Endocrine Abstracts

VEGF (vascular endothelial growth factor) is an endothelial cell mitogen responsible for physiological and pathophysiological angiogenesis. Abnormal regulation of VEGF expression in anterior pituitary folliculostellate (FS) cells has been implicated in pituitary tumour progression. PACAP is a well-documented stimulant of VEGF secretion from the pituitary in which both FS and endocrine cells express VEGF. TtT/GF cells are a mouse pituitary cell line with many characteristics of native FS cells. We have therefore used TtT/GF cells to investigate the control of VEGF secretion from FS cells, assaying secretion by ELISA. VEGF is thought to be secreted by the constitutive pathway, via electron-electron lucent vesicles which are present in FS and TtT/GF cells. Controls on VEGF secretion are therefore likely to be at the level of transcription.

TtT/GF cells were shown to express VEGF164, the most potent and bioavailable isoform. PACAP caused a robust 2-fold increase in VEGF secretion over 24h. Glyburide, an ABC1 and KATP channel blocker, caused a 3-fold increase in VEGF secretion when applied alone, and greatly amplified (3-fold) the response to PACAP. Other ABC blockers did not affect VEGF secretion. Exposure of TtT/GF cells to cycloheximide prior to PACAP or glyburide abolished the increased secretion of VEGF, consistent with control of secretion via transcription. RT-PCR demonstrated that the SUR2B/Kir6.1 form of K_ATP channels is expressed by TtT/GF cells. Diazoxide, a K_ATP activator, inhibited PACAP-stimulated VEGF secretion. These data suggest that K_ATP channels play a substantial role in the control of VEGF secretion and that activation of K_ATP channels, via cell depolarisation and calcium influx, inhibits the secretion of VEGF at the level of transcription.