Umbilical cord blood-derived mesenchymal stem cells are a promising source of cells for regeneration therapy due to their multipotency, high proliferative capacity, relatively noninvasive collection, and ready availability. However, extended cell culture inevitably triggers cellular senescence-the irreversible arrest of cell divisionthereby limiting the proliferative lifespan of adult stem cells. Wnt/b-catenin signaling plays a functional role as a key regulator of self-renewal and differentiation in mesenchymal stem cells (MSCs), and thus Wnt/b-catenin signaling and cellular senescence might be closely connected. Here, we show that the expression levels of canonical Wnt families decrease as MSCs age during subculture. Activation of the Wnt pathway by treatment with Wnt3a-conditioned medium or glycogen synthase kinase 3b inhibitors, such as SB-216763 and 6-bromoindirubin-3¢-oxime, delays the progression of cellular senescence as shown by the decrease in the senescence effectors p53 and pRb, lowered senescence-associated b-galactosidase activity, and increased telomerase activity. In contrast, suppression of the Wnt pathway by treatment with dickkopf-1 (an antagonist of the Wnt coreceptor) and bcatenin siRNA transfection promotes senescence in MSCs. Interestingly, the magnitude of the response to enhanced Wnt3a/b-catenin signaling appears to depend on the senescent state during extended culture, particularly after multiple passages. These results suggest that Wnt3a signaling might be a predominant factor that could be used to overcome senescence in long-term cultured MSCs by directly intervening in the proliferative capacity and MSC senescence. The functional role of Wnt3a/b-catenin signaling in hedging cellular senescence may allow the development of new approaches for stem cell-based therapies.

Autophagy is frequently activated in radioresistant cancer cells. Rapamycin, mammalian target of rapamycin (mTOR) inhibitor, activates autophagy but enhances radiosensitivity. The mechanism of these actions by which such opposing functions coexist was investigated on radiation-resistant cancer cell lines (AMC-HN-9 and U-87) and the antitumor activity was evaluated in mice bearing xenografts of the cancer cells. Enhanced autophagic flux induced by radiation returned to untreated control levels. Treatment of the cancer cells with rapamycin leads to the potentiation and prolongation of radiation-induced autophagy, the increases in senescence-associated β-galactosidase activity, heterochromatin formation, and irreversible growth arrest. Furthermore, rapamycine resulted in a tumor regrowth delay and increased the level of β-galactosidase staining and the expression of heterochromatin markers in irradiated xenografts. These results suggest that even though autophagy is a survival mechanism in radioresistant cells, a persistent activation of autophagy by mTOR inhibitor induces premature senescence in these cells, eventually making the cells radiosensitive. Our data suggest a novel mechanism by which an inhibition of mTOR pathway increases autophagy but paradoxically increases radiosensitivity in radioresistant cancer cells. Citation Format: Hae Yun Nam, Myung Woul Han, Hyo Won Chang, Yoon Sun Lee, Myungjin Lee, Mi Ra Kim, Hyang Ju Lee, Ji Yung Jeoung, So Young Moon, Hyo Jung Kim, Sang Yoon Kim, Seong Who Kim. Rapamycin increases radiosensitivity of cancer cells by induction of premature senescence. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 81. doi:10.1158/1538-7445.AM2013-81