(A) Dose-dependent expression of CASTOR1 and CASTOR2 in MM and KMM cells. Western blot analysis of CASTOR1 and CASTOR2 in MM and KMM cells transduced with increasing doses of lentiviruses of CASTOR1, CASTOR2, or vector control at 2, 4, or 6 MOI. Three independent experiments were repeated with similar results, and results from 1 representative experiment are shown. (B and C) Overexpression of either CASTOR1 or CASTOR2 impaired the proliferation and cellular transformation of KMM but not MM cells. MM and KMM cells transduced with different MOI of lentiviruses of CASTOR1, CASTOR2, or vector control, as described in A, were assessed for cell proliferation (B) and colony formation in soft agar (C). Representative images acquired with a x4 objective are shown. Graph shows the quantification of colonies with a diameter of greater than 50 μm. Three independent experiments were repeated with similar results, and results from 1 representative experiment with 4 biological replicates (B) or 3 combined experiments (C) are shown as the mean ± SEM. (D and E) Overexpression of either CASTOR1 or CASTOR2 induced cell-cycle arrest in KMM cells but had a weak effect on MM cells and weak apoptosis in KMM but not MM cells. MM and KMM cells transduced with increasing doses of lentiviruses of CASTOR1, CASTOR2, or a vector control at 2, 4, or 6 MOI for 48 hours were examined for cell-cycle progression (D) and apoptosis (E). Three independent experiments were repeated with similar results, and results from 1 representative experiment with 3 biological replicates are shown as the mean ± SEM. Data were analyzed by 1-way ANOVA followed by Tukey’s post hoc test for P values below 0.05. *P < 0.05, **P < 0.01, and ***P < 0.001.