Disabled-2 (Dab2) mediates transforming growth factor β (TGFβ)-stimulated fibronectin synthesis through TGFβ-activated kinase 1 and activation of the JNK pathway

The multifunctional cytokine transforming growth factor beta ( TGF beta) exerts many of its effects through its regulation of extracellular matrix components, including fibronectin ( FN). Although expression of both TGF beta and FN are essential for embryonic development and wound healing in the adult, overexpression leads to excessive deposition of extracellular matrix observed in many fibroproliferative disorders. We previously have demonstrated that TGF beta- stimulated FN induction requires activation of the c- Jun N- terminal kinase ( JNK) pathway; however, the signaling molecules that link the TGF beta receptors to the JNK pathway remain unknown. We show here that the cytosolic adaptor protein disabled2 ( Dab2) directly stimulates JNK activity, whereas stable small interfering RNA- mediated ablation of Dab2 in NIH3T3 mouse fibroblasts and A10 rat aortic smooth muscle cells demonstrates that its expression is required for TGF beta- mediated FN induction. We demonstrate that TGF beta treatment stimulates the association of Dab2 with the mitogen- activated protein kinase kinase kinase, TAK1. Attenuation of cellular TAK1 levels by transient double- stranded RNA oligonucleotide transfection as well as overexpression of kinase- deficient TAK1 leads to abrogation of TGF beta- stimulated FN induction. Furthermore, cell migration, another JNK- dependent response, is attenuated in NIH3T3- siDab2- expressing clones. We, therefore, delineate a signaling pathway proceeding from the TGF beta receptors to Dab2 and TAK1, leading to TGF beta- stimulated JNK activation, FN expression, and cell migration.