CDKN2B's effects on remodeling and apoptosis are dependent on p53. In vitro, inhibition of both CDKN2B and p53 eliminated the difference in apoptosis while enhancing the difference in proliferation. (A) Inhibition of p53 with pifithrin-α accentuated the ratio of cell proliferation in CDKN2B deficient HCASMC relative to control transfected cells (siCDKN2B/siCtrl), compared to the baseline ratio. In contrast, treatment with pifithrin-α eliminated the difference in staurosporine-induced apoptosis that was noted at baseline, as assessed by (B) caspase 3/7 activity, (C) TUNEL positivity, and (D) annexin-V positivity by FACS analysis. In vivo, p53 inhibition also neutralized the difference in apoptosis between Cdkn2b-/- (n= 6) and Cdkn2b+/+ mice (n= 7), and led to a reversal of the remodeling phenotype compared to baseline. (E) One week after carotid ligation, the ratio of TUNEL positive cells between theCdkn2b-/- and Cdkn2b+/+ genotypes (Cdkn2b-/-/Cdkn2b+/+) had normalized in mice treated with intraperitoneal pifithrin-α, compared to baseline where the knockout mice had nearly double the rate of apoptosis (n= 5 for each). (F) Four weeks after ligation, Cdkn2b-/- mice treated with intraperitoneal pifithrin-α had larger neointimal areas and I/M ratios than wild type mice also treated with the p53 inhibitor, showing a reversal of phenotype after inhibiting the p53 pathway. (G) Similarly, aneurysm expansion was no longer different between genotypes after pifithrin-α treatment, (P > 0.7) at each timepoint (n=6 vs. 5).