HEDGEHOG-GLI1 signaling regulates human glioma growth, cancer stem cell self-renewal, and tumorigenicity

Cancer stem cells are rare tumor cells characterized by their ability to self-renew and to induce tumorigenesis. They are present in gliomas and may be responsible for the lethality of these incurable brain tumors. In the most aggressive and invasive type, glioblastoma multiforme (GBM), an average of about one year spans the period between detection and death [1]. The resistence of gliomas to current therapies may be related to the existence of cancer stem cells [2-6]. We find that human gliomas display a stemness signature and demonstrate that HEDGEHOG (HH)-GLI signaling regulates the expression of stemness genes in and the self-renewal of CD133(+) glioma cancer stem cells. HH-GLI signaling is also required for sustained glioma growth and survival. It displays additive and synergistic effects with temozolomide (TMZ), the current chemotherapeutic agent of choice. TMZ, however, does not block glioma stem cell self-renewal. Finally, interference of HH-GLI signaling with cyclopamine or through lentiviral-mediated silencing demonstrates that the tumorigenicity of human gliomas in mice requires an active pathway. Our results reveal the essential role of HH-GLI signaling in controlling the behavior of human glioma cancer stem cells and offer new therapeutic possibilities.     

Pathways and consequences: Hedgehog signaling in human disease

Signaling pathways that play a fundamental role during development are turning out to underlie many disease states when misregulated. Here, we review some of the recent findings in the Hedgehog (Hh) pathway and the role it plays in different human diseases. We present a summary of the diseases that result from the inactivation or inappropriate activation of the Hh pathway. The human phenotypes generally fit the findings in model organisms and help to identify some potential targets for therapy.     

Wnt signals are targets and mediators of Gli function

There is growing evidence that Gli proteins participate in the mediation of Hedgehog and FGF signaling in neural and mesodermal development. However, little is known about which genes act downstream of Gli proteins. Here we show the regulation of members of the Wnt family by Gli proteins in different contexts. Our findings indicate that Gli2 regulates Wnt8 expression in the ventral marginal zone of the early frog embryo: activating Gli2 constructs induce ectopic Wnt8 expression in animal cap explants, whereas repressor forms inhibit its endogenous expression in the marginal zone. Using truncated Frizzled and dominant-negative Wnt constructs, we then show the requirement of at least two Wnt proteins, Wnt8 and Wnt11, for Gli2/3-induced posterior mesodermal development. Blocking Wnt signals, however, inhibits Gli2/3-induced morphogenesis, but not mesodermal specification. Gli2/3 may therefore normally coordinate the action of these two Wnt proteins, which regulate distinct downstream pathways. In addition, the finding that Gli1 consistently induces a distinct set of Wnt genes in animal cap explants and in skin tumors suggests that Wnt regulation by Gli proteins is general. Such a mechanism may link signals that induce Gli activity, such as FGFs and Hedgehogs, with Wnt function.     

Growth, hedgehog and the price of GAS.

Embryonic development in a given species is orchestrated by genes regulating growth and differentiation in a stereotyped and conserved manner, resulting in embryos of consistent size and shape. Several signaling pathways, including that of Sonic Hedgehog (SHH), have been implicated in these processes. Recent experiments with Gas1 indicate that it may act as a growth-inducing gene, challenging its previous function as a gene specifically involved in growth arrest. Moreover, GAS1, a GPI-linked membrane protein, can bind SHH, suggesting an interacting link between growth and patterning through SHH and GAS1.     

Expression of the Xhox3 Homeobox Protein in Xenopus Embryos: Blocking Its Early Function Suggests the Requirement of Xhox3 for Normal Posterior Development

Antibodies directed against the product of the Xenopus homeobox gene Xhox3 were raised and used to localize the expression of Xhox3 in the embryo at different stages of development. These studies suggest that endogenous Xhox3 protein is distributed in a graded fashion in the nuclei of mesodermal cells along the anterior-posterior (A-P) and dorso-ventral (D-V) axes in the postgastrula embryo with low levels in anterior and ventral regions and higher levels in posterior and dorsal regions. Xhox3 protein is also detected at different times in the midbrain, spinal cord and hindbrain. In the hindbrain, Xhox3 displays different metameric expression patterns in dorsal and ventral regions during early embryogenesis and metamorphosis. We have tested for the early function of Xhox3 by injecting antibodies against the Xhox3 protein into the cytoplasm of developing embryos. A significant number of embryos injected with Xhox3 antibodies show posterior (trunk and tail) deficiencies. This posterior deficient phenotype constitutes the opposite of the anterior (head) deficient phenotype obtained after overexpresson of Xhox3 reported previously. These results suggest that expression of Xhox3 in the posterior mesoderm is necessary for posterior development and that the graded distribution of Xhox3 in the embryonic mesoderm is required for the development of normal embryonic axial pattern.     

Neural expression of the Xenopus homeobox gene Xhox3: evidence for a patterning neural signal that spreads through the ectoderm

The Xenopus laevis homeobox gene Xhox3 is expressed in the axial mesoderm of gastrula and neurula stage embryos. By the late neurula-early tailbud stage, mesodermal expression is no longer detectable and expression appears in the growing tailbud and in neural tissue. In situ hybridization analysis of the expression of Xhox3 in neural tissue shows that it is restricted within the neural tube and the cranial neural crest during the tailbud-early tadpole stages. In late tadpole stages, Xhox3 is only expressed in the mid/hindbrain area and can therefore be considered a marker of anterior neural development. To investigate the mechanism responsible for the anterior-posterior (A-P) regionalization of the neural tissue, the expression of Xhox3 has been analysed in total exogastrula. In situ hybridization analyses of exogastrulated embryos show that Xhox3 is expressed in the apical ectoderm of total exogastrulae, a region that develops in the absence of anterior axial mesoderm. The results provide further support for the existence of a neuralizing signal, which originates from the organizer region and spreads through the ectoderm. Moreover, the data suggest that this neural signal also has a role in A-P patterning the neural ectoderm.