Lnk Expression Research Articles

Negative Hematopoietic Scaffold Lnk Upregulates Integrin Outside-In Signaling in Platelets.

The adaptor molecule Lnk negatively regulates signaling downstream of cytokine receptors in self-renewal of hematopoietic stem cells, proliferation of B precursors and differentiation/maturation of megakaryocytes. The circulating number of platelets in Lnk-deficient mice is increased by 5-folds to control, presumably owing to its negative effects on TPO-elicited signaling. Lnk expression is maintained in platelets, however, the precise functions in platelets remain unknown. Here we report that Lnk deficiency results in the prolonged bleeding time (2-folds, p<0.01) due to, at least in part, defective the outside-in signaling from α IIbβ 3 integrin. Lnk-null platelets exhibited less activation of PAK kinase, a Rac effector in fibrinogen-adherent platelets. These were concomitant with the defect of lamellipodial formation upon adhesion to fibrinogen in the absence of Lnk, compared to control platelets (p<0.01). Defective Lnk-null platelet functions were also demonstrated by the reduced three-dimensional growth (height) of platelet thrombi on the collagen surface under whole blood flow condition with the adjusted same platelet number at high wall shear rate of 1,500 s-1 (8.1±1.4 μ m in Lnk-deficient vs. 12.1±2.2 μ m in control, p<0.001), while the two-dimensional surface coverage was not impaired. On the other hand, there was no difference between Lnk-null and control platelets in aggregation and α IIbβ 3 integrin activation in response to stimulation with thrombin or collagen. Taken together, implication of Lnk to sites of adhesion may explain the phenotype of Lnk-deficient platelet function. The findings indicate that Lnk is a unique “dual-functioning” scaffolding molecule, which upregulates thrombus formation, while suppressing cytokine-mediated signaling.

La protéine adaptatrice Lnk module l'activation des cellules endothéliales

Lnk is an adaptator protein involved in B lymphocytes and platelet differentiation and in T lymphocyte activation. We previously reported on Lnk expression and regulation in endothelial cells (ECs) upon activation. In the present study, the involvement of Lnk in the tumor necrosis factor alpha (TNFα) pathway was investigated in vitro through Lnk overexpression in primary cultures of human endothelial cells. Using a recombinant adenovirus encoding human Lnk, we first demonstrated that Lnk overexpression does not induce vascular cell adhesion molecule-1 (VCAM-1) suggesting that Lnk does not promote ECs activation. However, Lnk overexpression significantly reduced TNFα-mediated expression of VCAM-1 (at mRNA and protein levels) in activated EC as compared with controls. Western blot analysis showed that Lnk overexpression in HUVEC was associated with phosphorylation of Akt kinase (at Ser 473) with no effect on IκBα, the specific inhibitor of NFκB, indicating that Lnk promotes activation of the phosphatidylinositol 3-kinase (PI3-kinase) pathway in ECs. Altogether, these results suggest that, in ECs, Lnk may participate to a regulatory pathway involving the PI3-kinase and modulating the inflammatory response.