MtTW-10 pituitary tumor cells: galanin gene expression and peptide secretion

The peptide galanin is synthesized within and secreted from specific cell types of the rat anterior pituitary gland. The small size of the rat anterior pituitary gland is somewhat limiting for studying the regulation of galanin gene expression and peptide synthesis/secretion. We examined the mammotropic rat pituitary tumor MtTW-10 as a possible model system to study galanin. The objectives of this study were to 1) determine if galanin is secreted from MtTW-10 cells in vitro in a regulated manner, 2) characterize the molecular forms of immunoreactive galanin secreted by MtTW-10 cells, and 3) assess whether galanin gene expression in MtTW-10 tumors is regulated by estradiol. MtTW-10 pituitary tumors were transplanted to female Wistar-Furth rats that were implanted with estradiol-filled capsules. Cells were harvested from the MtTW-10 tumors and cultured for 4 days. When examined by electron microscopy, the MtTW-10 cells maintained in culture were irregular in shape with microvilli on their surface and contained numerous large secretory granules. Immunoreactive galanin, PRL, and GH were secreted from the cells in a time-dependent fashion during static incubations. LH, ACTH, and TSH were undetectable in the culture medium. Somatostatin (10 and 100 nM) inhibited galanin, PRL, and GH release in a dose-dependent manner. In contrast, dopamine, TRH, LH-releasing hormone, CRH, and GH-releasing hormone at concentrations of 10-100 nM failed to alter hormone secretion. Only high concentrations of dopamine (1 microM) inhibited the secretion of galanin, PRL, and GH. HPLC fractionation of peptides secreted by MtTW-10 cells cultures showed that approximately 84% of the galanin immunoreactivity coeluted with synthetic rat galanin. In tumor-bearing rats, plasma levels of immunoreactive galanin were 10-fold higher after estradiol treatment than levels in ovariectomized controls. Galanin mRNA levels were increased 20-fold by estradiol in MtTW-10 tumors, as determined by solution hybridization, and peptide levels were elevated nearly 100-fold. We conclude that 1) galanin is secreted from MtTW-10 cells in vitro, and its secretion is inhibited by somatostatin; and 2) estradiol increases galanin gene expression and peptide secretion in MtTW-10 tumors in vivo. These data show that MtTW-10 tumors may be useful to study the regulation of pituitary galanin gene expression, peptide synthesis, and secretion.