The extracellular matrix is a three-dimensional network of proteins, glycosaminoglycans and other macromolecules. It has a structural support function as well as a role in cell adhesion, migration, proliferation, differentiation, and survival. The extracellular matrix conveys signals through membrane receptors called integrins and plays an important role in pituitary physiology and tumorigenesis. There is a differential expression of extracellular matrix components and integrins during the pituitary development in the embryo and during tumorigenesis in the adult. Different extracellular matrix components regulate adrenocorticotropin at the level of the proopiomelanocortin gene transcription. The extracellular matrix also controls the proliferation of adrenocorticotropin-secreting tumor cells. On the other hand, laminin regulates the production of prolactin. Laminin has a dynamic pattern of expression during prolactinoma development with lower levels in the early pituitary hyperplasia and a strong reduction in fully grown prolactinomas. Therefore, the expression of extracellular matrix components plays a role in pituitary tumorigenesis. On the other hand, the remodeling of the extracellular matrix affects pituitary cell proliferation. Matrix metalloproteinase activity is very high in all types of human pituitary adenomas. Matrix metalloproteinase secreted by pituitary cells can release growth factors from the extracellular matrix that, in turn, control pituitary cell proliferation and hormone secretion. In summary, the differential expression of extracellular matrix components, integrins and matrix metalloproteinase contributes to the control of pituitary hormone production and cell proliferation during tumorigenesis.

We studied Smad-4dn tumors generated from lactosomatotrophic GH3 cells stably transfected with a dominant negative form of Smad-4 (a bone morphogenetic protein-4, BMP-4, signal co-transducer) which had reduced tumorigenicity in nude mice, but had showed a late increase in tumor size. We found that they had lost in vivo the expression of Smad-4dn and had recovered c-Myc expression. In accordance, BMP-4 is overexpressed and stimulates the expression of c-Myc in human prolactinomas, but not in other human pituitary adenomas or normal pituitary. In adittion ICI 182,780 inhibited BMP-4 stimulated c-Myc expression and BMP-4 and 17b-estradiol in combination had an additive effect on GH3 cell proliferation. Their action was inhibited by blocking BMP-4 with ICI 182,780 or 17b-estradiol with Smad-4dn. Furthermore, co-immunoprecipitation studies demonstrate that Smad-4 physically interacts with the ERa/ERb. We show for the first time the role of BMP-4 in prolactinoma pathogenesis, involving a functional cross-talk BMP-4/E2.

Analizamos molecularmente tumores formados por clones estables de la línea lactosomatotrofa GH3 Smad-4 (transductor de señales de la proteína morfogénica de hueso-4, BMP-4) dominante negativo (Smad-4dn que bloquea el transductor Smad) que desarrollan tumores de menor tamaño que los controles en ratones nude, pero que presentaron un crecimiento tardío. Encontramos que en éstos la expresión/control de Smad-4dn se perdió y que sobreexpresan c-Myc. Concordantemente, BMP-4 se sobreexpresa y estimula la expresión de c-Myc en prolactinomas humanos pero no en otros adenomas o en hipófisis normales. Además en células GH3, ICI 182,780 (bloqueante de estrógenos) inhibe la estimulación de c-Myc por BMP-4 y el cotratamiento BMP-4/estrógenos posee un efecto aditivo sobre la proliferación celular. Al bloquear BMP-4 con ICI y estrógenos (E2) con Smad-4dn se bloquea significativamente cada efecto estimulatorio sobre la proliferación. A su vez Smad-4 interacciona físicamente con los dos subtipos de receptores de estrógenos, ERa/ERb. Demostramos por primera vez el rol de BMP-4 en la tumorigénesis de prolactinomas, involucrando un crosstalk funcional BMP-4/estrógenos.