Raf in skin

The Raf family of cytosolic serinehreonine kinases plays a central role in evolutionarily conserved signal transduction pathways. A vaste amount of literature implicates these enzymes in regulated and deregulated proliferation, differentiation, apoptosis, but also in cell adhesion and migration. Ablation of either Raf-1 or B-Raf results in embryonic lethality. To gain information about the function of Raf in specific tissues and in the adult animal, we are using conditional knock-out mouse strains established in our lab (Raf-1) or obtained as a result of collaboration (B-Raf). In the course of a previous project, we have started to elucidate the role of Raf-1 in the epidermis. We chose this experimental system because it is eminently amenable to the study of tissue remodeling and neoplasia, and because constitutive activation of the pathway at study is reportedly associated with regulated and deregulated skin proliferation. We have shown that Raf-1 is not necessary for normal skin homeostasis, whereas it is essential for wound healing and for the development of Ras-induced tumors. The present project has two specific aims: 1) to identify the molecular changes responsible for the delayed wound healing and the lack of tumor development in the Raf-1Dskin mice; and 2) to investigate the impact, if any, of B-Raf ablation on skin development, remodeling, and neoplasia and thereby to examine whether Raf-1 and B-Raf have distinct or redundant functions in this tissue. To achieve these goals, we will combine phenotype analysis in the whole organism (in vivo) as well as in cells (ex vivo) with biochemical experiments in ablated cells to elucidate the molecular basis of the phenotype. The results will help defining the function(s) of Raf-1 and B-Raf in this specific system, as well as their relevant biological targets. The information obtained will be instrumental in assessing the potential of these molecules as (anticancer) therapeutic targets and in directing the design and use of pharmacological kinase inhibitors.

Raf kinases play a central role in signal transduction pathways conserved from flies, or worms, to man. They have been implicated in a variety of biological processes, including cell proliferation, differentiation, death, and motility. Because of this, and because some of the pathway`s components are mutated in human cancer, they are considered attractive therapeutic targets. We are interested in defining the essential biological functions of Raf and the molecules mediating them, to inform the design and use of prospective drugs targeting the pathway. In this project, we have examined the connection between Raf and Ras-mediated tumorigenesis. We have found that two Raf proteins, Raf-1 and B-Raf, counteracts Ras-induced tumorigenesis by two distinct mechanisms. Raf-1 controls cell shape, motility, and tumorigenesis by binding to, and inhibiting, a kinase that controls the cytoskeleton. Remarkably, the presence of Raf-1, but not its function as a kinase, is essential to promote motility, maintain cell shape, and inhibit tumorigenesis. The investigation of the second Raf protein, B-Raf, has yielded the first in vivo demonstration that this kinase is essential for Ras-induced ERK activation and tumorigenesis in the context of an immunocompetent animal. In skin, the cascade is essential to stimulate proliferation. Our data confirm that B-Raf is a candidate for inhibitor therapy, while knockdown therapy maybe a more appropriate approach in the case of Raf-1.