Phosphatidylinositol-(4,5)-Bisphosphate Regulates Plasma Cholesterol Through LDL (Low-Density Lipoprotein) Receptor Lysosomal Degradation

Objective: TMEM55B (transmembrane protein 55B) is a phosphatidylinositol-(4,5)-bisphosphate (PI[4,5]P2 ) phosphatase that regulates cellular cholesterol, modulates LDLR (low-density lipoprotein receptor) decay, and lysosome function. We tested the effects ofTmem55b knockdown on plasma lipids in mice and assessed the roles of LDLR lysosomal degradation and change in (PI[4,5]P2 ) in mediating these effects.Approach and Results: Western diet–fed C57BL/6J mice were treated with antisense oligonucleotides againstTmem55b or a nontargeting control for 3 to 4 weeks. HepaticTmem55b transcript and protein levels were reduced by ≈70%, and plasma non-HDL (high-density lipoprotein) cholesterol was increased ≈1.8-fold (P <0.0001). Immunoblot analysis of fast protein liquid chromatography (FPLC) fractions revealed enrichment of ApoE-containing particles in the LDL size range. In contrast,Tmem55b knockdown had no effect on plasma cholesterol inLdlr −/− mice. In primary hepatocytes and liver tissues fromTmem55b knockdown mice, there was decreased LDLR protein. In the hepatocytes, there was increased lysosome staining and increased LDLR-lysosome colocalization. Impairment of lysosome function (incubation with NH4 Cl or knockdown of the lysosomal proteinsLAMP1 orRAB7 ) abolished the effect ofTMEM55B knockdown on LDLR in HepG2 (human hepatoma) cells. Colocalization of the recycling endosome marker RAB11 (Ras-related protein 11) with LDLR in HepG2 cells was reduced by 50% uponTMEM55B knockdown. Finally, knockdown increased hepatic PI(4,5)P2 levels in vivo and in HepG2 cells, whileTMEM55B overexpression in vitro decreased PI(4,5)P2 .TMEM55B knockdown decreased, whereas overexpression increased, LDL uptake in HepG2 cells. Notably, theTMEM55B overexpression effect was reversed by incubation with PI(4,5)P2. Conclusions: These findings indicate a role for TMEM55B in regulating plasma cholesterol levels by affecting PI(4,5)P2 -mediated LDLR lysosomal degradation.