3.1. Tumor-Initiating Capacity
In general, CSCs are derived from normal TSCs or re-differentiate and/or de-differentiate from progenitor/differentiated cells. Some studies have discussed the relationship between genetic alterations in human PGSCs and tumors formed by pituitary adenoma stem cells (PASCs) [36
The relationship between stem cells and PAs has been demonstrated in PA model mice. Gleiberman et al. crossed Rb (+/−) mice, which develop PA in almost 100% of cases, with nestin-GFP mice, and the crossed mice developed PA nodules surrounded by nestin-GFP-positive cells [37
]. Dopamine receptor D2 knockout [Drd2 (−/−)] mice, prolactinoma model mice, contain more SOX2-positive cells in their anterior pituitary than control mice [38
Human PASCs were first isolated from somatotropinomas and non-functioning adenomas using the sphere-forming system in 2009, which initiated hormone-producing PAs as xenografts. The initiating PAs produced GH, PRL, FSH, LH and TSH, thereby responding to hypothalamic stimulation [39
] (Figure 2
Since then, some studies have demonstrated that stem cell side populations can be isolated from human PAs (mainly somatotropinomas and non-functional PAs) using several stem cell markers, such as SOX2, CD133 and CD15 and successful tumor cell proliferation as xenografts has been shown [40
]. In particular, the relationship between CXCR4 expression and the tumor-initiating capacity of PASCs has been discussed [42
]. Tunici et al. clearly revealed the multilineage potential of PASCs isolated from human PAs. In their report, PASCs treated with epidermal growth factor and bFGF expressed S-100 (molecular marker of folliculostellate cells) and synaptophysin in vitro [14
In contrast, some studies have demonstrated the difficulty of tumorigenesis as xenografts even if stem cell side populations can be isolated successfully [38
]. Wurth et al. demonstrated that the CD133-positive side population isolated from human PAs proliferates for only 2 months [43
]. The cause of the discrepancy in the status of tumorigenesis among these studies remains unclear. It may be attributed to the heterogeneity of different tumor types, including somatotropinomas and non-functioning adenomas and the difference in culture methodologies.
Recently, some factors promoting the tumorigenesis of PASCs have been identified. Mertens et al. demonstrated that upregulated stem cell markers (CD44, CXCR4, KIT, KLF4, Nestin and SOX2) and mesenchymal markers (VIM and fibronectin 1), as well as downregulated epithelial markers, including E-cadherin (CDH1) and claudin-1 (CLDN1), were associated with the tumorigenesis of human PASCs as xenografts [38
]. This suggests that epithelial–mesenchymal transition (EMT) is an important process for PAs. EMT is involved in morphological changes in tumors, from tightly packed columnar-type cells to more loosely distributed cells [44
]. In fact, a recent study of human PA specimens demonstrated that SNAIL1, an EMT-related transcriptional factor, is associated with the suprasellar expansion of human PAs [45
]. Other signaling pathways, such as CXCL12 CXCR4 and the activation of mutated β-catenin, Wnt, Notch and MAPK/ERK, have also been reported to promote the proliferation of PASCs [36
]. It is known that angiogenesis promotes PA progression [48
]. Interestingly, PASCs themselves promote angiogenesis in PAs by upregulated expression of PECAM1 and VCAM1 [38
Overactivation of the WNT pathway in SOX2-positive cells is involved in the tumor initiation of both pituitary adenoma and craniopharyngioma in mice and humans [19
]. In craniopharyngioma, the proliferation of SOX2-mutated cells results in forming β-catenin-accumulating cell clusters that secrete tumorigenic factors such as SHH, BMP4, WNT, interleukins, chemokines and growth factors. In contrast to the classical paradigm of CSCs, which cell-autonomously generate tumors, the cell clusters act as paracrine tumorigenesis paradigm which promote tumorigenesis either directly or indirectly through cell–cell signaling interactions or microenvironmental changes [19
]. This paracrine tumorigenesis paradigm may be applicable to PAs.
Donangelo et al. first isolated PASCs from murine PAs in 2014. SCA1-positive tumor cells expressing SOX2 and Nestin were isolated from Rb+/− mice with PAs, which exhibited a tumor-forming capacity when re-transplanted into the mouse brain [50
] (Figure 2
). Further studies of these genetically engineered mouse models with PAs may provide information on the detailed characteristics of PASCs.
3.2. Treatment Strategies for PASCs
Recently, the relationship between upregulated PASC markers and high proliferative activity was evaluated using human PA specimens [40
]. Therefore, PASC-targeted therapy has attracted attention. Zubeldia-Brenner et al. demonstrated that a γ-secretase inhibitor downregulates stemness by suppressing Notch signaling, resulting in the reduction of prolactin-producing xenografted tumors [51
]. Mertens et al. reported that the injection of AMD3100, which is a CXCR4 antagonist, suppresses the tumor growth of PASCs isolated from murine corticotropinoma in vivo [38
]. Inhibitors that target TGF-β receptor I kinase [52
] and Wnt/β-catenin [53
] might be useful to treat progressive PAs.
Treatment strategies for PASCs might be applied to multiple endocrine neoplasia type 1 (MEN1) that is characterized by a combination of tumors of the anterior pituitary, parathyroid, gastrointestinal tract and pancreas [54
]. MEN1-related PAs are often invasive and resistant to treatment [55
]. Menin deficiency is the consequence of a MEN1 mutation. It is associated with Notch signaling, which maintains PASCs [56
] and TGFβ signaling that promotes EMT of PASCs [57
]. Thus, PASC-targeted therapy may be a novel treatment strategy for aggressive MEN-related PAs.
Specific markers are needed to selectively target PASCs, which have not been established to date. Although SOX2 is frequently used as a representative PASC marker, it is also expressed on neural stem cells [58
]. Recently, Horiguchi et al. demonstrated that CD9 is coexpressed in most S100β/SOX2-positive cells in PAs, which might be a potent new marker of PASCs [59