Regulation of hepatic LDL receptors by mTORC1 and PCSK9 in mice
Ding Ai, Chiyuan Chen, Seongah Han, Anjali Ganda, Andrew J. Murphy, Rebecca Haeusler, Edward Thorp, Domenico Accili, Jay D. Horton, Alan R. Tall
Ding Ai, Chiyuan Chen, Seongah Han, Anjali Ganda, Andrew J. Murphy, Rebecca Haeusler, Edward Thorp, Domenico Accili, Jay D. Horton, Alan R. Tall
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Research Article Metabolism

Regulation of hepatic LDL receptors by mTORC1 and PCSK9 in mice

Abstract

Individuals with type 2 diabetes have an increased risk of atherosclerosis. One factor underlying this is dyslipidemia, which in hyperinsulinemic subjects with early type 2 diabetes is typically characterized by increased VLDL secretion but normal LDL cholesterol levels, possibly reflecting enhanced catabolism of LDL via hepatic LDLRs. Recent studies have also suggested that hepatic insulin signaling sustains LDLR levels. We therefore sought to elucidate the mechanisms linking hepatic insulin signaling to regulation of LDLR levels. In WT mice, insulin receptor knockdown by shRNA resulted in decreased hepatic mTORC1 signaling and LDLR protein levels. It also led to increased expression of PCSK9, a known post-transcriptional regulator of LDLR expression. Administration of the mTORC1 inhibitor rapamycin caused increased expression of PCSK9, decreased levels of hepatic LDLR protein, and increased levels of VLDL/LDL cholesterol in WT but not Pcsk9–/– mice. Conversely, mice with increased hepatic mTORC1 activity exhibited decreased expression of PCSK9 and increased levels of hepatic LDLR protein levels. Pcsk9 is regulated by the transcription factor HNF1α, and our further detailed analyses suggest that increased mTORC1 activity leads to activation of PKCδ, reduced activity of HNF4α and HNF1α, decreased PCSK9 expression, and ultimately increased hepatic LDLR protein levels, which result in decreased circulating LDL levels. We therefore suggest that PCSK9 inhibition could be an effective way to reduce the adverse side effect of increased LDL levels that is observed in transplant patients taking rapamycin as immunosuppressive therapy.

Authors

Ding Ai, Chiyuan Chen, Seongah Han, Anjali Ganda, Andrew J. Murphy, Rebecca Haeusler, Edward Thorp, Domenico Accili, Jay D. Horton, Alan R. Tall

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Figure 3

Effect of the mTOR inhibitor rapamycin on LDLR expression.

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Effect of the mTOR inhibitor rapamycin on LDLR expression. (A) Immunoblo...
(A) Immunoblot analysis of LDLR, p-S6KT389, total S6K, PCSK9, and β-actin in mice treated with rapamycin. C57BL/6 mice fed a chow diet received saline/DMSO with or without rapamycin (2 mg/kg body weight) for 1 week. (B) Plasma total cholesterol (chol), VLDL plus LDL cholesterol, and triglyceride (TG) levels from A were measured. (C) Immunoblot analysis of LDLR and LRP in the membrane fraction of the liver of the mice treated with rapamycin. C57BL/6 or Pcsk9–/– mice fed a chow diet received saline/DMSO (control [c]) with or without rapamycin (r; 2 mg/kg body weight) for 1 week. n = 5. Right panel: Quantification of the Western blot results; *P < 0.05. (D) Plasma VLDL plus LDL cholesterol and total cholesterol were measured in the mice described in C. (E) Immunoblot analysis of LDLR, p-S6 kinaseT389, total S6K, PCSK9, and β-actin. (F) Hepatic mRNA levels of Ldlr and Pcsk9 in the mice. n = 5–6 mice per group. *P < 0.05.