Rac1 GTPase Targeting Inhibits p53-Deficiency Mediated Lymphomagenesis.

Mutation of the p53 tumor suppressor is linked to therapeutic resistance in several tumor types. p53 mutation is generally associated with disease progression and poor prognosis in patients with lymphoid malignancies and can occur in approximately 50% of Burkitt’s lymphomas [1, 2]. Thus, new therapies are needed to specifically target p53-deficient lymphomas with increased efficacy. Rac1 small GTPase signaling is linked to tumorigenesis and Rac1 activity is increased by p53 loss of function. Previous studies in primary cell culture have shown that suppression of Rac1 activity can preferentially inhibit p53-deficiency induced hyperproliferation and cause increase apoptosis [3], indicating that targeting Rac1 may have therapeutic potential for p53-deficient malignancies. In the current study, the specific impact of inhibition of Rac1 on p53-deficient B and T lymphoma cell proliferation, apoptosis, tumor formation, and spontaneous lymphoma development in in vivo mouse lymphoma models was interrogated. p53-deficiency resulted in increased Rac1 activity in both human B and murine T lymphoma lines. Suppression of Rac activity using a dominant negative mutant or the small molecule inhibitor NSC23766 was able to abrogate p53-deficient lymphoma cell proliferation through a G1 checkpoint as determined by cell cycle analysis. Immunoblot analysis revealed that the anti-proliferative effect of Rac1 targeting in lymphoma cells may involve PAK, LIMK, and Akt signaling pathways rather than the MAP kinase pathway. Further, Rac1 targeting by shRNA resulted in an increase in expression of cleaved caspase 3 and cytochrome c by immunoblot concurrent with an elevation in the annexinV/7-AAD positive apoptotic cell population. These data indicate that loss of Rac1 activity can increase apoptosis via a p53-independent mechanism in p53 mutated lymphoma lines. These effects of inhibition of active Rac1 were extended in vivo where Rac1 targeting was able to impair p53-deficient lymphoma cell growth in xenograft models and postpone lymphomagenesis onset in murine transplant models. Taken together, our studies demonstrate that Rac1 activity is inversely regulated by functional p53 and that Rac1 contributes to p53-deficiency induced hyperproliferation by modulating both cell cycle and apoptosis in B and T lymphomas. Because the Rac signaling axis constitutes a critical determinant of cell survival and tumorigenesis associated with p53 defects, it may represent an important target for therapy in the treatment of p53-deficient lymphomas.