Localization and functional study of VEGF receptors in normal and adenomatous pituitary: evidence for a non-angiogenic role of VEGF

The role of members of the Vascular Endothelial Growth Factor
(VEGF) family and their receptors in angiogenesis, progression and
pathophysiology of pituitary tumours is still poorly understood. In
the present work, the expression and localization of the angiogenic
factor VEGF-A and the lymphangiogenic factor VEGF-C, as well as
VEGF receptors (VEGFR-1, VEGFR-2, VEGFR-3 and neuropilin-1), have
been studied in normal and tumoural pituitary tissue and in
transformed pituitary tumour cell lines. In addition, the role and
mechanism of action of VEGFR-1 ligands have been investigated in
normal and transformed rat pituitary cells. Immunohistochemical
investigations in 3 normal human adenohypophyses showed that
VEGFR-2 and neuropilin-1 were localized in blood vessel endothelial
cells, while VEGFR-1 was found in endocrine cells. VEGF-A
significantly induced ACTH and prolactin secretion in normal rat
pituitary cell cultures, indicating a role of VEGF-A and VEGFR-1 in
the regulation of the secretion of these pituitary hormones. In
contrast, VEGFR-2 and its co-receptor neuropilin-1 may be needed to
maintain optimal intrapituitary vascularization and blood vessel
permeability. Although no lymphatic vessels were identified in
normal adenohypophysis, the lymphangiogenic factor VEGF-C and its
receptor VEGFR-3 were detected by immunohistochemistry, suggesting
the involvement of the VEGF-C/VEGFR-3 system, usually implicated in
lymphangiogenesis, in the maintenance of blood vessel permeability.
The expression of VEGFR-1, VEGFR-2 and neuropilin-1 in a series of
39 pituitary adenomas reflected the same immunohistochemical
localization pattern as observed in the normal adenohypophysis
tissue. It was highly heterogeneous and mostly no significant
correlation with different parameters, such as: tumour type, tumour
grade, proliferation index (PI) and blood vessel number, was
noticed. Only the absence of VEGFR-2 and neuropilin-1 correlated
with a low PI, suggesting a role of these two receptors in
increasing vessel permeability and consequently the availability of
nutrients and oxygen for tumour cells. Functional studies, with the
VEGFR-1-positive somatotrophinoma rat pituitary cell line MtT-S,
showed that VEGF-A and the VEGFR-1 specific ligand PlGF,
significantly stimulated the cell proliferation, through the
activation of PI3K pathway and the induction of the anti-apoptotic
factor Bcl-2 and the cell cycle promoter cyclin D1. VEGF-C
immunostaining was detected in endocrine tumour cells of 10
adenomas and VEGFR-3 immunopositive vessels were found in 22
tumours, even if only 9 of them were positive for both VEGFR-3 and
LYVE-1 (specific lymphatic vessel marker), suggesting that the
VEGF-C/VEGFR-3 system may have a role in the regulation of tumour
angiogenesis of pituitary adenomas, rather than in
lymphangiogenesis, as already shown in other tumour types. In
conclusion, the results of the present study provide strong
evidence that VEGF may not only have a role in regulating pituitary
adenoma neovascularization but also, through VEGFR-1, may affect
pituitary adenoma pathophysiology by modulating growth, cell cycle
progression and survival of the adenoma cells.