HEDGEHOG-GLI Signaling Drives Self-Renewal and Tumorigenicity of Human Melanoma-Initiating Cells

Abstract

The question of whether cancer stem/tumor-initiating cells (CSC/TIC) exist in human melanomas has arisen in the last few years. Here, we have used nonadherent spheres and the aldehyde dehydrogenase (ALDH) enzymatic activity to enrich for CSC/TIC in a collection of human melanomas obtained from a broad spectrum of sites and stages. We find that melanomaspheres display extensive in vitro self-renewal ability and sustain tumor growth in vivo, generating human melanoma xenografts that recapitulate the phenotypic composition of the parental tumor. Melanomaspheres express high levels of Hedgehog (HH) pathway components and of embryonic pluripotent stem cell factors SOX2, NANOG, OCT4, and KLF4. We show that human melanomas contain a subset of cells expressing high ALDH activity (ALDH high ), which is endowed with higher self-renewal and tumorigenic abilities than the ALDH low population. A good correlation between the number of ALDH high cells and sphere formation efficiency was observed. Notably, both pharmacological inhibition of HH signaling by the SMOOTHENED (SMO) antagonist cyclopamine and GLI antagonist GANT61 and stable expression of shRNA targeting either SMO or GLI1 result in a significant decrease in melanoma stem cell self-renewal in vitro and a reduction in the number of ALDH high melanoma stem cells. Finally, we show that interference with the HH-GLI pathway through lentiviralmediated silencing of SMO and GLI1 drastically diminishes tumor initiation of ALDH high melanoma stem cells. In conclusion, our data indicate an essential role of the HH-GLI1 signaling in controlling self-renewal and tumor initiation of melanoma CSC/TIC. Targeting HH-GLI1 is thus predicted to reduce the melanoma stem cell compartment.

Endocannabinoids are conserved inhibitors of the Hedgehog pathway

“…Cannabinoids and Hh signaling regulate many of the same processes-e.g., angiogenesis (6,38), hair follicle development (39,40), nocioception (5,41), bone formation (42,43), and energy metabolism (3,4,44). Indeed, cannabinoids block growth of tumors known to depend on Hh signaling (45)(46)(47)(48)(49)(50). Interestingly, Cannabis exposure in utero inhibits fetal growth and alters brain development (51); whether impaired Shh signaling contributes to these problems is worth investigating.…”

Breast cancer lung metastasis: Molecular biology and therapeutic implications

“…It also regulates BCSCs and plays a crucial role in carcinogenesis. [78][79][80][81] Several recent studies have provided evidence that paracrine Hh signaling appears to be an important mechanism in breast cancer growth. 82,83 Moreover, Hh has been shown to regulate breast cancer cell migration.…”

Function of KLF4 in Stem Cell Biology

“…Both pharmacological inhibition of HH signaling by the SMO antagonist cyclopamine and GLI antagonist GANT61, and stable expression of shRNA targeting either SMO or GLI1 result in a significant decrease in melanoma stem cell self-renewal in vitro and a reduction in the number of ALDH high melanoma stem cells, indicating an essential role of the HH-GLI1 signaling in of melanoma CSC/TIC. Notably, melanomaspheres express not only high levels of Hedgehog pathway components, but also high levels of embryonic pluripotent stem cell factors Sox2, Nanog, Oct4 and KLF4 [129]. This is the first report that reveals a possible correlation of HH signaling and KLF4 in CSC, though the underlying mechanism appears entirely unknown.…”

“…Activating events in the HH pathway are involved in numerous human cancers, including melanoma [129], glioma [130], and basal cell carcinoma (BCC) [131]. Mammalian HH signaling is initiated when one of three HH ligands (Sonic, Indian, and Desert HH) binds the dodecatransmembrane receptor Patched (Ptch1).…”