Purpose: Mixed lineage leukemia (MLL) is a histone methyltransferase that activates gene transcription and associates with menin. In multiple endocrine neoplasia type 1 (Men1), a mutation of menin caused decreased expression of the p27(Kip1) and p18(Ink4C) genes and deregulated cell growth. We hypothesized that the same pathway might be involved in sporadic pituitary adenomas.
Experimental design: mRNA levels for MLL, Men1, p27(Kip1), and p18(Ink4C) were measured in specimens of several sporadic pituitary adenomas, and a search for clinical parameters revealed that octreotide treatment affected the level of expression of some genes tested. To study molecular mechanisms, we cloned and characterized the MLL promoter region and used small interfering RNA for MLL and specific inhibitors for signal transduction pathways.
Results: A strong correlation between MLL and p27(Kip1) mRNA levels was observed in prolactinomas and growth hormone-secreting adenomas, and these levels were attenuated except in growth hormone-secreting adenomas treated with a somatostatin analogue, octreotide. Conversely, the patients treated with octreotide showed high levels of MLL-p27(Kip1) mRNA. Experiments in vitro showed that octreotide increased MLL and p27(Kip1) protein and mRNA levels, and overexpression of MLL induced a marked increase in p27(Kip1)promoter activity. Furthermore, octreotide stimulated the promoter activity of the MLL gene through phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways. In addition, incubation with an inhibitor for methyltransferase, MTA, and knockdown of MLL completely inhibited the octreotide-induced expression of p27(Kip1).
Conclusions: The MLL-p27(Kip1) pathway was down-regulated in the pituitary adenomas, and octreotide increased the p27(Kip1) level, at least in part, by sequential transcriptional stimulation of the MLL and p27(Kip1) genes through phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways.