Willemien van Zwol, Antoine Rimbert, Justina C. Wolters, Marieke Smit, Vincent W. Bloks, Niels J. Kloosterhuis, Nicolette C. A. Huijkman, Mirjam H. Koster, Umesh Tharehalli, Simon M. de Neck, Colin Bournez, Marceline M. Fuh, Jeroen Kuipers, Sujith Rajan, Alain de Bruin, Henry N. Ginsberg, Gerard J. P. van Westen, M. Mahmood Hussain, Ludger Scheja, Joerg Heeren, Philip Zimmerman, Bart van de Sluis, Jan Albert Kuivenhoven
Background and Aims: The assembly and secretion of VLDL from the liver, a pathway that affects hepatic and plasma lipids, remains incompletely un-derstood. We set out to identify players in the VLDL biogenesis pathway by identifying genes that are co-expressed with the MTTP gene that encodes for microsomal triglyceride transfer protein, key to the lipidation of apolipo-protein B, the core protein of VLDL. Using human and murine transcriptomic data sets, we identified small leucine-rich protein 1 (SMLR1), encoding for small leucine-rich protein 1, a protein of unknown function that is exclusively expressed in liver and small intestine.
METHODS AND RESULTS:
To assess the role of SMLR1 in the liver, we used somatic CRISPR/CRISPR-associated protein 9 gene editing to silence murine Smlr1 in hepatocytes (Smlr1- LKO). When fed a chow diet, male and female mice show hepatic steatosis, reduced plasma apolipoprotein B and triglycerides, and reduced VLDL secretion without affecting microsomal tri-glyceride transfer protein activity. Immunofluorescence studies show that SMLR1 is in the endoplasmic reticulum and Cis-Golgi complex. The loss of hepatic SMLR1 in female mice protects against diet-induced hyperlipidemia and atherosclerosis but causes NASH. On a high-fat, high-cholesterol diet, insulin and glucose tolerance tests did not reveal differences in male Smlr1- LKO mice versus controls.
We propose a role for SMLR1 in the trafficking of VLDL from the endoplasmic reticulum to the Cis-Golgi complex. While this study un-covers SMLR1 as a player in the VLDL assembly, trafficking, and secretion pathway, it also shows that NASH can occur with undisturbed glucose ho-meostasis and atheroprotection.