Regulation of angiogenesis by CPEB-mediated translational control

Abstract

[spa]En muchas enfermedades hepáticas cr6nicas, la angiogénesis es una importante característica patológica y juega un papel crucial en la progresión de la fibrogénesis hepática a cirrosis, y en la aparición y agravamiento de la hipertensión portal, la cual determina las principales complicaciones de la enfermedad. A pesar de que es evidente que VEGF es el principal efector de la angiogénesis patológica, los mecanismos moleculares que gobiernan la activación post-transcripcional de su síntesis durante la cirrosis hepática son en gran parte desconocidos. En este trabajo se muestra que la síntesis de VEGF está regulada a través de funciones secuenciales y no redundantes de dos miembros de la familia de las proteínas CPEB:. CPEB1 y CPEB4. Por 10 tanto, CPEB1 promueve el procesamiento alternativo de ambos los pre-ARNm de CPEB4 y VEGF, acortando las 3'UTRs y excluyendo elementos de inhibición de la traducción de los transcritos maduros. Como resultado de este procesamiento alternativo, CPEB4 se sobreexpresa, y polyadenyla el ARNm de VEGF, aumentando aún más su traducción. Entonces, se requieren tanto CPEB1 como CPEB4 para la síntesis de VEGF y la consecuente angiogénesis. Por tanto, todas las proteínas se sobreexpresan de forma secuencial en pacientes y en modelos animales de cirrosis hepática e hipertensión portal, y ambos ratones knock-out para CPEB1 y CPEB4 no lograron activar la angiogénesis tras la inducción de hipertensión portal. A través del análisis de la angiogénesis en ensayos in vitro, las muestras de humanos y modelos animales, nuestros resultados ponen de relieve el papel crucial de CPEBs en la neovascularización patol6gica, en el marco de la hipertensión portal y cirrosis, e identifican CPEBs como potenciales nuevas dianas moleculares para el tratamiento de la enfermedad hepática cr6nica y otras enfermedades dependientes de la neovascularización, como el cáncer.

[eng]Hepatic cirrhosis is a largely diffused pathology caused by alcohol abuse and hepatitis C in developed countries, whereas hepatitis B is the cause in most parts of Asia and sub-Saharan Africa. It's characterized by development of regenerative nodules delimited by fibrous septa. Regenerative nodules are composed by proliferating hepatocytes, entrapped by deposition of extracellular matrix, and have been shown to be involved in growth factor production, in particular VEGF, which is responsible of angiogenic induction that culminates with formation of a dense network of blood vessels into fibrous septa. The newly formed blood vessels of fibrous septa connect the vessels of the portal region with terminal hepatic veins, determining an alternative route of the blood flow that, in this way, is redirected into the systemic circulation bypassing the liver. At pre-hepatic level, the cirrhosisrelated portal hypertension induces development of new blood vessels that shunt the portal blood into the systemic circulation. As consequence of both intrahepatic and pre-hepatic angiogenesis, the liver cannot metabolize several blood components such as drugs, nutrients, toxins, and bacteria, with obvious deleterious effects. Despite angiogenesis is one of the main complications of liver cirrhosis and VEGF has a pivotal role in the control of blood vessels formation, the molecular mechanisms that govern VEGF expression during angiogenesis and hepatic cirrhosis are poorly understood. In order to address whether CPEB family of proteins may have a function in the regulation of angiogenesis in liver diseases, we analysed the expression of CPEBl and CPEB4 in liver of patients affected by hepatic cirrhosis. The expression of CPEBl and CPEB4 was increased in regenerative nodules, compared with basal levels of expression detected in healthy hepatic parenchyma. Interestingly, also VEGF expression was higher in regenerative nodules. The numerous fibrous septa that characterized cirrhotic livers resulted highly vascularized, and the endothelium of newly formed blood vessels expressed high levels of CPEB1, CPEB4 and VEGF. The description of CPEB1, CPEB4 and VEGF expression in healthy and cirrhotic conditions represented a first cue of CPEB-mediated translational regulation of VEGF mRNA during angiogenesis. To prove the accuracy of this hypothesis we implemented two animal models that allowed the study of intrahepatic and prehepatic angiogenesis correlated with liver cirrhosis. CBDL experiments performed in rats enabled to recapitulate the histopathological conditions observed in human hepatic cirrhosis. Cirrhotic rat livers showed deep histological perturbations, with high proliferation of blood vessels and biliary ducts. Compared with control healthy samples, the expression of CPEB1, CPEB4 and VEGF in pathological liver was strongly increased. These proteins localized at level of both, blood vessels and biliary ducts. Partial portal vein ligation (PPVL) experiments performed in rats enabled to show an important correlation between CPEBl and CPEB4 expression with VEGF synthesis and consequent high vascularization of mesentery. In angiogenic condition, both pre-existing and newly formed blood vessels expressed CPEB1, CPEB4 and VEGF at level of endothelium, smooth muscle and adventitia, tissues that playa pivotal role in the development of the vascular net. To better define the mechanistic relevance of these correlations, we characterized the endothelial cell line HSV. In this in vitro model we modulated the levels of CPEBl and CPEB4, and showed a direct involvement of this two proteins in the translational regulation of VEGF mRNA. Taking advance of the ability of HSV cells to form blood vessel like structure in an in vitro angiogenesis assay, we were able to show that CPEBl and CPEB4 are required for VEGF synthesis and secretion, which in turn are essential to create the correct microenvironment necessary to activate the cells and induce the formation of a dense network of vascular structures on Matrigel. Our results suggest that CPEBl drives the nuclear cleavage of VEGF 3'UTR while CPEB4 is responsible of its cytoplasmic polyadenylation.