Studies on the controlling mechanism of endocrine cell proliferation mediated by the MEN1 gene product menin

点击翻译按钮获取中文摘要

1. To elucidate the controlling mechanism for endocrine cell proliferation mediated by menin, the protein product of the causative gene for multiple endocrine neoplasia type 1, proteins binding to menin in endocrine cells were explored by immunoprecipitation. Insulin-secreting culture cells derived from a rat pancreatic islet tumor were stably transfected with plasmid expressing Flag-tagged menin. Immunoprecipitation of the cell lysate with anti-Flag antibody yeilded several proteins coprecipitated with menin. Mass spectrometric analyses revealed that the coprecipitated proteins included heat shock proteins, ribosomal proteins and transcription factor-like proteins. The latter has been shown to bind weakly to menin by the mammalian cell two-hybrid analysis. Immunocytochemical analyses demonstrated that this menin-binding protein is localized to juxtanuclear area in a spotted pattern.2. Menin has previously been demonstrated to have GTPase activity. Menin synthesized by the in vitro transcription and translation system was tested for GTPase activity. The in vitrosynthesized menin failed to show GTPase activity, suggesting that the post-translational modification may be required to have the enzyme activity of menin.3. By the experiments with parathyroid tumor cells derived from a patient of multiple endocrine neoplasia type 1, menin has been shown to mediate cell growth-inhibiting effects of TGF-β on parathyroid cells.4. Immunoblotting analyses of various missense mutant menin proteins that are forced to be expressed in culture cells demonstrated that almost all pathogenic missense MEN1 gene mutations caused rapid degradation of menin. These findings suggest that instability of the mutant menin is involved in the pathogenesis of multiple endocrine neoplasia type 1 caused by the missense mutations of the MEN1 gene.