Loss of Therapy-Induced Senescence in Myc-Driven Lymphomas Compromises Treatment Outcome In Vivo.

Premature senescence, a terminal cell-cycle arrest condition, reflects an acute cellular stress response upon a variety of cellular insults including oncogene activation and chemotherapeutic DNA damage. Therefore, senescence potentially complements apoptosis as a safeguard program and tumor-suppressive mechanism. Although it is frequently observed in the clinic that patients who only achieve disease stabilization or partial remission in response to chemotherapy may experience lasting freedom from progression, no tumor model has shown that therapy-induced senescence (TIS) significantly contributes to treatment outcome. Here, we employ the Eμ-myc mouse lymphoma model with and without intact alleles of the histone H3 lysine 9 (H3K9) methyltransferase Suv39h1 (controlling senescence) and of p53 (mediating both apoptosis and senescence) to demonstrate a critical role for senescence in cancer therapy in vivo.

Lymphoma cells (LCs), retrovirally transduced with bcl2 to block apoptosis, were treated with the DNA damaging anticancer agent adriamycin (ADR) in vitro, or were exposed to the alkylating agent cyclophosphamide upon lymphoma formation in normal immunocompetent mice in vivo. TIS was analyzed by senescence-associated β-galactosidase activity (SA-β-gal), Ki67 staining and BrdU incorporation. Tumor formation, therapy and progression in vivo was monitored by whole body fluorescence and luciferase imaging and 18F-fluoro-deoxyglucose (FDG) and 18F-fluoro-deoxythymidine (FLT) positron emission tomography (PET). Time-course analysis of glucose and oxygen consumption rates and NAD(P)/NAD(P)H ratios over time were used to define a senescence energy consumption profile. Progression-free and overall survival was evaluated using the Kaplan-Meier method.

Bcl2-protected control (i.e. no further defined genetic defects) LCs, but not Suv39h1- or p53-deficient LCs, enter TIS in vivo as evidenced by uniform SA-β-gal reactivity, high frequency of H3K9-trimethylation-positive cells, and loss of Ki67 staining and BrdU incorporation due to a cell-cycle block in the G1-phase. Notably, Suv39h1- lymphomas recapitulate the proliferation rate and sensitivity to drug-induced apoptosis of control lymphomas, but display significantly shorter progression-free and overall survival after chemotherapy in vivo. Despite their stable growth arrest ADR-senescent control;bcl2 lymphomas exhibited higher glucose metabolism and energy consumption in vitro when compared to their untreated counterparts or senescence-refractory, ADR-treated Suv39h1-;bcl2 lymphomas. Accordingly, TIS LCs can be non-invasively detected by a discordant positive FDG- but negative FLT-PET scan. Discussion: The study demonstrates that Suv39h1 acts as an essential mediator of TIS without compromising apoptosis or altering the proliferative capacity of lymphoma cells. Myc-driven lymphomas senesce in response to DNA damaging anticancer therapies, but display high levels of glucose metabolism. Therefore, they can be detected by FDG-PET despite their resting condition, indicating that a positive post-therapy FDG-PET scan in the clinic does not necessarily reflect a growing tumor lesion. In vivo, Bcl2-protected Suv39h1- lymphoma-bearing mice succumb dramatically fast to their disease after chemotherapy reminiscent of p53null lymphoma-bearing mice. Hence, Suv39h1-controlled TIS is a critical component of anticancer drug therapy in vivo and significantly extends progression-free and overall survival of the host.

No relevant conflicts of interest to declare.