Senescence in Breast Cancer Figure 7. Senescence cell progenitor and immortal cell progenitor subtype cell lines correlate with distinct breast tumor subtypes. (A) Unsupervised hierarchical clustering of breast tumor and cell line gene expression data were obtained from Sorlie et al [2] and Charafe-Jauffret et al [25]. Dendrogram displaying the relative organization of tumor and cell line data demonstrated that ICP cell lines cluster with basal and luminal A tumors in the same branch, except for MDA-MB-453. In contrast, SCP cell lines clustered with luminal A (BT-474, CAMA-1, MCF-7, T47D) and ‘‘normal- like’’ tumors (ZR-75-1). No data was available for MDA-MB-468. A dendrogram with sample IDs was provided in Fig. S6. The ‘‘intrinsic gene set’’ data generated by Sorlie et al. [2] was used to filter cell line data generated by Charafe-Jauffret et al. [13]. A set of 175 genes was common between the two data sets. Sixty-eight tumors and 31 cell lines were subjected to pair-wise complete-linkage hierarchical clustering and distance measurements. (B) Immortal cell progenitor MDA-MB-231 was more tumorigenic than the SCP type T47D cell line. Female (n = 5 per cell line) and male (n = 4 per cell line) CD1nude mice received 56106 cells by subcutaneous injection and observed up to 47 days for tumor formation. Chart displays mean tumor sizes (6 S.D.) generated by MDA-MB-231 and T47D cell lines, respectively in female (top) and mala (bottom) mice. T47D cells formed smaller tumors that regressed and completely resolved within 30 days. Tumors formed by MDA-MB-231 were larger in size and did not show total resolution. Note that this cell line was more tumorigenic in male mice. doi:10.1371/journal.pone.0011288.g007 of a large number of progeny for each cell line studied. From these approaches, we draw several important conclusions. First, breast cancer cell lines form two distinct groups: SCP and ICP subtypes. The SCP cell lines produce non-senescent and senescent progeny, whereas the ICP cell lines produce only non-senescent progeny. Second, SCP and ICP cell lines are exclusively ER+ and ER- cell lines, respectively. Senescence occurs as a result of ER loss associated with p21Cip1 induction in SCP cells. Inversely, experimental activation of ER by E2 protects from senescence, whereas its inactivation by tamoxifen aggravates it. Thus, senescence in ER-dependent cells appears to result from the loss of survival signals generated by transcriptional activity of ER. A similar type of senescence has been reported for lymphoma, osteosarcoma, and hepatocellular carcinoma tumors upon c-MYC inactivation [31]. Third, SCP cells generate ER+, CD24+, or CK19+ luminal-like, as well as ASMA+ myoepithelial-like progeny. These findings strongly suggest that most, if not all, SCP cells have the capacity to give rise to two major types of differentiated cells that are found in normal mammary epithelium. In sharp contrast, ICP cells never produce ER+ luminal-like or PLoS ONE | www.plosone.org 10 June 2010 | Volume 5 | Issue 6 | e11288