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.

FoxO3 Transcription Factor and Sirt6 Deacetylase Regulate Low Density Lipoprotein (LDL)-cholesterol Homeostasis via Control of the Proprotein Convertase Subtilisin/Kexin Type 9 (Pcsk9) Gene Expression

“…First, why does FoxO3 play a major role rather than FoxO1, as FoxO1 is also highly abundant in the liver as well? In agreement with our data, previous reports have also shown that FoxO1 does not play a significant role in LDL-cholesterol regulation (36,51,52). Second, what is the role of FoxO3 in the regulation of the Pcsk9 gene in obese and diabetic conditions?…”

“…With regard to the Pcsk9 gene, SREBP-1/2 and HNF1A have been shown to play significant regulatory roles (29,31,33,34,36). Our data suggest that Sirt6 may be recruited by FoxO3 to the Pcsk9 gene promoter to suppress the gene expression.…”

“…Whereas several reports have shown that feeding or insulin can induce the Pcsk9 gene expression (23,29,53), another one has documented an increase in the Pcsk9 gene expression in the liver of insulin receptor knockdown mice (36). In insulin-deficient type 1 diabetic rats, hepatic Pcsk9 mRNAs are dramatically decreased (53); however, in ob/ob leptin-deficient obese mice, hepatic Pcsk9 mRNAs are also decreased by 2-fold (36). Further investigation is needed to clarify what causes differential regulation of the Pcsk9 gene expression under those conditions.…”

“…PCSK9 protein levels decrease in the course of fasting and increase after feeding (22,27,29,(32)(33)(34). A number of transcription factors or cofactors have been shown to regulate the PCSK9 gene expression, including sterol-response element binding proteins (SREBP-1/ 2), hepatocyte nuclear factor 1A (HNF1A), farnesoid X receptor, peroxisome proliferator-activated receptor ␥, liver X receptor, and histone nuclear factor P (24,28,29,(33)(34)(35)(36)(37). However, how the PCSK9 gene expression is controlled by epigenetic chromatin remodeling is not clear.…”

ENDOCRINE SIDE EFFECTS OF ANTI-CANCER DRUGS: Effects of anti-cancer targeted therapies on lipid and glucose metabolism

“…Proprotein convertase subtilisin kexin type 9 (PCSK9) is an endogenous inhibitor of the LDLr (for review see (59)). Interestingly, a recent report has demonstrated that rapamycin increased expression of PCSK9, decreased levels of hepatic LDLr protein, and finally increased levels of VLDL/LDL cholesterol in WT but not in PCSK9 knockout mice (60). These results suggest that PCSK9 could be a molecular link for the mTOR inhibitor-induced dyslipidemia.…”