Role of activating transcription factor 4 in guiding the liver response to amino acid depletion by asparaginase

Asparaginase (ASNase) is widely used to treat acute lymphoblastic leukemia (ALL) in children but it causes metabolic complications related to liver toxicity. ASNase depletes circulating asparagine and glutamine, activating the homeostatic amino acid response (AAR) via phosphorylation of eukaryotic initiation factor 2 (eIF2) and resultant synthesis of activating transcription factor 4 (ATF4). The eIF2-ATF4 pathway is essential for cell survival during amino acid starvation conditions. Activation of the AAR in liver requires the eIF2 kinase called general control nonderepressible 2 kinase (GCN2). This pathway is vital to prevent hepatic failure during ASNase treatment. To what extent activation of the GCN2-eIF2-AAR is mediated by ATF4 is unknown. My dissertation objective is to assess the role of ATF4 in directing the hepatic response to ASNase. The overarching hypothesis is that the AAR protects the liver during ASNase treatment. My objective and hypothesis are addressed in three aims: (1) Describe the liver response to ASNase in mice deleted for Atf4; (2) Determine if Atf4 heterozygosity alters the liver response to ASNase; (3) Examine the hepatic response to ASNase in mice with a liver-specific deletion of Atf4. RNA sequencing alongside complementary biochemical and histological approaches were performed in the livers of mice treated with 8 daily injections of ASNase or saline excipient. Cellular pathways examined in detail included the AAR, endoplasmic reticulum (ER) stress response, and the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway. In Aim 1, I discovered that global hepatic gene expression patterns in Atf4 knockout mice overlapped with Gcn2 knockout mice. Shared hepatic pathways or processes altered during ASNase included nuclear receptor activation, mTOR signaling, and xenobiotic metabolism. On the other hand, loss of Atf4 during ASNase uniquely altered gene expression signatures reflecting signaling via eIF2 and ER stress. Further exploration at the level of protein expression and activity in liver revealed that during ASNase Gcn2 deletion stimulated mTORC1 activity whereas Atf4 deletion induced ER stress. In Aim 2, I found that Atf4 heterozygosity compromised the hepatic AAR to ASNase, resulting in greater DNA fragmentation and hepatotoxicity. In Aim 3, I discovered that global hepatic gene expression patterns in nonstressed Atf4 knockout mice reflected many of the same processes and pathways altered in nonstressed mice with a liver-specific deletion of Atf4. Furthermore, the AAR and ER stress profiles in ASNase-treated mice with liver specific deletion of Atf4 were similar in pattern and direction to whole body Atf4 deletion, supporting a role for hepatic ATF4 in directing the adaptive AAR and preventing maladaptive ER stress to ASNase. This research provides insight into the importance of genetic background of patients in choosing ASNase as a treatment. These findings may be used to help predict which patients diagnosed with ALL may be susceptible to adverse metabolic events during ASNase. Alongside that, I established that global or partial loss of ATF4 influences liver toxicity in ASNase-treated mice.Ph.D.Includes bibliographical referencesby Rana Jaber Taris Al-Baghdad