Stem cells in the pituitary gland and candidate cancer stem cells in pituitary tumors

It has repeatedly been postulated that the pituitary gland contains stem cells, putatively involved in the generation of new hormonal cells during homeostatic turnover as well as during plastic remodelling in response to changing endocrine needs of the body. In previous studies, our group clearly designated a candidate stem cell population in the anterior pituitary (AP) lobe of the adult mouse using the side population (SP) approach. In the first part of this thesis, we followed upon these studies and succeeded in identifying the stem/progenitor cells by fractionating the SP according to stem cell antigen-1 (Sca1) expression level. Detailed analysis of whole-genome expression profile, and of the capacity to form pituispheres and generate hormonal lineages (multipotency), tracked down the pituitary stem/progenitor cells in the non-Sca1high SP fraction. Sox2 emerged as one of the important markers of the cells . In situ examination localized the Sox2+ cells in the marginal zone around the cleft (periluminal layer), but additionally in clusters scattered over the gland, thus suggesting the existence of multiple niches which is considered favorably for swift and/or subtle adaptation of cellular composition. To probe the functional significance of the stem/progenitor cells, we meticulously examined their phenotype during neonatal maturation of the pituitary as a (re-)modelling paradigm. Higher abundance and proliferative activity , swifter differentiation competence and more prominent stemness-expression characteristics all point to an activated state of the stem/progenitor cells during the neonatal maturation phase. Moreover, in situ scrutiny revealed some remarkable topography including accumulation of Sox2+ cells in the wedge region of AP and intermediate lobe, and connection of the Sox2+ cell clusters to the periluminal layer, suggesting the idea that Sox2+ cell groups in the parenchyma originate from the cleft . Also these topographical peculiarities were more pronounced at neonatal than adult age. Together, our observations argue for functional involvement of the pituitary stem/progenitor cells in plastic (re-)modelling of the gland. There is growing evidence that tumors contain a subset of cells that drive their growth, maintenance, progression, therapy resistance and/or recurrence. Because knowledge on pituitary tumor pathogenesis is very limited, we introduced this appealing concept of cancer stem cells (CSC) in the field. In the second part of this thesis, we started to search for CSC(-like) cells by applying SP analysis,in other tumors shown to enrich for tumorigenic (CSC) activity. A SP was identified in all human pituitary adenomas analyzed, and whole-genome expression profiling revealed CSC-associated expression characteristics including EMT (epithelial-mesenchymal transition), a process recently shown to drive the generation and activity of CSC. Because microarray analysis indicated that the adenoma SP still included endothelial and immunecells, we further purified the SP from these cells and reached similar conclusions regarding transcriptomal profile. To assess functional CSC properties, the in vivo xenograft assay was tried but did not work forpituitary adenomas, probably because they are typically benign and slow-growing. Therefore, we set up in vitro tumorigenic (CSC) assays which are currently tested on adenoma SP. Moreover, we implemented complementary pituitary tumor models including (rodent) cell lines and a transgenic mouse model in which pituitary tumor develops de novo in the normal microenvironment of the gland. A SP was identified in all these modelsand is now examined for molecular and functional CSC characteristics. In conclusion, this thesis study essentially contributed to the final identification of stem/progenitor cells in the pituitary gland and strongly argues for the functional implication of these cells in plastic cell (re-)modeling of the gland. Moreover, we identified a SP as candidate CSC in pituitary adenomas. In particular, EMT - in connection with SP/CSC -, may play an important role in pituitary tumor pathogenesis and represent an interesting mechanistic and therapeutic concept to advance the fieldstatus: publishe