Raf-1 Keeps Developing Blood Vessel Cells Grouped Together

The elegant interaction between two molecular signaling pathways is critical to the process by which new blood vessels are formed (sprouting angiogenesis), and drugs that interfere with this interaction are being evaluated as possible chemotherapeutic agents.

Investigators at the University of Vienna (Austria) and the Medical University of Vienna (Austria) used video microscopy of developing blood vessels in vitro to study the interaction between the enzymes Raf-1(rapidly accelerated fibrosarcoma protein) and Rok-alpha.

Raf-1 is a MAP (mitogen-activated protein) kinase kinase kinase (MAP3K), which functions downstream of the Ras family of membrane associated GTPases to which it binds directly. Once activated, the cellular RAF1protein can phosphorylate to activate the dual specificity protein kinases MEK1 and MEK2, which in turn phosphorylate to activate the serine/threonine specific protein kinases, ERK1 and ERK2. Activated ERKs are effectors of cell physiology and play an important role in the control of gene expression involved in the cell division cycle, apoptosis, cell differentiation, and cell migration.

Rok-alpha is a serine/threonine kinase that regulates cytokinesis, smooth muscle contraction, the formation of actin stress fibers and focal adhesions, and the activation of the c-fos serum response element.

New findings published in the December 29, 2011, online edition of the journal Developmental Cell revealed that that Raf-1 was an essential component of the angiogenesis regulatory network, and that its removal impaired endothelial cell cohesion, sprouting, and tumor-induced angiogenesis. Mechanistically, Raf-1 was recruited to VE-cadherin complexes by a mechanism involving the small G protein Rap1 and was required to bring the Rho effector Rok-alpha to emerging adherens junctions (sites of connection between cells). This Raf-1-mediated fine-tuning of Rok-alpha signaling allowed the activation of junctional myosin and the timely maturation of adherens junctions essential for maintaining cell cohesion during sprouting angiogenesis.

“The real breakthrough came when we were able to use video microscopy on the developing vessels in vitro,” explained first author Reiner Wimmer, a researcher at the University of Vienna. “We realized that the cells without Raf-1 were actively migrating, but only as single cells. They could not migrate in a group.”

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University of Vienna
Medical University of Vienna