Hedgehog (HH)/GLI signaling plays a critical role in epidermal development and basal cell carcinoma. Here, we provide evidence that epidermal growth factor receptor (EGFR) signaling modulates the target gene expression profile of GLI transcription factors in epidermal cells. Using expression profiling and quantitative reverse transcriptase PCR, we identified a set of 19 genes whose transcription is synergistically induced by GLI1 and parallel EGF treatment. Promoter studies of a subset of GLI/EGF-regulated genes, including the genes encoding interleukin-1 antagonist IL1R2, Jagged 2, cyclin D1, S100A7, and S100A9, suggest convergence of EGFR and HH/GLI signaling at the level of promoters of selected direct GLI target genes. Inhibition of EGFR and MEK/ERK but not of phosphatidylinositol 3-kinase/AKT abrogated synergistic activation of GLI/EGF target genes, showing that EGFR can signal via RAF/MEK/ ERK to cooperate with GLI proteins in selective target gene regulation. Coexpression of the GLI/EGF target IL1R2, EGFR, and activated ERK1/2 in human anagen hair follicles argues for a cooperative role of EGFR and HH/GLI signaling in specifying the fate of outer root sheath (ORS) cells. We also show that EGF treatment neutralizes GLI-mediated induction of epidermal stem cell marker expression and provide evidence that EGFR signaling is essential for GLI-induced cell cycle progression in epidermal cells. The results suggest that EGFR signaling modulates GLI target gene profiles which may play an important regulatory role in ORS specification, hair growth, and possibly cancer.

No abstract

No abstract

In future, the characterization and isolation of different human stem cells will allow the detailed molecular investigation of cell differentiation processes and the establishment of new therapeutic concepts for a wide variety of diseases. Since the first successful isolation and cultivation of human embryonic stem cells about 10 years ago, their usage for research and therapy has been constrained by complex ethical consideration as well as by the risk of malignant development of undifferentiated embryonic stem cells after transplantation into the patient's body. Adult stem cells are ethically acceptable and harbor a low risk of tumor development. However, their differentiation potential and their proliferative capacity are limited. About 4 years ago, the discovery of amniotic fluid stem cells, expressing Oct-4, a specific marker of pluripotent stem cells, and harboring a high proliferative capacity and multilineage differentiation potential, initiated a new and promising stem cell research field. In between, amniotic fluid stem cells have been demonstrated to harbor the potential to differentiate into cells of all three embryonic germlayers. These stem cells do not form tumors in vivo and do not raise the ethical concerns associated with human embryonic stem cells. Further investigations will reveal whether amniotic fluid stem cells really represent an intermediate cell type with advantages over both, adult stem cells and embryonic stem cells. The approach to generate clonal amniotic fluid stem cell lines as new tools to investigate molecular and cell biological consequences of human natural occurring disease causing mutations is discussed.

p27(Kip1) plays an important role in cell cycle regulation by inhibiting cyclin-CDK complex activity in the nucleus. p27(Kip1) is regulated by its concentration as well as by its subcellular localization. Tuberin, encoded by the tuberous sclerosis tumor suppressor gene TSC2, is a potent negative cell cycle regulator. We show herein, that tuberin induces nuclear p27 localization by inhibiting its 14-3-3-mediated cytoplasmic retention. Tuberin interferes with 14-3-3's counteracting effects on p27-mediated cell cycle arrest. Akt-mediated phosphorylation of p27, but not of tuberin, negatively regulates tuberin's potential to trigger p27 nuclear localization. In G0 cells, tuberin binds p27 triggering downregulation of p27's binding to 14-3-3 and of its cytoplasmic retention. At transition to S phase p27 is phosphorylated by Akt, tuberin/p27 complex levels are downregulated and binding of p27 to 14-3-3 increases triggering cytoplasmic retention of p27. These findings demonstrate p27 localization during the mammalian cell cycle to be under the control of the tumor suppressor tuberin.