CDK4 kinase activity is required for enhanced mitochondrial respiration and mitochondria-mediated anti-apoptosis. (A) HK18 cells were co-transfected with MTS-CDK4-wt or MTS-CDK4-dn along with Cyclin D1, and p-serine-MnSOD levels were measured at 6h after radiation in stable transfectants further transfected with MnSOD-flag. (B-G) HK18 cells co-transfected with MTS-CDK4-wt or MTS-CDK4-dn along with Cyclin D1 were treated with 10 cGy radiation and collected at 6h after radiation to measure (B) MnSOD activity, (C) mitochondria superoxide, (D) apoptosis, (E) mitochondrial membrane potential, (F) clonogenic survival assay, (G) oxygen consumption and (H) mitochondrial ATP production. MTS, mitochondria targeting sequence; wt, wild type CDK4; dn, D158N dominant negative mutant. GFP and RFP were used to confirm equal amount of inputs. Mean ± S.E.M.; *P < 0.05; n ≥ 3.