Variable cartilage degradation in mice with diet-induced metabolic dysfunction: food for thought.

OBJECTIVE: Human cohort studies have demonstrated a role for systemic metabolic dysfunction in osteoarthritis (OA) pathogenesis in obese patients. To explore the mechanisms underlying this metabolic phenotype of OA, we examined cartilage degradation in the knees of mice from different genetic backgrounds in which a metabolic phenotype was established by various dietary approaches. DESIGN: Wild-type C57BL/6J mice and genetically modified mice (hCRP, LDLr-/-. Leiden and ApoE*3Leiden.CETP mice) based on C57BL/6J background were used to investigate the contribution of inflammation and altered lipoprotein handling on diet-induced cartilage degradation. High-caloric diets of different macronutrient composition (i.e., high-carbohydrate or high-fat) were given in regimens of varying duration to induce a metabolic phenotype with aggravated cartilage degradation relative to controls. RESULTS: Metabolic phenotypes were confirmed in all studies as mice developed obesity, hypercholesteremia, glucose intolerance and/or insulin resistance. Aggravated cartilage degradation was only observed in two out of the twelve experimental setups, specifically in long-term studies in male hCRP and female ApoE*3Leiden.CETP mice. C57BL/6J and LDLr-/-. Leiden mice did not develop HFD-induced OA under the conditions studied. Osteophyte formation and synovitis scores showed variable results between studies, but also between strains and gender. CONCLUSIONS: Long-term feeding of high-caloric diets consistently induced a metabolic phenotype in various C57BL/6J (-based) mouse strains. In contrast, the induction of articular cartilage degradation proved variable, which suggests that an additional trigger might be necessary to accelerate diet-induced OA progression. Gender and genetic modifications that result in a humanized pro-inflammatory state (human CRP) or lipoprotein metabolism (human-E3L.CETP) were identified as important contributing factors.

Molecular mechanisms responsible for the differential effects of apoE3 and apoE4 on plasma lipoprotein-cholesterol levels.

OBJECTIVE: The goal of this study was to understand the molecular basis of how the amino acid substitution C112R that distinguishes human apolipoprotein (apo) E4 from apoE3 causes the more proatherogenic plasma lipoprotein-cholesterol distribution that is known to be associated with the expression of apoE4. APPROACH AND RESULTS: Adeno-associated viruses, serotype 8 (AAV8), were used to express different levels of human apoE3, apoE4, and several C-terminal truncation and internal deletion variants in C57BL/6 apoE-null mice, which exhibit marked dysbetalipoproteinemia. Plasma obtained from these mice 2 weeks after the AAV8 treatment was analyzed for cholesterol and triglyceride levels, as well as for the distribution of cholesterol between the lipoprotein fractions. Hepatic expression of apoE3 and apoE4 induced similar dose-dependent decreases in plasma cholesterol and triglyceride to the levels seen in control C57BL/6 mice. Importantly, at the same reduction in plasma total cholesterol, expression of apoE4 gave rise to higher very low-density lipoprotein-cholesterol (VLDL-C) and lower high-density lipoprotein-cholesterol levels relative to the apoE3 situation. The C-terminal domain and residues 261 to 272 in particular play a critical role, because deleting them markedly affected the performance of both isoforms. CONCLUSIONS: ApoE4 possesses enhanced lipid and VLDL-binding ability relative to apoE3, which gives rise to impaired lipolytic processing of VLDL in apoE4-expressing mice. These effects reduce VLDL remnant clearance from the plasma compartment and decrease the amount of VLDL surface components available for incorporation into the high-density lipoprotein pool, accounting for the more proatherogenic lipoprotein profile (higher VLDL-C/high-density lipoprotein-cholesterol ratio) occurring in apoE4-expressing animals compared with their apoE3 counterparts.