Abstract
Premature senescence is a permanent proliferative arrest that occurs in response to oncogenic signaling or DNA-damaging chemotherapy. Although tumor cell senescence has been recognized as a prognostically relevant contribution to long-term outcome post-therapy in hematological tumor models, therapeutic utilization of senescence is still hampered by an incomplete understanding of biological properties and long-term fate of senescent tumor cells in patients. Interestingly, senescence-regulating factors have recently been shown to limit reprogramming of somatic cells to pluripotency, and to protect stem cell compartments from premature exhaustion. Hence, we explore here whether cellular senescence and stemness may functionally overlap, thereby potentially equipping arrested cells with latent self-renewing potential.
Stem cell-related features (stem cell gene signatures, Sca-1 expression, ALDH and ABC transporter activity) were analyzed in primary apoptosis-blocked Eµ-myc transgenic B-cell lymphomas, which enter treatment-induced senescence (TIS) in response to standard antineoplastic agents. Several (e.g. Suv39h1- or p53-based) genetic models were established, in which TIS occurred in a conditional and reversible fashion. Clonogenicity, proliferative and repopulating assays were performed in vitro and in vivo, comparing individual lymphomas that grew out of senescence (“Previously Senescent”, PS) with matched lymphoma cells that equally received chemotherapy, but were incapable of entering TIS (“Never Senescent”, NS). In a mouse model of T-cell acute lymphoblastic leukemia (T-ALL), stem cell markers and tumor initiating potential were assessed in a flow-sorted non-self-renewing leukemia cell population after senescence induction by chemotherapy. Human hematological cancer cell lines and tumor samples obtained from B-cell lymphoma and acute myeloid leukemia (AML) patients were analyzed for stem cell features after exposure to senescence-inducing chemotherapy in vitro.
Senescent mouse lymphomas were strongly skewed towards an increased expression of an adult tissue stem cell signature, distinct stem cell markers and functional stemness properties. Upon release from conditional senescence, PS cells resumed proliferation and rapidly exceeded the proliferative, clonogenic and tumor-initiating capacity of NS cells. Interrogation of self-renewal-relevant cascades revealed activation of and dependence on canonical Wnt signaling in senescence, as blocking of this pathway reduced the growth of PS, but not NS cells. Moreover, TIS-related stemness occurred independent of secretable factors. Strikingly, in a murine T-ALL model, temporary senescence enforcement re-programmed non-stem leukemia cells into leukemia stem cells, allowing PS bulk leukemia cells to de novo initiate leukemias in recipient mice. These results were supported by consistent findings in human hematological cancer cell lines as well as primary human B-cell lymphoma and acute myeloid leukemia samples.
Our findings uncover senescence-associated stemness as a detrimental capability which is latently enriched for by chemotherapy in lymphoma and leukemia. The aggressive growth potential might become evident when senescent cells occasionally acquire alterations that allow them to re-enter the cell-cycle, thereby unleashing the tumor-promoting potential of a biological program so far considered to operate as a tumor-suppressive mechanism. However, targeted intervention at stemness-related signaling cascades in senescence may open novel therapeutic options for apoptosis-resistant lymphoma and leukemia.
No relevant conflicts of interest to declare.
Author notes
Asterisk with author names denotes non-ASH members.
