Abstract PR02: C/EBPG: A critical stress response regulator with a pro-oncogenic role
Authors: Mayekar Manasi K. , Huggins Christopher J. , Martin Nancy , Saylor Karen L. , Gonit Mesfin , Jailwala Parthav , Kasoji Manjula , Haines Diana C. , Quinones Octavio A. , Johnson Peter F. 
Published: May 2016

Abstract
Stress signals such as amino acid deprivation and redox imbalances activate the integrated stress response (ISR), which allows cells to alleviate the stress or to undergo apoptosis if the stress is unresolved. The ISR is also important for the survival of cancer cells, since they experience stress in the form of nutrient and oxygen deprivation and hence frequently activate stress-response pathways. Cellular stresses trigger up-regulation of the transcription factor ATF4, which subsequently activates stress response genes through recruitment to cis-regulatory sites known as C/EBP:ATF response elements (CAREs). Although the role of ATF4 in regulating stress response is well established, the identity of the C/EBP partner that heterodimerizes with ATF4 to execute this crucial function remains obscure. Here we show that the transcription factor C/EBPG is a critical partner of ATF4 and that C/EBPG:ATF4 heterodimers are the predominant CARE-binding species in stressed cells. Similar to ATF4, C/EBPG is necessary for resistance of MEFs to oxidative stress. MEFs lacking C/EBPG show increased levels of reactive oxygen species (ROS) as a consequence of impaired glutathione biosynthesis, as was also seen in ATF4-deficient MEFs. C/EBPG is required for stress-induced association of ATF4 with CAREs and the subsequent activation of several critical stress-responsive genes, including those with known pro-oncogenic functions. Accordingly, resistance to stress conferred by C/EBPG also facilitates the growth of cancer cells. Depletion of C/EBPG impairs the proliferation of cancer cells in vitro and elevates ROS levels. These effects can be suppressed by addition of the antioxidant, N-acetylcysteine. Mice lacking C/EBPG are smaller in size and show defective eye lens formation, similar to ATF4-deficient mice. The absence of C/EBPG also causes perinatal mortality due to pulmonary atelectasis and respiratory failure. This mortality can be rescued by in utero exposure to N-acetylcysteine. Accordingly, gene expression analysis suggests the presence of increased oxidative stress and impaired expression of stress-responsive genes in the lungs of Cebpg-/- newborn mice. Interestingly, Cebpg-/- mice on a mixed strain background, which do not show perinatal lethality, are resistant to the development of solid malignant tumors. These findings suggest that the role of C/EBPG as a stress response regulator may be important for tumor development/progression. A pro-oncogenic function for C/EBPG is also suggested by the observation that elevated C/EBPG levels are associated with poor patient prognosis in several clinical cancer studies. Activation of stress-responsive genes through upregulation of C/EBPG could be a mechanism deployed by cancer cells to mitigate the high levels of ROS and metabolic stresses that they experience. The importance of C/EBPG and its targets in tumor cells could potentially be exploited to devise novel anti-cancer therapies.
Citation Format: Manasi K. Mayekar, Christopher J. Huggins, Nancy Martin, Karen L. Saylor, Mesfin Gonit, Parthav Jailwala, Manjula Kasoji, Diana C. Haines, Octavio A. Quinones, Peter F. Johnson. C/EBPG: A critical stress response regulator with a pro-oncogenic role. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr PR02.