Purification of a 72-kDa protein-tyrosine kinase from rat liver and its identification as Syk: Involvement of Syk in signaling events of hepatocytes

Syk protein-tyrosine kinase (PTK) has been implicated in a variety of hemat opoietic cell responses including immunoreceptor signaling. However, so far , there has been no evidence of the expression of Syk or Syk-related PTK in non-hematopoietic tissues. In this study, we have purified from blood cell -depleted rat liver a 72-kDa cytoplasmic PTK which shows cross-reactivity w ith anti-Syk antibody, Partial amino acid sequence analysis revealed that t his 72-kDa PTK is identical to Syk, Immunohistochemical and RT-PCR analyses demonstrated that Syk is expressed in human hepatocytes and two rat liver- derived cell lines, JTC-27 and RLC-16. Furthermore, Syk is significantly ty rosine-phosphorylated in response to angiotensin II in JTC-27 cells, and an giotensin II-induced MAP kinase activation is blocked by the treatment of c ells with a Syk-selective inhibitor piceatannol. These results suggest that Syk plays an important role in signaling events of hepatocytes, such as si gnaling steps leading to MAP kinase activation by G-protein-coupled recepto rs, This is the first report of the expression of Syk in non-hematopoietic tissue.