Data from Noncanonical Regulation of the Hedgehog Mediator <i>GLI1</i> by c-MYC in Burkitt Lymphoma

Abstract

<div>Abstract<p>Although Hedgehog signaling plays a major role in <i>GLI1</i> transcription, there is now evidence suggesting that other pathways/genes, such as <i>c-MYC</i>, may also regulate <i>GLI1</i> expression. We initiated studies in Burkitt lymphoma cells, which constitutively express <i>c-MYC</i> due to a chromosomal translocation, to determine whether Hedgehog or c-MYC regulates <i>GLI1</i> expression. We show that all Burkitt lymphoma cell lines tested express <i>GLI1</i>, <i>PTCH1</i>, and <i>SMO</i> and that five of six Burkitt lymphomas express GLI1. Exposure to Sonic or Indian Hedgehog or cyclopamine (SMO inhibitor) does not modulate <i>GLI1</i> expression, cell proliferation, or apoptosis in most Burkitt lymphoma cell lines. Sequence analysis of <i>PTCH1</i>, <i>SMO</i>, and <i>SuFu</i> failed to show mutations that might explain the lack of Hedgehog responsiveness, and we did not detect primary cilia, which may contribute to it. We show that c-MYC interacts with the 5′-regulatory region of <i>GLI1</i>, using chromatin immunoprecipitation (ChIP) assay, and E-box–dependent transcriptional activation of <i>GLI1</i> by c-MYC in NIH3T3 and HeLa cells. The c-MYC small-molecule inhibitor 10058-F4 downregulates <i>GLI1</i> mRNA and protein and reduces the viability of Burkitt lymphoma cells. Inhibition of GLI1 by GANT61 increases apoptosis and reduces viability of some Burkitt lymphoma cells. Collectively, our data provide evidence that c-MYC directly regulates <i>GLI1</i> and support an antiapoptotic role for GLI1 in Burkitt lymphoma. Burkitt lymphoma cells do not seem to be Hedgehog responsive. These findings suggest a mechanism for resistance to SMO inhibitors and have implications for using SMO inhibitors to treat human cancers. <i>Mol Cancer Res; 11(6); 604–15. ©2013 AACR</i>.</p></div>