Genetic background determines if Stat5b suppresses or enhances murine hepatocarcinogenesis.

Murine hepatocarcinogenesis requires growth hormone (GH). To determine if the GH-responsive transcription factor STAT5b (signal transducer and activator of transcription 5b) is also required, we compared the hepatic gene expression profiles of global Stat5b null mice to cancer-resistant mice mutant in the GH pathway-GH-deficient little and androgen receptor-null Tfm males. We found a high degree of overlap among Tfm, little, and Stat5b null males. The liver cancer susceptibility of global Stat5b null mice was assessed on three distinct genetic backgrounds: BALB/cJ (BALB), C57BL/6J (B6), and C3H/HeJ (C3H). The effect of Stat5b on hepatocarcinogenesis depended on the genetic background. B6 Stat5b null congenic males and females developed 2.4 times as many tumors as wild-type (WT) controls (P < 0.002) and the tumors were larger (P < 0.003). In BALB/c congenics, loss of STAT5b had no effect on either sex. C3H Stat5b null congenic males and females were resistant to liver cancer, developing 2.7- and 6-fold fewer tumors, respectively (P < 0.02, 0.01). These results provide the first example of a single gene behaving as both oncogene and tumor suppressor in a given tissue, depending only on the endogenous modifier alleles carried by different genetic backgrounds.

Antioxidant proteins and reactive oxygen species are decreased in a murine epidermal side population with stem cell-like characteristics.

Reactive oxygen species (ROS) and antioxidants are essential to maintain a redox balance within tissues and cells. Intracellular ROS regulate key cellular functions such as proliferation, differentiation and apoptosis through cellular signaling, and response to injury. The redox environment is particularly important for stem/progenitor cells, as their self-renewal and differentiation has been shown to be redox sensitive. However, not much is known about ROS and antioxidant protein function in freshly isolated keratinocytes, notably the different keratinocyte subpopulations. Immunostaining of neonatal cutaneous sections revealed that antioxidant enzymes [catalase, SOD2, gluthatione peroxidase-1 (GPx)] and ROS are localized predominantly to the epidermis. We isolated keratinocyte subpopulations and found lower levels of SOD2, catalase and GPx, as well as decreased SOD and catalase activity in an epidermal side population with stem cell-like characteristics (EpSPs) compared to more differentiated (Non-SP) keratinocytes. EpSPs also exhibited less mitochondrial area, fewer peroxisomes and produced lower levels of ROS than Non-SPs. Finally, EpSPs were more resistant to UV radiation than their progeny. Together, our data indicate ROS and antioxidant levels are decreased in stem-like EpSPs.

Murine epidermal side population possesses unique angiogenic properties.

Total epidermal keratinocytes are a heterogeneous population of cells, including undifferentiated stem/progenitor cells (EpSPs) and their more differentiated progeny (Non-SP cells). Our previous in vivo data showed that EpSPs enhanced blood flow restoration when injected into an ischemic limb, whereas Non-SP cells had no significant effect on in vivo blood flow restoration. However, the cellular and molecular mechanisms of this observation remain largely unknown. Therefore, the aim of this study was to investigate the angiogenic properties of different epidermal subpopulations in vitro and the mechanism by which EpSPs enhanced blood flow in vivo. Using migration assay and capillary network formation, we show that EpSPs secrete higher levels of pro-angiogenic molecules compared to Non-SP cells, unsorted keratinocytes and fibroblasts in vitro. Secretion of vascular endothelial growth factor (VEGF) was detected at higher levels in EpSP conditioned medium than the medium conditioned by other epidermal subpopulations and fibroblasts. Also, RT-PCR analyses revealed a unique angiogenic gene profile for EpSPs. Finally, gene array data indicate significant changes in angiogenic gene expression six days after cell injection in murine ischemic limbs. Therefore, we conclude that EpSPs possess unique angiogenic properties and that these cells may be indirectly responsible for the angiogenic response previously observed in our ischemic limb model.