We found that Plin2 is significantly overexpressed in monocytes of children with ow/obesity as compared with controls, likely owing to an impairment of the proteasome dependent degradative pathway. Likewise, levels of Plin2 protein were significantly correlated with cIMT, a surrogate of atherosclerosis. This is the first report in individuals with obesity.
The role of Plin2 has been investigated in relation to two conditions that are strongly associated, i.e. the atherosclerosis and the non-alcoholic fatty liver disease (NAFLD) [37]. It is likely that the dysregulation of the protein both in macrophages and in liver is pivotal to explain the co-occurrence of the various obesity co-morbidities, in particular atherosclerosis and NAFLD.
Over-expression of Plin2 is associated with early atherosclerosis.
Human and murine models of atherosclerosis support strongly the pivotal role of Plin2 in the formation of foam cells. Very recently, we found that upregulation of Plin2 is associated with neoatherogenesis in patients with restenosis after drug-eluting stent implantation [38].
Our study supports the role of Plin2 in the pathogenesis of human early atherosclerosis given the correlation between Plin2 protein level and cIMT.
Dyslipidemia of people with obesity is mainly characterized by reduced HDL-C. In our series, levels of Plin2 expression were correlated with HDL-C but not with LDL-C, triglycerides and ApoB. A correlation trend was observed between Plin2 protein expression and ApoA1 levels, too. These data could suggest a compensative response, favoring reverse cholesterol transport in monocytes/macrophages of children with obesity who have increased intracellular levels of Plin2.
Such as Plin2 has been proposed as a therapeutic target to prevent atherosclerosis. In double ApoE and Plin2 knockout mouse model, the increase of plasma ApoA1 enhanced athero-protection and influenced positively remodeling of the lesion and reversal of the cholesterol transport [39].
Impairment of Plin2 degradation in children with obesity.
Inside the mechanism, our findings demonstrate that the degradation of Plin2 is impaired in children with obesity and causes the upregulation of the protein. On the contrary, there is no transcriptional dysregulation, since the mRNA expression was not different in normal-weight and children with obesity. We investigated first CMA in monocytes of healthy donors demonstrating that this pathway regulates Plin2 protein level in myeloid cells similarly to what it does in the liver [15]. In fact, disruption of CMA downstream, by LAMP2A silencing, and upstream, by AMPK signaling inhibition, led to a significant increase of Plin2 protein level. Thus, both the CMA and the proteasome degradation pathways regulate Plin2 in monocytes of healthy individuals.
Then, we evaluated LAMP2A and Hsc70 protein expression in children normal-weight and with obesity but we did not find a significant difference between the two groups; while there was a significant inverse correlation between Plin2 and LAMP2A in normal-weight individuals but not in those with obesity.
After having ruled out the role of deregulated CMA as cause of Plin2 overexpression in children with obesity, we tested the hypothesis of an impairment of the proteasome degradation pathway.
We analyzed mRNA expression of the proteasome regulatory subunits PSMD3 and PSMC4 and the catalytic subunits PSMA6 and PSMB7. We found that expression of regulatory subunits was significantly downregulated in obese children in keeping with previous observation in obese adults [40] and that levels of Plin2 protein level and mRNA expression of proteasome regulatory subunits were inversely correlated. In addition, the proteasome activity was lower in children with ow/obesity (Fig. 6).
Free fatty acids, insulin and oxidative stress can all affect proteasome functionality in liver and adipose tissue of individuals with obesity, worsening insulin resistance [41].
In our setting, down-regulation of the proteasome could favor Plin2 accumulation in circulating monocytes of children with obesity contributing to early onset of the atherogenic process.