures and cellular functions. To date only ␣4 and ␣5 types of laminins have been identified in the BMs of blood vessels. Our recent study suggested the presence of novel ␣3B-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 ␣3B, ␤1, ␤2, and ␥1 chains. In the culture of MVE cells, ␣3B was associated with ␤1 and ␥1, 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 ␣3␤1 and ␣6␤1 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 integrinderived signaling is mediated by these factors. The unique activity of Lm3B11 appears to be favorable to the branching of capillaries and venules.Laminin, a family of large heterotrimeric glycoproteins consisting of ␣, ␤, and ␥ chains, is a major cell-adhesive component present in the basement membranes (BMs) 2 of various tissues. It plays critical roles in the maintenance of tissue architectures and the regulation of cellular functions such as cell adhesion, migration, proliferation, apoptosis, and differentiation (1). Different combinations of five ␣, three ␤, and three ␥ chains give rise to more than 15 laminin isoforms (2). These laminins are distributed in a tissue-specific manner and are thought to play differential roles in the individual tissues. Such specific distribution and function of laminins are mainly determined by the ␣ chains. For example, ␣2 laminins (laminin-211 and -221) are localized in the BMs of muscle and neurons, whereas ␣4 laminins (laminin-411 and -421) are localized in vascular BMs.There are two splicing variants of laminin ␣3 chain, the truncated form ␣3 (or ␣3A) and the full-sized form ␣3B (3, 4), gene expression of which is regulated by alternative splicing and different promoters (5). Laminin-3A32 (Lm3A32), which is previously known as laminin-5 and consists of all truncated chains (␣3A, ␤3, and ␥2), is a major laminin isoform in the skin, esophagus, lung, breast, and other epithelial tissues. This laminin has extensively been studied because it has unique structure and biological activity as compared with other laminins (6). In the skin, Lm3A32 in the BM associates with integrin ␣6␤4 of basal keratinocytes to form the hemidesmosome structures and contributes to their stable anchorage to the underlying connective tissues (7,8). The laminin ␣3A chain also associates with the full-siz...

Functional Diversity of Laminins

“…In certain tissues, such as renal glomeruli, lung, and pancreatic islets, podocytes, alveolar epithelial cells, or islet β-cells, respectively, also rest on the subendothelial BM. The subendothelial BMs of most vessels contain two main laminin types, LM-511 and LM-411 (Hallmann et al 2005, Simon-Assmann et al 2011), but Mori et al (2010) recently identified a third less common isoform, LM-3B11, expressed by vascular endothelial cells. Strict developmental regulation of laminin expression appears to occur.…”

Integrin α 6 β 1 Is the Main Receptor for Vascular Laminins and Plays a Role in Platelet Adhesion, Activation, and Arterial Thrombosis

“…22 Recently, laminin-3B11 (α 3B β 1 γ 1 , LM3B11) was identified as a novel vascular laminin predominantly expressed in capillaries and small veins. 23 The α 3B chain has the highest homology to α 5 , 23 but whether LM3B11 allows platelet adhesion remains to be established.…”

Downregulation of a newly identified laminin, laminin‐3B11, in vascular basement membranes of invasive human breast cancers

“…( 19 ) Furthermore, it was found that the laminin α3B chain is expressed as a novel laminin isoform, laminin‐3B11 (Lm3B11; α3B/β1/γ1), in vascular endothelial cells. ( 20 ) This new vascular laminin was expressed as a recombinant protein in HEK‐293 cells. Although the purified Lm3B11 showed moderate cell adhesion and motility activity toward epithelial cells, it strongly stimulated vascular endothelial cells to extend multiple protrusions, suggesting its involvement in the formation or maintenance of complex vascular network.…”