Abstract 3978: Reactive oxygen species in Sonic hedgehog-driven proliferation of cerebellar granule neuron precursors
Authors: Potts Chad R. , Rotenberry Rachel D. , Kenney Anna M. 
Published: May 2015

Medulloblastoma is the most common solid malignant pediatric brain tumor. These tumors arise in the cerebellum and can be molecularly subdivided into 4 consensus subgroups, one of which is marked by amplification and activation of Sonic hedgehog (Shh) pathway components and downstream targets. This subclass is proposed to arise from oncogenic transformation of cerebellar granule neuron precursors (CGNPs), whose expansion during post-natal brain development is driven by and requires activation of the Shh pathway. These tumors often demonstrate similarities with normal cerebellar development at the molecular level, thus allowing us to use primary CGNP cultures as a model system for the Sonic hedgehog (Shh) driven subclass of medulloblastoma.

In addition to mitogens driving proliferation, it has been shown in the past that low levels of intracellular reactive oxygen species (ROS) are required for proliferation, through mechanisms as diverse as inhibition of receptor tyrosine phosphatases, stabilization of proliferation proteins, and modifications of metabolites, thus indicating additional roles for ROS in proliferation and perhaps tumor growth beyond their known capacity to cause DNA damage, thereby contributing to genomic instability and apoptosis.

Although the Shh ligand does not bind to a receptor tyrosine kinase (RTK), it is known that Shh signaling cooperates with RTK- activated pathways such as the insulin-like growth factor pathway to drive proliferation. To determine whether intracellular ROS play a role in Shh-driven CGNP proliferation, we treated CGNPs with the ROS scavenger lipoic acid (LA) in the presence of Shh, and observed a significant decrease in proliferation. Conversely, addition of the ROS inducer tert-Butyl hydroperoxide to Shh treated CGNPs led to enhanced proliferation over Shh treatment alone. These results indicate that a certain level of ROS are required to support Shh-driven CGNP proliferation, and enhancing their levels can increase proliferation.

To investigate whether Shh signaling may affect expression of ROS regulatory genes we carried out a qPCR analysis. We identified up-regulation of sod2, gstm1, and gsto1: genes known to respond to ROS and whose products neutralize ROS, over vehicle-treated CGNPs. Paradoxically, when we examined ROS regulatory enzyme expression in Shh-driven mouse medulloblastomas, we noted sharp drop in the expression of gstm1, gsto1, and sod2 compared to the adjacent cerebellum suggesting a reduced ability to inactivate ROS, which could be contributing to proliferation or DNA damage in this transgenic model. These findings suggest that normal Sonic hedgehog pathway activation contributes to the production and tight regulation of ROS via downstream effectors in addition to synergizing with ROS to drive proliferation, a highly regulated balance that may be lost in medulloblastoma.

Citation Format: Chad R. Potts, Rachel D. Rotenberry, Anna M. Kenney. Reactive oxygen species in Sonic hedgehog-driven proliferation of cerebellar granule neuron precursors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3978. doi:10.1158/1538-7445.AM2014-3978