Gas6 induces proliferation of tumor cells in vitro and in vivo. (A) Morphometric analysis of proliferation on BrdU-stained CT26 tumor sections revealing reduced proliferation of tumor cells in Gas6^−/− vs WT mice (n = 8; P < .005). (B) Morphometric analysis of Caspase 3 (Casp3) staining showing similar tumor cell apoptosis in CT26 tumors in both genotypes (n = 8; P = NS). (C-D) Representative micrographs of immunofluorescent BrdU stainings (BrdU⁺ nuclei stained in green, all nuclei stained in blue with DAPI) of CT26 tumors grown in WT mice and Gas6^−/− mice indicate reduced proliferation of tumor cells in Gas6^−/− mice. (E-F) Representative micrographs of immunofluorescent stainings of activated Caspase 3 (activated Caspase 3⁺ nuclei stained in green, all nuclei stained in blue with DAPI) indicating similar numbers of apoptotic cells in CT26 tumors grown in WT mice and Gas6^−/− mice. (G) Recombinant human Gas6 (hrGas6) dose-dependently induces proliferation of CT26 tumor cells in vitro after 48 hours (n = 3; P < .05). (H) Axl-Fc reduced proliferation of CT26 tumor cells induced by 400 ng/mL Gas6 compared with control without Fc (bar 4 vs bar 2; P = .001; n = 3), whereas the addition of control sFC led to a nonsignificant inhibition (bar 5 vs bar 2; P = NS; n = 3). Scale bars represent 100 μm in panels C-F.