As illustrated in Supporting Information Fig. 3A, in wild-type embryonic fibroblasts, that express very low level of Abl (data not shown), podosome rosettes contained the product of the Abl-related gene (Arg), the other member of the Abl kinase family. Notably, fibroblasts with the triple deficiency of Src, Yes, and Fyn (SYF fibroblasts) were unable to form podosomal rosettes (compare Supporting Information
Fig. 3A and C with B and D) thus implicating a SFK/Abl kinase as a signaling module indispensable for podosome formation in different selleck kinase inhibitor cell types. Having established that Abl is a macrophage podosome component, we addressed whether this kinase is indispensable for podosome formation. As shown in Fig. 2A and C (central panel), siRNA-induced silencing of Abl expression in BMDMs resulted in disassembly of podosome rosettes. This effect was dependent on a selective silencing of Abl, and not the Abl-related kinase Arg, expression because the siRNA we used did not decrease expression
of Arg Fig. 2B. Notably, reduced expression of Abl by siRNA also resulted in a marked reduction in phosphorylation GPCR Compound Library order of the Abl substrate CrkL both constitutively and upon plating of BMDMs on fibronectin Fig. 2B. Similar results were obtained inhibiting Abl kinase activity with imatinib mesylate (STI) Fig. 2C, right panel). In fact, whereas BMDMs formed several podosome rosettes when plated on fibronectin (Fig. 2C, left panel, yellow arrows) even a short (30 min) treatment with STI resulted in rosette disassembly Fig. 2 C, right panel. In order to establish a relationship between Abl-dependent podosome formation and myeloid cell invasive ability, we plated BMDMs on gelatin-FITC-coated coverslips for 24 h and examined gelatin degradation (Fig. 2D). siRNA-induced silencing of Abl expression resulted in an almost total suppression of matrix degradation (Fig. 2D, right panel). That Abl is required for matrix degradation by myeloid leukocytes was also demonstrated by the finding that the capability of human monocyte-derived macrophages to degrade gelatin was markedly anti-EGFR antibody inhibitor inhibited by imatinib mesylate
(Fig. 2E). Considering that macrophage migration in 3-dimension (3D) and trans-endothelial migration of leukocytes from blood to the interstitium [[3, 17]] require podosome formation we addressed whether silencing of Abl resulted in a reduced migration through an extracellular matrix or an endothelial cell monolayer. As shown in Fig. 3A and B silencing of Abl resulted in a significant inhibition of both type of migratory forms. Although there is not a simple correlation between podosome formation and cell migration, at least in two dimensions [], the efficacy of siRNA in suppressing Abl expression in BMDMs allowed us to address whether the indispensability of Abl in regulating BMDM migration demonstrated with inhibitory drugs [] could be strengthen by studies with Abl-deficient cells.