Although we have described signaling pathways that regulate stem cell self-renewal, individually it is clear that in vivo
there are extensive interactions between the pathways. For instance, there is evidence for interaction between Hedgehog signaling and Notch signaling. One study provided evidence that secreted Shh might be involved in reinforcing the cell fate switch executed by Notch [62
]. Moreover, a recent study presented intriguing evidence that Notch signaling regulates Gli-2 expression in mouse skin, and inactivation of the Notch-1 gene in epidermis induces sustained expression of Gli-2 resulting in the formation of basal carcinoma-like tumors [63
Recently we used our mammosphere-derived culture systems to examine the relationship between the hedgehog pathway and the Notch pathway, and we found that the activation of the Notch pathway resulted in the subsequent activation of the hedgehog pathway, including increased expression of Ptch and Gli. This activation could be blocked by γ-secretase inhibitor, which inhibits Notch signaling (Liu S et al
., manuscript in preparation). These studies suggest that hedgehog acts downstream of Notch. In contrast, one study showed that Shh acts upstream of Notch to determine arterial cell fate during arterial endothelial differentiation [64
]. Furthermore, we have evidence that activation of hedgehog pathway by the hedgehog ligands (Shh or Ihh) increased the expression of the Notch pathway target, HES1, in the mammospheres, and this effect could be blocked by the hedgehog inhibitor cyclopamine (Liu S et al
., manuscript in preparation). Together, these studies indicate that Hedgehog and Notch might form a feedback loop regulating normal development.
Furthermore, deregulation of this loop might be involved in cancer formation. In the skin, the activation of two markers of active Wnt signaling, β-catenin and LEF-1, are associated with Notch-dependent transformation [65
]. The activation of Smo might initiate processes during which transcription factors belonging to the Gli family are activated, and modify the transcription of Ptch and Wnt [65
]. Wnt regulation has previously been observed in human basal carcinomas, indicating that tumor progression is mediated by interactions of distinct signaling pathways that regulate organ development during embryogenesis. All of these pathways are also intimately involved in the regulation of stem cell self-renewal. Interestingly, Bmi-1 expression was rapidly increased after the addition of Shh or after the overexpression of the Shh target Gli in cerebellar granular cells, which implies that Bmi-1 is a downstream target in the Shh pathway [66
]. Overexpression of Bmi-1 correlated with overexpression of Ptch and SuFu, which suggests at least a partial activation of the Hedgehog pathway in Bmi-1 overexpression tumors [66
]. In our preliminary data we showed that both the activation of Hedgehog pathway by Shh or Ihh and the activation of the Notch pathway by DSL resulted in the expression of Bmi-1 in the mammosphere culture system, and the induction of Bmi-1 expression could be blocked by the pathway-specific inhibitors cyclopamine in the Hedgehog pathway and γ-secretase inhibitor in the Notch pathway, respectively (Liu S et al
., manuscript in preparation).
Together, these studies demonstrate extensive interaction between the signaling pathways that regulate stem cell self-renewal. These interactions are depicted graphically in Fig. . In this model, Hedgehog and Notch signalings form a loop regulating normal development; both of these pathways might regulate the stem cell self-renewal by upregulating the expression of Bmi-1, which has been identified as a regulator of stem cell self-renewal. It has also been shown that the Wnt pathway can act downstream of both the Hedgehog pathway and the Notch pathway, and the Wnt pathway has been shown to be a regulator of stem cell self-renewal. However, it has not been determined whether the Hedgehog pathway and the Notch pathway can regulate stem cell self-renewal through downstream targets other than Bmi-1. Further elucidation of this model will be required for an understanding of the elements that regulate normal and malignant mammary stem cell self-renewal.
Figure 5 A hypothetic interacting model in the regulation of stem cell self-renewal by the Hedgehog signaling pathway, the Notch signaling pathway, the Wnt signaling pathway, and B lymphoma Mo-MLV insertion region 1 (Bmi-1). Interactions between the Hedgehog, (more ...)