We have shown that a constitutively active G(alpha13) (G(alpha13Q226L)) induces differentiation in P19 embryonic carcinoma cells to an endodermal phenotype. In this report, we demonstrate that Ku, a heterodimer of p80 (Ku80) and p70 (Ku70), is upregulated ill P19 cells overexpressing G(alpha13Q226L). Ku is the regulatory subunit of the DNA-dependent protein kinase and is primarily involved in DNA repair and recombination. Ku80 also is a somatostatin receptor. We show that while overexpression of Ku80 drastically reduced P19 cell proliferation, it was not sufficient to induce endodermal differentiation. However, coexpression of G(alpha13Q226L) and an antisense Ku8O abrogated the retarded growth rate and endodermal differentiation observed in cells expressing only G(13Q226L). Overexpression of G(13Q226L_) or Ku80 downregulated RNA polymerase 1-mediated transcriptional activity and overexpression of antisense Ku80 restored the activity to control level. These results suggest that Ku80 is required for Galpha13-mediated endodermal differentiation in P19 cells. (C) 2004 Elsevier Inc. All rights reserved.