Lipid Metabolism

Lipids are omnipresent in a multitude of vital cell processes. Cells use lipids to store energy, build membranes, signal within and between cells, sense the environment, covalently modify proteins, form specialized permeability barriers (e.g., in skin), and protect cells from highly reactive chemicals.

To utilize lipids available from food, adipose tissue or synthesis by the liver, a complex transport system for lipids, primarily in the form of triglycerides has been established. Lipoproteins enable cells to transport hydrophobic lipid molecules via the aqueous blood and tissue lymph environment. Lipoprotein particles are defined by their complement of associated apolipoproteins (Apo) and their content of cholesterol (CHOL), triglyceride (TG) and phospholipid that each particle carries. Specific apolipoprotein constituents control the lipoprotein metabolism and are involved in the transport and redistribution of lipids among various cells and tissues.

Lipoproteins have been grouped into four major classes: high density lipoprotein (HDL), low density lipoprotein (LDL), very low density lipoprotein (VLDL), and chylomicrons1. First step to form HDL is the transfer of cholesterol and phospholipids onto apolipoproteinA-1 (ApoA-1) to generate nascent HDL. NPC1 is responsible for the intracellular transport of cholesterol, the transfer to HDL is catalysed by the ATP-binding cassette (ABC) A1 (ABCA1) transporter 2. ABCG1, another ABC transporter, is able to load more cholesterol onto HDL from peripheral tissues as well as intestines (dietary cholesterol) which is then esterified by Lysolecithin cholesterol acyltransferase (LCAT) in order to generate the mature HDL3. HDL cholesterol-esters are taken up by scavenger receptor SCARB1 in liver and after hydrolysis the resulting free cholesterol is metabolized to bile acids (BA). The bile acids are excreted into the digestive tract in a process that again utilizes ABC transporters (ABCG5/ABCG8, ABC11, ABCB4 and ABCC2).

In the small intestine absorbed dietary fatty acids are converted into TGs and together with ApoB, ApoE and ApoC2 subsequently packaged and secreted into the bloodstream as chylomicrons. Lipoprotein lipase (LPL) hydrolizes the TG, and chylomicron remnants emerge, which are taken up by the liver via the apoE receptor (ApoER). Cholesterol ester from HDL are also transferred to VLDL remnant particles (IDL) via the cholesteryl ester transfer protein (CETP)4. IDL on the other hand loses the majority of apolipoprotein and is converted to LDL by the action of hepatic lipase (LIPC). This converts IDL into LDL, which is taken up by cells that require cholesterol for incorporation into their cell membranes or for synthetic purposes (e.g. the formation of the steroid hormones). The remainder of the LDLs is removed by the liver. Uptake is mediated by SCARB3 (also known as CD36) and LDL receptor (LDLR) in both cases. Proprotein convertase subtilisin/kexin type 9 (PCSK9) can bind and inhibit the LDLR.


References:

  1. Braiman, A and Isakov, N (2009): "Triglycerides and HDL Cholesterol”. Diabetes Care., S373–S377.[PMID: 2811435]
  2. Gong x (2016): “Structural Insights into the Niemann-Pick C1 (NPC1)-Mediated Cholesterol Transfer and Ebola Infection.” Cell, 165(6):1467-1478. [PMID: 27238017]
  3. Rousset (2009): “Lecithin:Cholesterol Acyltransferase: From Biochemistry to Role in Cardiovascular Disease”. Curr Opin Endocrinol Diabetes Obes, 16(2): 163–171. [PMID: 2910390]
  4. Zhang (2015): “HDL surface lipids mediate CETP binding as revealed by electron microscopy and molecular dynamics simulation”. Sci Rep, 5: 8741. [PMID: 4348656]


Apolipoproteins

APOA1 - Apolipoprotein A-I

This gene encodes apolipoprotein A-I, which is the major protein component of high density lipoprotein (HDL) in plasma. The protein promotes cholesterol efflux from tissues to the liver for excretion, and it is a cofactor for lecithin cholesterolacyltransferase (LCAT) which is responsible for the formation of most plasma cholesteryl esters. This gene is closely linked with two other...   More...

APOE (Apolipoprotein E):

Antigens in this category:
APOC4 (Apolipoprotein C-IV): APOC4 antibodies APOC4 ELISA Kits APOC4 Proteins
ApoE3 (Apolipoprotein E3): ApoE3 antibodies ApoE3 ELISA Kits  
APOA5 - Apolipoprotein A-V: APOA5 antibodies APOA5 ELISA Kits APOA5 Proteins
APOB (Apolipoprotein B): APOB antibodies APOB ELISA Kits APOB Proteins
APOA2 - Apolipoprotein A-II: APOA2 antibodies APOA2 ELISA Kits APOA2 Proteins
APOC3 (Apolipoprotein C-III): APOC3 antibodies APOC3 ELISA Kits APOC3 Proteins
APOC2 - Apolipoprotein C-II: APOC2 antibodies APOC2 ELISA Kits APOC2 Proteins
APOA4 (Apolipoprotein A-IV): APOA4 antibodies APOA4 ELISA Kits APOA4 Proteins

ATP-Binding Cassette

Antigens in this category:
ABCG1 (ATP-Binding Cassette, Sub-Family G (WHITE), Member 1): ABCG1 antibodies ABCG1 ELISA Kits ABCG1 Proteins
ABCA1 (ATP-Binding Cassette, Sub-Family A (ABC1), Member 1): ABCA1 antibodies ABCA1 ELISA Kits ABCA1 Proteins
ABCG5 (ATP-Binding Cassette, Sub-Family G (WHITE), Member 5): ABCG5 antibodies ABCG5 ELISA Kits ABCG5 Proteins
ABCG8 (ATP-Binding Cassette, Sub-Family G (WHITE), Member 8): ABCG8 antibodies   ABCG8 Proteins

