GFP-expressing S100β-positive cells of the rat anterior pituitary differentiate into hormone-producing cells

Some non-endocrine cells in the pituitary anterior lobe are responsible for providing stem/progenitor cells to maintain hormone-producing cells. In particular, cells expressing S100β protein, a calcium-binding protein, have been hypothesized to be a pituitary cell resource. Accumulating data have revealed that S100β-positive cells comprise heterogeneous populations and some of them certainly show stem/progenitor characteristics in vivo. Hence, we examine whether S100β-positive cells have the capacity to differentiate into endocrine cells, by means of in vivo and in vitro experiments on transgenic rats expressing enhanced green fluorescent protein (EGFP) under the control of the S100β promoter. Immunohistochemistry of the pituitary confirmed that some S100β-positive cells expressed SOX2 (SRY [sex-determining region Y]-box 2) and had proliferative activity. Dispersed anterior lobe cells were observed by time-lapse microscopy, followed by immunostaining for hormone and pituitary-transcription-factor1 (PIT1). First, the dispersed anterior lobe cells were immunostained by an antibody against SOX2. S100β-protein co-localizes with SOX2 (about 89 %). Although 44 of 134 S100β-positive cells traced were proliferative but negative to any hormones, 14 cells were positive for one of the pituitary hormones and/or PIT1, confirming the presence of all types of hormone-producing cells. Notably, GFP-fluorescence appeared in two hormone-positive cells during culture. On the other hand, we observed hormone-producing cells that were not positive for S100β at the end of the time-lapse study, despite being initially positive. These findings suggest that S100β-positive cells cultured from the anterior lobe are capable of developing into hormone-producing cells, although this happens relatively infrequently.