Tuesday, May 24, 2016
Cardiovascular disease is still the biggest cause of death in the developed world including the United States. One of the critical elements that influence this rate of death is the disruption of cholesterol homeostasis, especially in the context of increasing the risk of arteriosclerosis.1,2 One of the current principal medical therapies for managing high cholesterol is the administration of statins. However, while statins have demonstrated a relatively strong safety profile with minimal sides effects, there are individuals who are unresponsive to treatment or may prefer a different option. The chief influence of cholesterol concentrations is tied to both high-density lipoproteins (HDL) and low-density lipoproteins (LDL) concentrations. Stanins address the LDL side of the equation through their inhibition of HMG-CoA reductase; it makes sense that the next step in producing another effective form of cholesterol treatment is to focus on HDL.
HDL is one of five major lipoprotein groups that are responsible for transporting lipids like cholesterol, phospholipids and triglycerides. Both apolipoproteins, apoA-I and apoA-II, are required for normal HDL biosynthesis with apoA-I making up 70%.3 In contrast to LDL, HDL is responsible for moving lipids from cells, including within artery wall atheroma, to other organs for excretion or catabolism, most notably the liver.4 Both HDL and LDL concentrations are indirectly measured through the concentrations of HDL-C and LDL-C due to difficulties and costs associated with direct measurement. Since the 1970s HDL has been acknowledged as having a direct inverse relationship regarding risk for cardiovascular disease (CVD).5 This HDL-CVD relationship has also been conserved over different racial and ethnic populations.6 A seminal study known as the Framingham study also identified high LDL-C and low HDL-C levels as a strong predictor of CVD risk.7 Finally it has also been noted that close to 30% of lipids are transported by HDL in healthy individuals.8
The general belief is that HDL is able to lower the risk of cardiovascular disease through the inhibition and even reversal of atherogenesis via initiating the process of reverse cholesterol transport (RCT).9-11 RCT is the common term for the removal of cholesterol from peripheral cells and transport to the liver. While RCT involves multiple steps, the major ones involve the transfer of cholesterol from peripheral cells to HDL by ATP-binding cassette transporter (ABCA1) through apoA-I interaction and phospholipid interaction, conversion of cholesterol to cholesteryl esters by lecithin-cholesterol acyltransferase (LCAT) and the removal of these esters by interaction with either a direct removal pathway, scavenger receptor class BI (SR-BI), or the indirect removal pathway, cholesterylester transfer protein (CETP).4,10,11
CETP interaction involves the exchange of triglycerides of VLDLs with the cholesteryl esters of HDL resulting in VLDL being converted to LDL that later enters the LDL receptor pathway where the triglycerides are degraded due to their instability in HDL resulting in a smaller HDL lipoprotein that can begin to reabsorb new cholesterol molecules.12
The strategy of manipulating HDL concentrations or interactions to produce better health outcomes is certainly not unique and has not gone unnoticed by the pharmaceutical community. For example one of the initially more promising therapeutic treatments for high cholesterol was increasing expression of endogenous apoA-I due to its role in HDL synthesis. To this end some research has focused on using PPARgamma agonists to increase APOA1 gene transcription to eventually increase apoA-I concentration.13,14 ApoA-II has also received some attention because it appears required for normal HDL biosynthesis and metabolism. Increasing either apoA-I or apoA-II concentrations produce an increase in HDL-C levels and supposed HDL levels.
In contrast to increasing apoA synthesis rates, there is already effective means for increasing HDL-C levels via the supposed reduction of apoA catabolism through increasing nicotinic acid (niacin) concentrations.15 Niacin has demonstrated the ability to reduce HDL apoA-I uptake in hepatocytes in vitro.16 Whether or not this influence occurs via interaction with a HDL receptor or G protein-coupled receptors (most notably GPR109A), is unclear,16,17 but what is known is that niacin reduces apoA catabolism and increases HDL-C concentration.15
In addition to research on increasing HDL synthesis, other research has focused on reducing the degradation/loss of HDL through focusing on influencing the esterifcation and de-esterifaction HDL pathways. As mentioned above HDL-C is esterified to HDL-CE by LCAT. Low concentrations of LCAT in both humans and mice produce significant drops in HDL-C concentration and rapid catabolism of apoA-I and apo-II whereas high concentrations of LCAT result in significantly increased HDL-C concentrations.18,19 These results are more than likely due to feedback systems in that increased LCAT activity via higher LCAT concentrations increase conversion of HDL-C to HDL-CE, thus increasing the demand for HDL-C and its reactants (HDL and apoA-I/apoA-II).
