Ax1-Gas6 interaction counteracts E1A-mediated cell growth suppression and proapoptotic activity

The adenovirus type 5 early region 1A gene (E1A) has previously been known as an immortalization oncogene because E1A is required for transforming onc ogenes, such as pas and E1B, to transform cells in primary cultures. Howeve r, E1A has also been shown to downregulate the overexpression of the Her-2/ neu oncogene, resulting in suppression of transformation and tumorigenesis induced by that oncogene. In addition, E1A is able to promote apoptosis ind uced by anticancer drugs, irradiation, and serum deprivation. Many tyrosine kinases, such as the epidermal growth factor receptor, Her-2/Neu, Src, and Axl, are known to play a role in oncogenic signals in transformed cells. T o study the mechanism underlying the E1A-mediated tumor-suppressing functio n, we exploited a modified tyrosine kinase profile assay (D. Robinson, F. L ee, T. Pretlow, and H.-J. Kung, Proc. Natl. Acad. Sci. USA 93:5958-5962, 19 96) to identify potential tyrosine kinases regulated by E1A. Reverse transc ription (RT)-PCR products were synthesized with two degenerate primers deri ved from the conserved motifs of various tyrosine kinases. A tyrosine kinas e downregulated by E1A was identified by analyzing the AluI-digested RT-PCR products. We isolated the DNA fragment of interest and found that E1A nega tively regulated the expression of the transforming receptor tyrosine kinas e Axl at the transcriptional level. To study whether downregulation of the Axl receptor is involved in E1A-mediated growth suppression, we transfected axl cDNA into E1A-expressing cells (ipl-E1A) to establish cells that overe xpressed Axl. The Axl ligand Gas6 triggered a greater mitogenic effect in t hese ip1-E1A-Axl cells than in ipl-E1A control cells and protected the Art- expressing cells from serum deprivation-induced apoptosis. These results in dicate that downregulation of the Axl receptor by E1A is involved in E1A-me diated growth suppression and ELA-induced apoptosis and thereby contributes to E1A's antitumor activities.