Raf-1 regulates Rho signaling and cell migration

Fine regulation of RhoA and Rock is required for skeletal muscle differentiation

The RhoA GTPase controls a variety of cell functions such as cell motility, cell growth, and gene expression. Previous studies suggested that RhoA mediates signaling inputs that promote skeletal myogenic differentiation. We show here that levels and activity of RhoA protein are down-regulated in both primary avian myoblasts and mouse satellite cells undergoing differentiation, suggesting that a fine regulation of this GTPase is required. In addition, ectopic expression of activated RhoA in primary quail myocytes, but not in mouse myocytes, inhibits accumulation of muscle-specific proteins and cell fusion. By disrupting RhoA signaling with specific inhibitors, we have shown that this GTPase, although required for cell identity in proliferating myoblasts, is not essential for commitment to terminal differentiation and muscle gene expression. Ectopic expression of an activated form of its downstream effector, Rock, impairs differentiation of both avian and mouse myoblasts. Conversely, Rock inhibition with specific inhibitors and small interfering RNA-mediated gene silencing leads to accelerated progression in the lineage and enhanced cell fusion, underscoring a negative regulatory function of Rock in myogenesis. Finally, we have reported that Rock acts independently from RhoA in preventing myoblast exit from the cell cycle and commitment to differentiation and may receive signaling inputs from Raf-1 kinase.

A balance between Raf-1 and Fas expression sets the pace of erythroid differentiation

Normal erythropoiesis critically depends on the balance between the renewal of precursor cells and their differentiation. If the renewal phase is shortened, the decrease in the precursor pool results in anemia; conversely, impaired differentiation increases the number of proliferating progenitors and the potential risk of leukemic transformation. Using gene ablation, we have discovered 2 self-sustaining signal transduction loops that antagonize each other and regulate erythroid progenitor proliferation and differentiation, respectively. We identify Raf-1 as the main activator of the MEK/ERK cascade and as the key molecule in maintaining progenitor proliferation. Differentiation, in contrast, is mediated by Fas via the activation of both the ASK1/JNK/p38 module and the caspase cascade. The point of convergence between the 2 cascades is activated ERK, which positively feeds back on the proliferation pathway by maintaining the expression of Raf-1, while inhibiting the expression of Fas and therefore differentiation. In turn, Fas, once expressed, antagonizes proliferation by exerting a negative feedback on ERK activation and Raf-1 expression. Simultaneously, Fas-mediated caspase activation precipitates differentiation. These results identify Raf-1 and Fas as the key molecules whose expression finely tunes erythropoiesis and the extent of ERK activation as the switch that tips the balance between them.

Raf-1 sets the threshold of Fas sensitivity by modulating Rok-alpha signaling

Ablation of the Raf-1 protein causes fetal liver apoptosis, embryonic lethality, and selective hypersensitivity to Fas-induced cell death. Furthermore, Raf-1–deficient cells show defective migration as a result of the deregulation of the Rho effector kinase Rok-α. In this study, we show that the kinase-independent modulation of Rok-α signaling is also the basis of the antiapoptotic function of Raf-1. Fas activation stimulates the formation of Raf-1–Rok-α complexes, and Rok-α signaling is up-regulated in Raf-1–deficient cells. This leads to increased clustering and membrane expression of Fas, which is rescued both by kinase-dead Raf-1 and by interfering with Rok-α or its substrate ezrin. Increased Fas clustering and membrane expression are also evident in the livers of Raf-1–deficient embryos, and genetically reducing Fas expression counteracts fetal liver apoptosis, embryonic lethality, and the apoptotic defects of embryonic fibroblasts. Thus, Raf-1 has an essential function in regulating Fas expression and setting the threshold of Fas sensitivity during embryonic life.

A role for RKIP in cell motility

The target of locostatin, a small-molecule inhibitor of cell movement, has been identified as RKIP, a Raf-1 kinase modulator [1]. In addition to advancing our understanding of cell locomotion, this work represents a major landmark in the development of chemical genetics.

Essential role of B-Raf in ERK activation during extraembryonic development

The kinases of the Raf family have been intensively studied as activators of the mitogen-activated protein kinase kinase/extra-cellular signal-regulated kinase (ERK) module in regulated and deregulated proliferation. Genetic evidence that Raf is required for ERK activation in vivo has been obtained in lower organisms, which express only one Raf kinase, but was hitherto lacking in mammals, which express more than one Raf kinase. Ablation of the two best studied Raf kinases, B-Raf and Raf-1, is lethal at midgestation in mice, hampering the detailed study of the essential functions of these proteins. Here, we have combined conventional and conditional gene ablation to show that B-Raf is essential for ERK activation and for vascular development in the placenta. B-Raf-deficient placentae show complete absence of phosphorylated ERK and strongly reduced HIF-1alpha and VEGF levels, whereas all these parameters are normal in Raf-1-deficient placentae. In addition, neither ERK phosphorylation nor development are affected in B-raf-deficient embryos that are born alive obtained by epiblast-restricted gene inactivation. The data demonstrate that B-Raf plays a nonredundant role in ERK activation during extraembyronic mammalian development in vivo.

Coordinating ERK/MAPK signalling through scaffolds and inhibitors

The pathway from Ras through Raf and MEK (MAPK and ERK kinase) to ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) regulates many fundamental cellular processes. Recently, a number of scaffolding proteins and endogenous inhibitors have been identified, and their important roles in regulating signalling through this pathway are now emerging. Some scaffolds augment the signal flux, but also mediate crosstalk with other pathways; certain adaptors target MEK-ERK/MAPK complexes to subcellular localizations; others provide regulated inhibition. Computational modelling indicates that, together, these modulators can determine the dynamic biological behaviour of the pathway.

Second nature: Biological functions of the Raf-1 “kinase”

More than 20 years ago, Raf was discovered as a cellular oncogene transduced by transforming retroviruses. Since then, the three Raf isoforms have been intensively studied, mainly as the kinases linking Ras to the MEK/ERK signaling module. As this pathway is activated in human cancer, the Raf kinases are considered promising therapeutic targets, and we have learned a lot about their regulation, targets, and functions. Do they still hold surprises? Recent gene targeting studies indicate that they do. This review focuses on the regulation and biology of the best-studied Raf isoform, Raf-1, in the context of its kinase-independent functions.