Activation of the Rap1 guanine nucleotide exchange gene, CalDAG-GEF I, in BXH-2 murine myeloid leukemia

Here we report the recurrent proviral activation of the Rap1-specific guani ne nucleotide exchange factor CalDAG-GEF I (Kawasaki, H., Springett, G. M., Toki, S., Canales, J. J., Harlan, P., Blumenstiel, J. P., Chen, E. J., Ban y, I. A., Mochizuki, N., Ashbacher, A., Matsuda, M., Housman, D. E., and Gr aybiel, A. M. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 13288-13283; Correct ion (1999) Proc. Natl. Acad Sci. U.S.A. 96, 318) gene in BXH-2 acute myeloi d leukemia. We also show that CalDAG-GEF I encodes two protein isoforms, a full-length isoform (CalDAG-GEF Ia) and a C-terminally truncated isoform (C alDAG-GEF Ib). Expression of the full-length CalDAG-GEF la isoform in Rat2 fibroblasts enhances growth in low serum, whereas expression in Swiss 3T3 c ells causes morphological transformation and increased saturation density. In FDCP1 myeloid cells, CalDAG-GEF Pa expression increases growth and satur ation density in the presence of the diacylglycerol analogs phorbol 12-myri state 13-acetate (PMA), which activates CalDAG-GEF Pa exchange activity. Li kewise, in 32DcI3 myeloblast cells, CalDAG-GEF Ia expression increases cell adherence to fibronectin in response to PMA and calcium ionophore and allo ws higher saturation densities and prolonged growth on fibronectin-coated p lates. These effects were correlated with increased Rap1, but not Ras, prot ein activation following PMA. and calcium ionophore treatment. Our results suggest that Rap1-GTP delivers signals that favor progression through the c ell cycle and morphological transformation, The identification of CalDAG-GE F I as a proto-oncogene in BXH-2 acute myeloid leukemia is the first eviden ce implicating Rap1 signaling in myeloid leukemia.