Small intestine, Stat3 is absolutely required for survival of the stem cells near the base of the crypt [7] and expression of dominant negative Stat3 in hematopoietic stemcells results in a reduced lympho-myeloid reconstituting ability [8]. In the mammary gland Stat3 is activated early during postlactational regression and is a major regulator of the extensive cell death and tissue remodelling that occurs during this process [9,10]. Recently, we demonstrated that activation of Stat3 is required during mammary gland involution to upregulate the expression of the lysosomal proteases, cathepsins B and L, and to downregulate the expression of their endogenous cytoplasmic inhibitor (Spi2A) thereby mediating cell death [11]. However, a potential role for Stat3 in mammary stem cells has not been determined. Mammary epithelium consists of luminal (ductal and alveolar) and basal (myoepithelial) cells that are organised into a bi-layered structure with luminal cells 22948146 lining the lumen encased by an outer layer of basal cells [12]. It is presumed that both luminal and basal lineages originate from common embryonic stem and progenitor cells. Moreover, each pregnancy cycle is accompanied by the massive expansion of the mammary 
epithelial compartment which suggests that the adult mammary gland contains a population of stem/progenitor cells with long-term self-renewal potential [13]. Previous reports have confirmed that mammary stem cells transplanted into a cleared fat pad can regenerate 25837696 a functional mammary epithelial tree [14,15,16,17]. Moreover, each full-term pregnancy cycle generates so called parity-induced mammary epithelial cells (PI-MECs) that produce milk proteins during late gestation and lactation and do not undergo programmed cell death during involution. Some of these cells act as alveolar progenitors during subsequent pregnancies and in vivo transplantation experiments proved their multipotency and self renewalStat3 and Mammary Stem CellsFigure 1. Stat3fl/fl;BLG-Cre+ glands show incomplete involution and luminal progenitors have reduced proliferative capacity. (A) RTPCR analysis of Stat3 expression in FACS sorted populations of mammary epithelial cells. MRU: mammary repopulating units. (B, C) H E staining of sections of Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ mammary glands collected at day 5 of the second gestation (B) or four weeks after natural weaning (C). (D) Western blot analysis of four Stat3fl/fl;BLG-Cre2 and five Stat3fl/fl;BLG-Cre+ mammary glands four weeks after natural weaning for the expression or activation of Stat5, Erk, Akt, b-casein and WAP. b-actin was used as a loading control. (E) Immunohistochemistry staining for pStat5 (red) and E-cadherin (green) in mammary gland sections from Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ mice collected four weeks after natural weaning. Nuclei were stained with Hoechst 33342 (blue). (F) Flow cytometry analysis of luminal progenitors isolated from mammary glands of Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ females four weeks after natural weaning. (G) In vitro colony forming analysis performed on CD24+ CD49fhi CD61+ luminal progenitor cells sorted from Stat3fl/fl;BLG-Cre2 
and Stat3fl/fl;BLG-Cre+ mammary glands. Points represent the value for each mouse and lines depict mean values for each group. p value was determined using Student’s t test, * p,0.05. doi:10.1371/journal.pone.0052608.gcapacity [18,19]. Furthermore, these PI-MECs were shown to express cell surface markers that a.Small intestine, Stat3 is absolutely required for survival of the stem cells near the base of the crypt [7] and expression of dominant negative Stat3 in hematopoietic stemcells results in a reduced lympho-myeloid reconstituting ability [8]. In the mammary gland Stat3 is activated early during postlactational regression and is a major regulator of the extensive cell death and tissue remodelling that occurs during this process [9,10]. Recently, we demonstrated that activation of Stat3 is required during mammary gland involution to upregulate the expression of the lysosomal proteases, cathepsins B and L, and to downregulate the expression of their endogenous cytoplasmic inhibitor (Spi2A) thereby mediating cell death [11]. However, a potential role for Stat3 in mammary stem cells has not been determined. Mammary epithelium consists of luminal (ductal and alveolar) and basal (myoepithelial) cells that are organised into a bi-layered structure with luminal cells 22948146 lining the lumen encased by an outer layer of basal cells [12]. It is presumed that both luminal and basal lineages originate from common embryonic stem and progenitor cells. Moreover, each pregnancy cycle is accompanied by the massive expansion of the mammary epithelial compartment which suggests that the adult mammary gland contains a population of stem/progenitor cells with long-term self-renewal potential [13]. Previous reports have confirmed that mammary stem cells transplanted into a cleared fat pad can regenerate 25837696 a functional mammary epithelial tree [14,15,16,17]. Moreover, each full-term pregnancy cycle generates so called parity-induced mammary epithelial cells (PI-MECs) that produce milk proteins during late gestation and lactation and do not undergo programmed cell death during involution. Some of these cells act as alveolar progenitors during subsequent pregnancies and in vivo transplantation experiments proved their multipotency and self renewalStat3 and Mammary Stem CellsFigure 1. Stat3fl/fl;BLG-Cre+ glands show incomplete involution and luminal progenitors have reduced proliferative capacity. (A) RTPCR analysis of Stat3 expression in FACS sorted populations of mammary epithelial cells. MRU: mammary repopulating units. (B, C) H E staining of sections of Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ mammary glands collected at day 5 of the second gestation (B) or four weeks after natural weaning (C). (D) Western blot analysis of four Stat3fl/fl;BLG-Cre2 and five Stat3fl/fl;BLG-Cre+ mammary glands four weeks after natural weaning for the expression or activation of Stat5, Erk, Akt, b-casein and WAP. b-actin was used as a loading control. (E) Immunohistochemistry staining for pStat5 (red) and E-cadherin (green) in mammary gland sections from Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ mice collected four weeks after natural weaning. Nuclei were stained with Hoechst 33342 (blue). (F) Flow cytometry analysis of luminal progenitors isolated from mammary glands of Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ females four weeks after natural weaning. (G) In vitro colony forming analysis performed on CD24+ CD49fhi CD61+ luminal progenitor cells sorted from Stat3fl/fl;BLG-Cre2 and Stat3fl/fl;BLG-Cre+ mammary glands. Points represent the value for each mouse and lines depict mean values for each group. p value was determined using Student’s t test, * p,0.05. doi:10.1371/journal.pone.0052608.gcapacity [18,19]. Furthermore, these PI-MECs were shown to express cell surface markers that a.