Roles of MicroRNAs in Regulating the Biological Properties of Glioma-initiating Cells and Their Responses to Hypoxia

Abstract

Glioblastoma multiforme (GBM, WHO grade IV astrocytoma) is the most common and lethal primary brain tumor in adults, with an average survival of slightly more than one year after initial diagnosis. GBMs display significant heterogeneity within the tumor mass, among which a subpopulation of cells called glioma-initiating cells (GICs) are responsible for tumorigenesis and resistance to conventional therapies. However, the molecular mechanisms underlying the distinct properties of GICs and non-GICs remain largely unknown. Besides the intrinsic molecular features of these GICs, specific tumor microenvironment, such as hypoxia, has been recognized to enhance the abilities of self-renewal and tumor initiation in GICs, while the mechanisms regulating responses of GICs to hypoxia remain poorly understood. MicroRNAs are small non-coding RNAs that play important roles in the progression of various cancer types including GBM. To assess function of miRNAs in regulating GIC properties, we performed a candidate based miRNA qPCR array and found that miR-33a was up-regulated in GICs to promote GIC growth and self-renewal, consistent with its status as an onco-mir with a higher expression pattern associated with poor prognosis of GBM patients. Antagonizing miR-33a function in GICs led to reduced self-renewal and tumor progression in immune-compromised mice, whereas overexpression of miR-33a in non-GICs rendered them to display features associated with GICs. Mechanistically, miR-33a promoted the self-renewal of GICs via enhancing the activities of cAMP/PKA pathway and Notch signaling by targeting negative regulators of these two pathways, PDE8A and UVRAG. Together these findings reveal a miR-33a-centered signaling network that is required for GIC maintenance.In the meantime, deregulated miRNAs under hypoxia in solid tumors have been uncovered to affect cancer cell adaptation to the stress through repressing oncogenes or tumor suppressors. We next dissected roles of miRNAs in affecting the responses of GICs to hypoxia. Through combination of a miRNA qPCR array and in vivo functional screen, we identified that miR-215 was critical for the GICs maintenance and GBM progression in the hypoxic stress via modulating the activities of multiple pathways. In sum, this study has elucidated essential mechanisms in regulating the biological properties of GICs and their responses to the hypoxic stress through enhancing two distinct miRNA-mediated signal networks. Finding of this work will shed light on the development of specific therapies targeting GICs and the tumor microenvironment for GBM treatment.