Atherosclerotic cardiovascular disease (ASCVD) is the leading cause of morbidity and mortality worldwide. The initiation and progression of atherosclerosis are mediated by the deposition of lipids, in particular cholesterol, carried by blood-born lipoproteins, in vessel walls and the ensuing chronic inflammation [1,2]. Accordingly, lipoprotein cholesterol is the most established risk marker for ASCVD, while lipid-lowering drugs, such as statins, serve as the major therapeutic strategy. Besides lipoprotein cholesterol, the importance of residual inflammatory risk in ASCVD has also been increasingly recognized with C-reactive protein (CRP) as one of the most frequently used markers [2,3]. CRP is a canonical acute phase reactant secreted primarily by the liver, the blood levels of which can increase from less than 1 μg/ml at baseline to over 500 μg/ml upon infection or tissue injury [4]. Numerous clinical studies have further demonstrated that a subtle elevation of CRP levels independently predicts ASCVD risk with a power comparable to that of lipoprotein cholesterol [3]. Intriguingly, though, as a marker of residual inflammatory risk, levels of CRP are also closely associated with the therapeutic efficacy of stains in reducing the risk of ASCVD, wherein greater clinical benefits were observed in individuals with higher initial levels of CRP [3]. The underlying reason for that association, however, remains unclear.

As CRP is a powerful predictor of ASCVD risk and can regulate inflammatory responses by activating complement and immune cells, it is plausible that CRP may also be a direct mediator of atherosclerosis, whose targeting could provide additional benefits complementary to lipid-lowering therapy. However, this suggestion was not supported by studies conducted using animal models, which generate contrasting observations with a neutral effect of Crp most often observed [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. Of note, in most of these early studies, the effects of CRP in atherosclerosis were examined by transgenic expression [5,7,[9], [10], [11],13] or direct infusion of human or rabbit CRP [6,8] in rabbits or in Apoe−/−, Ldlr−/−, Lldlr−/− tg (Apob), or Apoe3-Leiden mice without first clarifying the role of endogenous Crp. Such a study design was probably based on the assumption that endogenous Crp in these animals is functionally negligible [4]. That assumption, however, has been proven to be incorrect [4]. As the in vivo functions of Crp are essential and conserved across species [4], transgenic expression or infusion of exogenous CRP in the presence of its endogenous counterpart will result in a much higher effective level that strongly deviates from the scenario wherein the effects of subtly elevated Crp in atherosclerosis are otherwise expected. There is also one previous study that examined the effects of Crp knockout (KO) on the development of spontaneous atherosclerosis in Apoe−/− or Ldlr−/− mice fed with a low-fat, semisynthetic diet containing 0.02 % cholesterol, concluding that Crp is not pro-atherosclerotic [12]. This study, however, did not examine the effects of Crp in high-fat diet (HFD; containing 0.15 % cholesterol)-induced atherosclerosis, which models a more prevalent scenario of the real world.

In the present study, we have examined the effects of Crp KO on the development of HFD-induced atherosclerosis in Apoe−/− mice. Our results reveal a pro-atherosclerotic effect of Crp secondary to its regulation on hepatic lipid accumulation via promoting the formation of enlarged lipid droplets. Such a regulation leads to increased levels of lipoprotein cholesterol that exacerbates lipid deposition and plaque formation in vessel walls. We further show that Crp KO abrogates rather than synergistically reinforces the therapeutic efficacy of Atorvastatin, a frequently used lipid-lowering drug, in HFD-induced atherosclerosis. These findings thus demonstrate an unexpected yet prominent action of Crp, a marker of residual inflammatory risk, in regulating the growth of lipid droplets, which might be the major effector mediating the beneficial effects of lipid-lowering drugs like statin. Given the central role of hepatic lipid droplets in metabolism [14,15], CRP may not only be a therapeutic target for diseases driven by hyperlipidemia, such as atherosclerosis and metabolic dysfunction-associated steatotic liver disease (MASLD), but serve as a marker to predict patient responses to lipid-lowering drugs.