Ras in epidermal proliferation

The Ras family of small GTPases (H-Ras, N-Ras and K-Ras) constitutes a central node in the transmission of mitogenic signals from cell surface receptors to the cell cycle machinery. In general, activation of membrane receptors causes GTP loading of Ras proteins via guanosine nucleotide exchange factor (GEF) stimulation. Loading of GTP promotes conformational changes that ultimately allow binding and subsequent activation of a battery of effector molecules, such as the Raf kinases, the catalytic subunits of phosphatidylinositol 3-kinase (PI3K) or the Ral-GEF family of proteins. Ras-mediated signaling is terminated upon GTP hydrolysis, a reaction that leads to an inactive conformation characterized by a GDP-bound state. This hydrolysis is catalyzed by the intrinsic GTPase activity of Ras proteins, a rather slow reaction that can be accelerated by a familiy of GTPase-activating proteins (GAPs) [1]. To date, little is known about the physiological functions of wild-type Ras proteins, since the majority of investigations focused on the role of their oncogenic counterparts [2]. In addition, the genetic analysis of the role of homeostatic Ras signaling has been complicated by the high redundancy between H-, Nand K-Ras proteins. To overcome this limitation, we have developed mice as well as cell lines that express conditional K-Ras alleles in the absence of H-Ras and N-Ras loci. Ablation of these conditional K-Ras alleles renders cells (and mice) Rasless, that is devoid of all Ras isoforms. Rasless mice die of multiorgan failure in less than two weeks (our unpublished observations). Likewise, Rasless fibroblasts are incapable of proliferating even in the presence of growth factors [3]. In an effort to better understand the role of Ras signaling in vivo, we have focused on the skin epidermis, since this tissue is highly proliferative, and there are well-defined markers for cell proliferation as well as for