A mutant form of p135tyk2, an interferon-alpha inducible tyrosine kinase, suppresses the transformed phenotype of Daudi cells.

The type I interferons induce an anti-viral state and suppress cell growth. The p135tyk2 non-receptor tyrosine kinase appears to initiate, at least in part, the type I interferon signal transduction pathway, and thereby activates type I interferon-dependent gene expression. To determine if p135tyk2 can suppress growth and/or tumorigenesis, derivatives of the tyk2 gene were introduced into the tumorigenic cell line Daudi. Transfectants expressing a tyk2 construct missing the carboxy-terminal 22 amino acids cloned with a greatly reduced efficiency in soft agar and displayed a partial decrease in the ability to form tumors in athymic mice. In addition, transfectants producing a kinase deficient version of tyk2 show an increase in both growth rate and agar cloning efficiency, suggesting that the inactive kinase can act in a dominant-negative manner. Surprisingly, the carboxyl-terminal deleted protein lacks both auto-kinase activity, and activity towards a putative substrate, even though it induces a phenotype which is precisely the opposite of that produced by another kinase-deficient tyk2 mutant containing an altered ATP binding site. Thus, while these results add tyk2 to a growing list of interferon-alpha regulated proteins that might be able to suppress tumor formation, the biochemical basis of this activity remains unknown.