Membrane Type 1 Matrix Metalloproteinase Cleaves Low Density Lipoprotein Receptor to Regulate Lipid Metabolism
Creators: Wang, Maggie
Publisher: name: University of Alberta Library
Year: 2022

<p>Low-density lipoprotein receptor (LDLR) mediates clearance of plasma LDL cholesterol<br>
(LDL-C) and protects against the development of atherosclerosis. Our previous studies<br>
demonstrated that membrane type-1 matrix metalloproteinase (MT1-MMP/MMP14) cleaves LDL<br>
receptor (LDLR) and exacerbates the development of atherosclerosis. In this study, we<br>
investigated the determinants in LDLR and MT1-MMP that were critical for MT1-MMP-induced<br>
LDLR cleavage. We performed site-directed mutagenesis to generate LDLR and MT1-MMP<br>
mutations and observed that deletion of various functional domains in LDLR, including the ligand binding repeats (LBR), the epidermal growth factor precursor homology domain, the clustered O-glycosylation region, and the C-terminal cytoplasmic tail, did not affect MT1-MMP induced<br>
cleavage of the receptor. Furthermore, removal of four predicted MT1-MMP cleavage sites on<br>
LDLR had no effect on cleavage by MT1-MMP. In addition, we observed multiple cleaved<br>
fragments of LDLR in the cell culture medium. In MT1-MMP, deletion of the catalytic domain or<br>
the MT-loop, but not the hemopexin (HPX) domain or the C-terminal cytoplasmic tail of MT1-<br>
MMP, significantly decreased MT1-MMP’s ability to cleave LDLR. Further Ala-scanning<br>
analysis revealed the important role of Ile at position 167 within the MT-loop in MT1-MMP’s<br>
action on LDLR. Replacement of Ile167 with polar residues in MT1-MMP caused a significant<br>
loss in LDLR cleavage, whereas mutation of Ile167 to a non-polar amino acid residue had no effect.<br>
In addition, MT1-MMP cleaves extracellular matrix (ECM) components, such as type I<br>
collagen, and plays a critical role in maintaining homeostasis of ECM. Considering the crucial role<br>
of ECM in the progression of liver fibrosis, we studied MT1-MMP’s effects on this process. To<br>
do so, we fed MT1-MMP hepatocyte specific knockout (Mmp14LKO) mice and the control</p>

<p>Mmp14Flox mice with a high fat, high fructose, and high cholesterol Gubra amylin liver NASH<br>
(GAN) diet for 42 weeks. We did not observe any significant difference between Mmp14LKO and<br>
Mmp14Flox mice in body weight, blood glucose levels, plasma levels of total cholesterol (TC), high<br>
density lipoprotein cholesterol (HDL-C), non-HDL-C, and triglyceride (TG). The ratio of liver<br>
weight to body weight, Oil-Red O staining, and hematoxylin and eosin (H&amp;E) staining were also<br>
comparable in the two genotypes. Histologically, the liver samples of both Mmp14LKO and<br>
Mmp14Flox mice did not show notable fibrosis even though they appeared to have serious steatosis.<br>
Analysis of liver lipid levels revealed that lacking hepatic MT1-MMP did not change TC and TG<br>
levels. mRNA levels of genes encoding factors important for lipid metabolism (Srebp1c, Fasn,<br>
Srebp2, Pcsk9, Ppara, and Cpt1a), inflammation (Tnfa, Il1b), and fibrosis (Tgfb, Col1a1) were<br>
also not altered in the liver of Mmp14LKO mice compared to that of Mmp14Flox mice.<br>
In conclusion, our studies indicate that MT1-MMP does not require a specific cleavage site<br>
on LDLR and may cleave LDLR at multiple sites. However, an amino acid residue with a<br>
hydrophobic side chain at position 167 in MT-loop is critical for MT1-MMP-induced LDLR<br>
cleavage. Furthermore, lacking hepatic MT1-MMP does not appear to alter the development of<br>
liver steatosis in mice or promote liver fibrosis in mice fed the GAN diet. These data combined<br>
with our previous findings suggest that MT1-MMP may be a promising therapeutic target to lower<br>
plasma cholesterol levels and pave the way for the design of highly selective MT1-MMP inhibitors.</p>