We previously identified km23 as a novel transforming growth factor Β (TGFΒ) receptor-interacting protein that is also a light chain of the motor protein dynein (DLC). Further, we have demonstrated that km23 is altered in 42% of cancer tissues from ovarian cancer patients. Here we demonstrate that the phosphorylation of km23 is TGFΒ-dependent, and that EGF is unable to phosphorylate km23. In addition, we show that km23 is co-localized with the TGFΒ signaling component Smad2 at early time periods after TGFΒ treatment of Madin Darby canine kidney (MDCK) epithelial cells, prior to translocation of Smad2 to the nucleus. Further, km23 interacted with Smad2, but not Smad3, in both immunoprecipitation (IP) /blot and glutathione-S-transferase (GST) pull-down assays. Blockade of km23 using small interfering RNA (siRNA) significantly reduced nuclear expression of Smad2 and TGFΒ-dependent Smad2 transcriptional activation of the activin-responsive element (ARE). Other TGFΒ responses were also suppressed by km23 siRNA, including TGFΒ inhibition of cell growth. Our findings demonstrate that km23 is required in a Smad2-dependent TGFΒ signaling pathway, and that it is necessary, but not sufficient, for TGFΒ-mediated growth inhibition. We also show that stable, inducible over-expression of km23 in a TGFΒ-resistant ovarian cancer cell line results in growth suppression in monolayer culture and in soft agar. Thus, km23 may function as a tumor suppressor, and appears to be a novel anti-cancer therapeutic target. This work was supported by NIH grants CA90765, CA92889, CA100239, and DAMD17-03-1-0287 to KMM

[Proc Amer Assoc Cancer Res, Volume 46, 2005]