Endocrine Abstracts

The pathogenesis of pituitary adenomas is poorly understood. One of the first genetic abnormalities identified in association with pituitary adenomas occurs in patients with multiple endocrine neoplasia type-1 (MEN-1), due to mutations in the MEN1 gene, encoding menin. A tumor suppressor, menin associates with histone methyltransferase complexes to change the expression of cyclin-dependent kinase (CDK) inhibitor genes, which may serve as an underlying epigenetic mechanism important in the control of pituitary cell proliferation. LSD1 and LSD2 are flavin-dependent monoamine oxidases, functioning to demethylate histone H3K4. We hypothesized that the histone methylation status of specific CDK inhibitor genes, modulated by LSD1 and LSD2, might serve as an underlying epigenetic mechanism important in pituitary tumor pathogenesis. We studied the expression of LSD1 and LSD2 in ten pituitary adenomas (from Partners Tissue Bank) and seven normal pituitaries (from autopsy specimens). LSD1 and LSD2 mRNA expression, as well as LSD1 protein levels, were significantly higher in the adenomas compared to controls. The LSD inhibitor, tranylcypromine, significantly reduced the proliferation rates of GH3 somatolactotrope- and LβT2 gonadotrope-derived cell lines. Specific, lentiviral-delivered shRNA against LSD1, but not LSD2, reduced LβT2 cell proliferation, compared to scramble shRNA controls, suggesting that the proliferation phenotype is LSD1 specific. After LSD1 knock-down (KD) in LβT2 cells, levels of the CIP/KIP family of CDK inhibitors, p27 and p57 (but not p21) were significantly increased. No changes in mRNA levels of members of the INK4a/ARF family of CDK inhibitors (p15, p16, and p18) were observed. ChIP analysis of LSD1 KD LβT2 cell chromatin, immunoprecipitated with an anti-H3K4Me2 antibody, revealed an increase in H3K4Me2 associated with the p27 gene promoter compared to control cells. These findings suggest a role for LSD1 in pituitary tumorigenesis through effects on cellular proliferation, likely mediated through changes in histone methylation status at the promoters of CDK inhibitors.