Identification of Scaffold Proteins RACK1 and Hic-5 as Regulators of Endothelial Sprouting Resoponses

Angiogenesis is defined as growth of new blood vessels from pre-existing ones and occurs during normal physiological development as well as pathological conditions like cancer. During angiogenesis, normally quiescent endothelial cells (ECs) are activated in response to external pro-angiogenic cues and undergo rapid morphogenic changes such as proliferation, migration, invasion, and lumen formation to extend new sprouts into surrounding three-dimensional (3D) matrix. To better understand angiogenic regulation, we utilized an in vitro model wherein sphingosine 1-phosphate (S1P) and pro-angiogenic growth factors (GF) synergize to induce rapid and robust endothelial sprouting in 3D collagen matrices. In a proteomic screen designed to identify molecules relevant to angiogenesis, we found up-regulated levels of receptor for activated C kinase 1 (RACK1) and the intermediate filament protein, vimentin. RACK1 depletion reduced EC invasion. Silencing of vimentin or RACK1 decreased cell adhesion and attenuated with focal adhesion kinase (FAK) activation, which is indispensable for successful angiogenesis. Moreover, pro-angiogenic GFs enhanced RACK1 and vimentin association. RACK1, vimentin, and FAK, formed an intermolecular complex during S1P- and GF- induced invasion. Also, depletion of RACK1 decreased vimentin and FAK association, suggesting a role for RACK1 in stabilizing vimentin-FAK interactions during sprouting. In an independent study, we identified focal adhesion (FA) scaffold protein, hydrogen peroxide inducible clone 5 (Hic-5), as a critical regulator of angiogenesis. Hic-5 depletion interfered with endothelial invasion and lumen formation. S1P induced rapid Hic-5 translocation to FAs and Hic-5 silencing attenuated FAK expression and activation. S1P induced a novel interaction between Hic-5 and membrane type 1 matrix-metalloproteinase (MT1-MMP). In vitro binding experiments revealed that LIM2 domain of Hic-5 was required for MT1-MMP binding. Moreover, Hic-5 and MT1-MMP levels were up-regulated in detergent resistant membrane fractions of invading ECs, indicative of their crosstalk. Hic-5 silencing interfered with S1P- induced MT1-MMP membrane translocation, a critical event for successful angiogenesis. Since MT1-MMP and FAK interaction has been reported to be essential for matrix degradation at FA sites, we further tested if Hic-5 mediated this interaction. Our results indicated that presence of Hic-5 significantly enhanced FAK and MT1-MMP complex formation. In conclusion, we report that scaffold proteins RACK1 and Hic-5 regulate successful endothelial sprouting responses in 3D matrices