Effect of PQQ on cell cycle proteins of tibiae in Bmi1-/- mice. (A) Representative tibiae western blot for expression of P16, P19, P21, P27 and P53. β-actin was used as loading control for western blot in WT mice, BKO mice and BKO+PQQ mice respectively. P16 (B), P19 (C), P21 (D), P27 (E) and P53 (F) proteins levels relative to β-actin protein levels were assessed by densitometric analysis and expressed relative to levels of WT mice. Each value is the mean ± SEM of determinations in six animals of the same groups. *, P<0.05; **, P<0.01; ***, P<0.001, compared with WT mice; #, P<0.05; ##, P<0.01; ###, P<0.001, compared with BKO mice.