Menin expression is regulated by transforming growth factor beta signaling in leukemia cells

Menin is a ubiquitously expressed protein encoded by the multiple endocrine neoplasia type 1 (MEN1) gene. Besides its importance in endocrine organs, menin has been shown to interact with the mixed lineage leukemia (MLL) protein, a histone H3 lysine 4 methyltransferase, and plays a critical role in hematopoiesis and leukemogenesis. Previous studies have shown that menin promotes transforming growth factor beta (TGF-β) signaling in endocrine cells. However, little is known regarding the impact of TGF-β pathway on menin in hematopoietic system. Here, with leukemia cell lines generated from conditional MEN1 or TGF-β receptor (TβRII) knockout mouse models, we investigated the possible cross-talk of these two pathways in leukemia cells.

MEN1 or TβRII conditional knockout mice were bred and the bone marrow cells were transduced with retroviruses expressing oncogeneic MLL-AF9 (a mixed lineage leukemia fusion protein) to generate two leukemia cell lines. Cell proliferation assays were performed to investigate the effect of TGF-β treatment on MLL-AF9 transformed leukemia cells with/without MEN1 or TβRII excision. Menin protein was detected with Western blotting and mRNA levels of cell proliferation-related genes Cyclin A(2) and Cyclin E(2) were examined with real-time RT-PCR for each treated sample. In vivo effect of TGF-β signal on menin expression was also investigated in mouse liver tissue after TβRII excision.

TGF-β not only inhibited the proliferation of wild type MLL-AF9 transformed mouse bone marrow cells, but also up-regulated menin expression in these cells. Moreover, TGF-β failed to further inhibit the proliferation of Men1-null cells as compared to Men1-expressing control cells. Furthermore, excision of TβRII, a vital component in TGF-β signaling pathway, down-regulated menin expression in MLL-AF9 transformed mouse bone marrow cells. In vivo data also confirmed that menin expression was decreased in liver samples of conditional TβRII knockout mice after TβRII excision.

These results provided the first piece of evidence of cross-talk between menin and TGF-β signaling pathways in regulating proliferation of leukemia cells, suggesting that manipulating the cross-talk of the two pathways may lead to a novel therapy for leukemia.