Angiogenesis in pituitary adenomas: human studies and new mutant mouse models

Abstract

The role of angiogenesis in pituitary tumor development has been questioned, as pituitary tumors have been usually found to be less vascularized than the normal pituitary tissue. Nevertheless, a significantly higher degree of vasculature has been shown in invasive or macropituitary prolactinomas when compared to noninvasive and microprolactinomas. Many growth factors and their receptors are involved in pituitary tumor development. For example, VEGF, FGF-2, FGFR1, and PTTG, which give a particular vascular phenotype, are modified in human and experimental pituitary adenomas of different histotypes. In particular, vascular endothelial growth factor, VEGF, the central mediator of angiogenesis in endocrine glands, was encountered in experimental and human pituitary tumors at different levels of expression and, in particular, was higher in dopamine agonist resistant prolactinomas. Furthermore, several anti-VEGF techniques lowered tumor burden in human and experimental pituitary adenomas. Therefore, even though the role of angiogenesis in pituitary adenomas is contentious, VEGF, making permeable pituitary endothelia, might contribute to adequate temporal vascular supply and mechanisms other than endothelial cell proliferation. The study of angiogenic factor expression in aggressive prolactinomas with resistance to dopamine agonists will yield important data in the search of therapeutical alternatives.

Figure 1

Disruption of the D2R from lactotropes induces paracrine VEGF secretion from follicle stellate cells. VEGF acts on VEGFR2 receptors on endothelial cells to induce pituitary angiogenesis.

Figure 2

Total (Drd2 ^−/−) or lactotrope (LacDrd2KO) disruption of D2R evokes and increases in pituitary VEGF and CD31, with no alteration of PTTG or FGF2 content. Pituitary angiogenesis correlates with pituitary hyperplasia and increased prolactin synthesis and release.