Hypoxia Inhibits Expression of Prolactin and Secretion of Cathepsin-D by the GH4C1 Pituitary Adenoma Cell Line

Abstract

SUMMARY:Diminished oxygen concentration within growing tumors may stimulate neovascularization by inducing both up-regulation of angiogenic factors and down-regulation of antiangiogenic agents. A potentially important molecule in the growth of pituitary adenomas is prolactin (PRL), which can be cleaved by cathepsin-D to yield a 16-kDa form (16K-PRL) with potent antiangiogenic effects. We examined the expression of PRL in cultured GH4C1 pituitary adenoma cells after exposure to hypoxia (0.1% oxygen) for periods of 12 to 36 hours. In contrast to increased expression of the angiogenic factor vascular endothelial growth factor in hypoxic cells, PRL mRNA and levels of intracellular and secreted PRL were significantly reduced under hypoxia. The reduction was not attributable to a general suppression of either transcription or protein synthesis. Although 16K-PRL was not evident in conditioned medium at physiologic pH, lowering the pH to mimic the acidic tumor microenvironment resulted in generation of 16K-PRL, which was sharply reduced in medium drawn from hypoxic cells. Production of 16K-PRL was blocked by the cathepsin-D inhibitor pepstatin-A, and the reduced 16K-PRL formation in hypoxic-conditioned medium correlated with a decrease in secretion of cathepsin-D and its precursor, procathepsin-D. Thus, hypoxia acts upon GH4C1 cells to increase vascular endothelial growth factor expression, decrease PRL synthesis, and suppress conversion of PRL to 16K-PRL via inhibition of cathepsin-D proteolysis. These mechanisms may act in concert to stimulate angiogenesis in prolactinomas. (Lab Invest 2003, 83:1627-1636.

Impact of Prolonged High Altitude Exposure on Plasma Prolactin in Men: Effect of Age and Ethnicity

“…Also, the PRL levels of HA natives in this study are close to the levels observed in male Sherpas, with a mean concentration of 11.0 ng=dL (Bangham and Hackett, 1978). Hypoxia, in general, has been shown to inhibit the expression of PRL mRNA and protein in the GH4C1 lactotroph cell line (Cosío et al, 2003). Normal PRL levels in HA native men are indicative of an evolutionary adaptation to reproductive stress under extreme environment.…”

Elevated Cell Invasion Is Induced by Hypoxia in a Human Pituitary Adenoma Cell Line

“…24 Then, down-regulation of PRL by hypoxia in GH4C1 cells supports the notion that hypoxia is an important factor for the hormonal moiety Figure 3A. Expressions of genes in hypoxia at 1% are compared with those at 21% oxygen by cDNA micro array analysis.…”

Regulation of Blood Vessels by Prolactin and Vasoinhibins

“…Cathepsin D cleaves PRL in the extracellular milieu of regressing corpus luteum [17] and mammary gland [16], and in cardiomyocytes under oxidative stress [13], conditions in which tissue remodelling and altered metabolic activity can acidify the pericellular pH. Cultured GH4C1 pituitary adenoma cells also secrete cathepsin D, and mimicking the tumor microenvironment by exposure to hypoxia reduces its release [26], suggesting that extracellular production of vasoinhibins could be decreased and favor the proangiogenic condition necessary for prolactinoma growth.…”