Lipase & Protease & Inhibitors

Antigens in this category:
ABHD5 (Abhydrolase Domain Containing 5): ABHD5 antibodies ABHD5 ELISA Kits ABHD5 Proteins
ANG - Angiostatin: ANG antibodies ANG ELISA Kits ANG Proteins
GIP (Gastric Inhibitory Polypeptide): GIP antibodies GIP ELISA Kits GIP Proteins
A2M - alpha 2 Macroglobulin: A2M antibodies A2M ELISA Kits A2M Proteins
LCAT (Lecithin-Cholesterol Acyltransferase): LCAT antibodies LCAT ELISA Kits LCAT Proteins
(Cholesterol Esterase):      
MGLL (Monoglyceride Lipase): MGLL antibodies MGLL ELISA Kits MGLL Proteins
PLG (Plasminogen): PLG antibodies PLG ELISA Kits PLG Proteins
PSMB3 - Proteasome (Prosome, Macropain) Subunit, beta Type, 3: PSMB3 antibodies   PSMB3 Proteins
CLPS - Colipase, Pancreatic: CLPS antibodies CLPS ELISA Kits CLPS Proteins
PNLIPRP2 - Pancreatic Lipase-Related Protein 2: PNLIPRP2 antibodies PNLIPRP2 ELISA Kits PNLIPRP2 Proteins
LIPE (Lipase, Hormone-Sensitive): LIPE antibodies LIPE ELISA Kits LIPE Proteins
LIPC - Lipase, Hepatic: LIPC antibodies LIPC ELISA Kits LIPC Proteins
LPL - Lipoprotein Lipase: LPL antibodies LPL ELISA Kits LPL Proteins

Lipidtransport

Antigens in this category:
PLTP (Phospholipid Transfer Protein): PLTP antibodies PLTP ELISA Kits PLTP Proteins
PLIN1 (Perilipin 1): PLIN1 antibodies PLIN1 ELISA Kits PLIN1 Proteins
HSPG2 (Heparan Sulfate Proteoglycan 2): HSPG2 antibodies HSPG2 ELISA Kits HSPG2 Proteins
P4HB - PDI: P4HB antibodies P4HB ELISA Kits P4HB Proteins
ALB - Albumin: ALB antibodies ALB ELISA Kits ALB Proteins
BSA - Bovine Serum Albumin: BSA antibodies BSA ELISA Kits BSA Proteins
SAR1B (SAR1 Homolog B (S. Cerevisiae)): SAR1B antibodies   SAR1B Proteins

Protein Kinase & Protein Phosphatase

Antigens in this category:
PRKACB (Protein Kinase, CAMP Dependent, Catalytic, beta): PRKACB antibodies PRKACB ELISA Kits PRKACB Proteins
PP1-BETA - Serine/threonine-Protein Phosphatase PP1-beta Catalytic Subunit: PP1-BETA antibodies    
PPP1CB - Protein Phosphatase 1, Catalytic Subunit, beta Isoform: PPP1CB antibodies   PPP1CB Proteins
ABL1 (C-Abl Oncogene 1, Non-Receptor tyrosine Kinase): ABL1 antibodies ABL1 ELISA Kits ABL1 Proteins
PRKACA (Protein Kinase A, alpha): PRKACA antibodies PRKACA ELISA Kits PRKACA Proteins
PPP1CC - Protein Phosphatase 1, Catalytic Subunit, gamma Isoform: PPP1CC antibodies   PPP1CC Proteins
PPP1CA - Protein Phosphatase 1, Catalytic Subunit, alpha Isoform: PPP1CA antibodies PPP1CA ELISA Kits PPP1CA Proteins
PKCd - PKC delta: PKCd antibodies PKCd ELISA Kits PKCd Proteins
PRKACG (Protein Kinase, CAMP-Dependent, Catalytic, gamma): PRKACG antibodies   PRKACG Proteins

Receptors & Signalling

Antigens in this category:
AMN (Amnionless): AMN antibodies AMN ELISA Kits AMN Proteins
TFAP2A (Transcription Factor AP-2 alpha (Activating Enhancer Binding Protein 2 Alpha)): TFAP2A antibodies TFAP2A ELISA Kits TFAP2A Proteins
BMP1 (Bone Morphogenetic Protein 1): BMP1 antibodies BMP1 ELISA Kits BMP1 Proteins
LDLR (Low Density Lipoprotein Receptor): LDLR antibodies LDLR ELISA Kits LDLR Proteins
SCARB1 (Scavenger Receptor Class B, Member 1): SCARB1 antibodies SCARB1 ELISA Kits SCARB1 Proteins
CD36 (CD36 Molecule (thrombospondin Receptor)): CD36 antibodies CD36 ELISA Kits CD36 Proteins
CUBN (Cubilin (Intrinsic Factor-Cobalamin Receptor)): CUBN antibodies CUBN ELISA Kits CUBN Proteins
SDC1 - Syndecan 1: SDC1 antibodies SDC1 ELISA Kits SDC1 Proteins
LDLRAP1 (Low Density Lipoprotein Receptor Adaptor Protein 1): LDLRAP1 antibodies LDLRAP1 ELISA Kits LDLRAP1 Proteins