Concurrent activation of growth factor and nutrient arms of mTORC1 induces oxidative liver injury

Abstract

mTORC1 is a protein kinase important for metabolism and is regulated by growth factor and nutrient signaling pathways, mediated by the Rheb and Rag GTPases, respectively. Here we provide the first animal model in which both pathways were upregulated through concurrent mutations in their GTPase-activating proteins, Tsc1 and Depdc5. Unlike former models that induced limited mTORC1 upregulation, hepatic deletion of both Tsc1 and Depdc5 (DKO) produced strong, synergistic activation of the mTORC1 pathway and provoked pronounced and widespread hepatocyte damage, leading to externally visible liver failure phenotypes, such as jaundice and systemic growth defects. The transcriptome profile of DKO was different from single knockout mutants but similar to those of diseased human livers with severe hepatitis and mouse livers challenged with oxidative stress-inducing chemicals. In addition, DKO liver cells exhibited prominent molecular pathologies associated with excessive endoplasmic reticulum (ER) stress, oxidative stress, DNA damage and inflammation. Although DKO liver pathologies were ameliorated by mTORC1 inhibition, ER stress suppression unexpectedly aggravated them, suggesting that ER stress signaling is not the major conduit of how hyperactive mTORC1 produces liver damage. Interestingly, superoxide scavengers N-acetylcysteine (NAC) and Tempol, chemicals that reduce oxidative stress, were able to recover liver phenotypes, indicating that mTORC1 hyperactivation induced liver damage mainly through oxidative stress pathways. Our study provides a new model of unregulated mTORC1 activation through concomitant upregulation of growth factor and nutrient signaling axes and shows that mTORC1 hyperactivation alone can provoke oxidative tissue injury.

Pathological Consequences of Hepatic mTORC1 Dysregulation

“…DEPDC5 is a protein in the GATOR1 complex that upregulates amino acid-dependent Rag signaling [ 22 ]. Recently, liver-specific DEPDC5 -knockout mice were described and analyzed [ 65 ]. DEPDC5 -knockout mice exhibited many phenotypes similar to Tsc1 -knockout mice, such as increased basal inflammation and resistance to HFD-induced steatosis [ 65 ].…”

“…Recently, liver-specific DEPDC5 -knockout mice were described and analyzed [ 65 ]. DEPDC5 -knockout mice exhibited many phenotypes similar to Tsc1 -knockout mice, such as increased basal inflammation and resistance to HFD-induced steatosis [ 65 ]. Transcriptome analysis of liver-specific single knockout mice of Tsc1 or DEPDC5 indeed revealed that they altered the liver transcriptome similarly [ 65 ].…”

“…DEPDC5 -knockout mice exhibited many phenotypes similar to Tsc1 -knockout mice, such as increased basal inflammation and resistance to HFD-induced steatosis [ 65 ]. Transcriptome analysis of liver-specific single knockout mice of Tsc1 or DEPDC5 indeed revealed that they altered the liver transcriptome similarly [ 65 ]. Therefore, even though the method of mTORC1 upregulation was different between the liver-specific knockouts of Tsc1 and DEPDC5 , consequences of mTORC1 activation appeared to be similar between the two models.…”

“…Even though the liver-specific knockout phenotypes of Tsc1 and Depdc5 were similar, they regulated mTORC1 through separate parallel pathways. Concurrent mutation of Tsc1 and Depdc5 in the liver produced prominent upregulation of mTORC1, which was not seen in the single knockout mutants [ 65 ]. Interestingly, mTORC1 hyperactivation in double knockouts severely injured the liver through a prominent accumulation of oxidative stress in hepatocytes [ 65 ].…”

“…Concurrent mutation of Tsc1 and Depdc5 in the liver produced prominent upregulation of mTORC1, which was not seen in the single knockout mutants [ 65 ]. Interestingly, mTORC1 hyperactivation in double knockouts severely injured the liver through a prominent accumulation of oxidative stress in hepatocytes [ 65 ]. This led to excessive hepatocellular injury and death, leading to functional liver failure, indicated by the dramatic elevation of serum liver enzymes and bilirubin concentrations [ 65 ].…”