Activated HSCs (aHSCs) participate in liver fibrogenesis and promote the development of liver tumor, the two most devastating sequela of chronic liver disease. Our rigorous efforts to screen Wnt- ?-catenin target genes in aHSCs, identified Scd (stearoyl-CoA desaturase) which encodes an ER enzyme responsible for the synthesis of mono-unsaturated fatty acids (MUFA) and is implicated in fatty liver, cancer and stemness. Scd is upregulated in a Wnt-dependent manner by the ability of ?- catenin to bind to SREBP-1c and potentiate nearly 10-fold SREBP-1c-induce transcription. Induction of Scd2, an isoform dominantly expressed in HSCs is required for HSC activation and provides a positive forward loop to amplify canonical Wnt pathway via Lrp5/6 mRNA stability. This novel loop is initiated by SCD-generated MUFA interfering nuclear import of the mRNA binding protein HuR via Transportin (TNPO) and Ran1 GTPase, resulting in HuR cytoplasmic accumulation, HuR binding to Lrp5/6 mRNA 3'UTR, and their stabilization. SCD inhibition or Scd2 silencing abrogates LRP5/6 expression and stabilization of ?-catenin in aHSCs, and attenuates liver fibrosis in mice. Intriguingly, the Wnt-SCD2-LRP5/6 loop also exists in liver tumor-initiating stem-cell like cells (TICs) and Huh7 cells and is required for their self-renewal and tumor-initiating activity, establishing this novel pathway as a functional core in both aHSCs and TICs/HCC cells. We further demonstrate that conditional Scd2 KO in aHSCs (Scd2 cKO) incapacitates aHSCs to promote TIC-initiated tumor growth in nude mice and suppresses tumor cell Scd2 expression and natural development of liver tumors induced by diethyl nitrosamine (DEN) in mice. Our most recent lipidomic and transcriptomic analyses of non-tumor liver tissues of Scd2 cKO mice, reveal global suppression of tumor-promoting lipid reprogramming in cholesterol synthesis and eicosanoid metabolism. Based on these results, we hypothesize that SCD2-HuR-Wnt positive loop established by aHSCs induces tumor-promoting lipid reprogramming in microenvironment by targeting genes involved in cholesterol synthesis and eicosanoid generation and metabolism. To test this hypothesis and the mechanisms of the tumor enhancer role of HSC SCD2, we will address the following aims:
Aim 1. To determine whether Scd2 conditional KO in hepatocytes prevents DEN-induced liver tumor development by using Scd2ff;Alb-Cre vs. Scd2ff mice.
Aim 2. To determine whether HSC SCD2 enhances tumor initiation vs. tumor promotion by timing Scd2 ablation using Scd2ff;Col1a1-Cre-ERT2.
Aim -3. To determine how SCD2 in aHSCs supports tumor promoting lipid microenvironment.
Aim 3 -1. To determine the mechanism of ?-catenin dependent regulation of HMGCR transcription.
Aim 3 -2. To determine whether ?-catenin transcriptionally stimulates FADS1/2 and LTA4H via TCF.
Aim 3 -3. To determine if LTA4H mRNA is up-regulated by HuR via SCD.
Aim 3 -4. To validate SCD-mediated lipid reprogramming in HCC-PDX (patient-derived xenograft) model. Collectively, these efforts will define the mechanisms of the novel tumor enhancer role of HSC SCD2 and help identify new therapeutic targets for HCC.
The proposed study is designed to better understand molecular mechanisms underlying activation of the liver cell type called hepatic stellate cells (HSCs) which cause cirrhosis. Our research has discovered HSCs promote liver tumor development by actions of the enzyme called stearoyl Co-A desaturase (SCD). Proposed research will aim to elucidate how this tumor promotion takes place using the genetic mouse models, global screening for lipids in tumor microenvironment, global gene expression profiling, isolation of HSCs from the liver tumor models and characterizing lipids and lipid-encapsulated molecules released by tumor-associated HSCs. These efforts will help identify new therapeutic targets for cirrhosis and liver cancer which are common complications of chronic liver disease among veteran patients.
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