Hematopoietic progenitor kinase 1 is a component of transforming growth factor beta-induced c-Jun N-terminal kinase signaling cascade

The c-Jun N-terminal kinase (JNK) signaling pathway is involved in transfor ming growth factor beta (TGF-beta) signaling in a variety of cell systems, We report here that hematopoietic progenitor kinase 1 (HPK1), a novel Ste20 -like protein serine/threonine kinase, serves as an upstream mediator for t he TGF-beta-activated JNK1 cas cede in 293T cells. TGF-beta treatment resul ted in a time-dependent activation of HPK1, which was accompanied by simila r kinetics of JNK1 activation. The activation of JNK1 by TGF-beta was abrog ated by a kinase-defective HPK1 mutant but not by a kinase-defective mutant of kinase homologous to Ste20/Sps1, This result indicates that HPK1 is spe cifically required for TGF-beta-induced activation of JNK1. We also found t hat TGF-beta-induced JNK1 activation was blocked by a kinase-defective muta nt of TGF-beta-activated kinase 1 (TAK1), In addition, interaction between HPK1 and TAK1 was observed in transient transfection assays, and this inter action was enhanced by TGF-beta treatment. Both stress-activated protein ki nase/extracellular signal-regulated kinase kinase (SEK) and mitogen-activat ed protein kinase kinase 7 (MKK7) are immediate upstream activators of JNK1 . Although SEK and MKK7 acted downstream of TAK1, only a kinase-defective S EK mutant blocked TGF-beta-induced activation of JNK1, indicating that the TGF-beta signal is relayed solely through SEK, but not MKK7, in vivo. Furth ermore, TGF-beta-induced activating protein 1 activation was blocked by a H PK1 mutant, as well as by TAK1 and SEK mutants. Taken together, these studi es establish a potential cascade of TGF-beta-activated interacting kinases beginning with HPK1, a Ste20 homolog, and ending in JNK1 activation: HPK1 - -> TAK1 --> SEK --> JNK1.