Laura Benjamin Lab
Our research has focused on the study of microvascular development and function. In angiogenesis blood vessels form first by creating an endothelial network for blood flow, which is later remodeled to its final shape and density depending on the metabolic needs of the surrounding tissue. This remodeling, similar to that of neurons in the nervous system, utilizes apoptosis to dispose of superfluous vessels. In previous studies we have investigated survival function and signaling downstream of several major angiogenic cytokines such as VEGF-A, PlGF and the Angiopoietins. Ongoing studies focus on the Akt survival kinase and related regulators of Akt signaling in remodeling and vascular function. To better understand effects of this signaling pathway in the absence of the other downstream effects of angiogenic cytokines, we have created a transgenic mouse model to activate Akt in endothelial cells. These studies utilized a binary transgenic mouse model where gene expression in endothelial cells is regulated by tetracycline. In our first published study using this model, we demonstrated that Akt was downregulated in retinas exposed to oxygen stress, and that similar to injection of VEGF, coincident expression of activated Akt in retinal blood vessels was sufficient to protect retinal blood vessels from oxygen-induced apoptosis (Sun et al, 2004). We have also studied Akt expression in tumor blood vessels and found that unlike the normal adjacent tissues, Akt in tumor endothelial cells is chronically activated. A more complete study of Akt function in the vasculature has recently been completed and we have found that Akt activation at levels equivalent to those in tumor blood vessels recapitulate the unusual features of tumor blood vessels even in normal tissues. For example, these vessels are enlarged and irregular, leaky, and lacking in pericyte support. Their pathological nature eventually leads to death in the animal but even at very late stages is completely reversible when expression is stopped by either tetracycline or rapamycin, an inhibitor of mTOR function. In related studies, we have uncovered a novel mechanisms for modulating Akt signaling in endothelial cells. We have shown that a little-known GTPase protein, RhoB, stabilizes Akt levels and facilitates Akt trafficking to the nucleus (Adini et al, 2003). In a continuation of these studies, we are investigating RhoB interactions with a zinc-finger transcription factor, DB1/VEZF1. Together these proteins regulate transcriptional activation of the VEGF receptor, neuropilin-1 (Nrp1). Both the GTPase function of RhoB and the RhoB-binding domain of DB1/VEZF1 are required for nrp1 expression. The outcome of these functions for RhoB is the regulation of normal vessel development. In developing vascular beds, such as the neonatal retina, RhoB enhances sprouting and early vessel formation. However, in a pathological retina, RhoB is essential for the abnormalities in neovascularization. We can inhibit the pathology of oxygen-induced retinopathy using RhoB null mice, or with a drug that reduces RhoB levels in endothelial cells. In summary, our lab studies the signaling mechanisms in endothelial cells as they relate to normal blood vessel development and physiology, as well as the role of those pathways in pathologies involving angiogenesis such as cancer and retinopathy.
Selected Recent Publication
Sun JF, Phung T, Shiojima I, Felske T, Feng D, Kornaga T, Dor T, Adini, I. Dvorak AM,, Walsh K, and Benjamin LE. Microvascular patterning is controlled by fine-tuning the Akt signal. Track II, PNAS 2005. Jan 4;102 (1):128-33