Of the two major ending points for HDL-CE, labeling studies suggest that a majority of HDL-CE is transported to the liver via CETP exchange instead of through direct liver uptake via SR-BI.20 Therefore, CETP inhibitors, like JTT-705 and torcetrapib, are also viewed as an effective means of increasing HDL-C (and by association) HDL concentrations.21-23 Interestingly enough there also appears to be a negative influence on LDL-C concentrations.4,21 However, despite this increase in HDL-C concentration from CETP inhibition, there is a question of whether or not this pathway actually reduces CVD. For example large genetic and observational studies have contrasting results,24 but lean towards increased CETP concentrations increasing CVD probability, but inhibition of CETP does not seem to reduce CVD beyond standard rates (the reduction seen from not having elevated concentrations); this behavior may occur due to CETP negatively interacting with RCT.20
Overall despite the notion that higher native HDL levels (and higher HDL-C levels) are associated with lower rates of CVD and that all of the above methods have some ability to increase HDL-C concentration levels, pharmaceutical derived increases of HDL-C levels, be it from HDL-C direct increases, niacin, or CETP inhibition, do not instill the same CVD health benefits as native levels.25,26 Isolated genetic variants also appear to have little to no effect; for example a loss-of-function variant in LIPG raises HDL-C, but did not change CVD probability.26,27 So what could be a reason behind this inability of HDL-C concentration alone to decrease CVD probability?
One important element in the HDL pathway that has only been alluded to so far with regards to pharmaceutical intervention is the expression of the direct removal pathway through SR-BI. Various studies have identified that overexpression of SR-BI reduces HDL-C concentration and under-expression of SR-BI increases HDL-C concentration.28-31 Neither of these two results should be surprising as SR-BI is an end point pathway for eliminating HDL-C and/or HDL-CE converting it back to HDL. However, the interesting aspect of this change in SR-BI expression is that increased SR-BI reduces the rate of arteriosclerosis and decreased SR-Bi expression increases it.26 So how could SR-BI have this effect?
SR-BI, which is encoded by the gene SCARB1, was identified as the primary liver related HDL receptor decades ago.32 The principal role of SR-BI is to selectively uptake HDL-CE into hepatocytes and steroidogenic cells as well as, to a lesser extent, HDL-C.4,32 Most importantly the interaction between SR-BI and HDL-C(E) results in the internalization of the whole HDL resulting in the removal of the cholesterol and return of the non-cholesterol carrying HDL into the bloodstream.33
This absorption of HDL-C(E) and associated return of HDL could explain the reduced rate of arteriosclerosis over CETP interaction for among other things the SR-BI and HDL relationship trigger macrophage derived RCT.34,35 Basically SR-BI is returning HDL, not HDL-C(E) to the bloodstream which is ready to absorb more cholesterol; this readiness somehow signals the associated macrophages to induce greater rates of RCT. Whereas CETP does not reduce the cholesterol load of HDL as much due to the reliance on other limiting factors making them less capable of increasing RCT rates due to reduce cholesterol absorption capacity.
With this information about the functionality of SR-BI, a theory can be posited regarding why increasing HDL-C does not result in improved health outcomes. It makes sense to consider the idea that SR-BI is a form of limiting factor in the capacity of HDL to reduce the risk of CVD. Due to the fact that CETP appears to manage a majority of HDL-C(E) reduction it stands to reason that SR-BI expression is not significantly tied to HDL, HDL-C or HDL-CE concentrations. Therefore, when HDL or its cholesterol variants increase in concentration there is no corresponding increase in SR-BI. One possible explanation for this outcome is that a certain minimum concentration of cholesterol is required to circulate in the blood, which is managed by a level of negative feedbacks that maintain SR-BI expression levels at a certain floor and ceiling.
So why is SR-BI more important overall than CETP if CETP manages a majority of HDL reduction/eliminations? Perhaps CETP has a limit to what type of HDL it can manage. If HDL gets too “big” via its total level of cholesterol absorption the only means to remove that cholesterol could come from the direct pathway, i.e. SR-BI. However, if HDL concentrations outpace SR-BI expression by a significantly higher than normal level then it stands to reason that significant amounts of HDL will become too big for CETP to manage. Eventually these HDL molecules can breakdown (i.e. explode in a sense) while still circulating in the blood stream releasing all of the previously absorbed cholesterol and transformed cholesterol-esters. If this happens the cholesterol is not properly managed and can result in the increased the rate of arteriosclerosis and associated CVD despite the higher HDL concentrations.
In the end while in general statins have been impressive at controlling high cholesterol and its associated detrimental side effects to health, it is always wise to have alternative strategies. The best sought alternative to statins is a pharmaceutical agent that increases HDL(-C) concentrations due to their positive relationship with quality health outcomes to cholesterol related events. However, numerous studies have produced disappointing results for agents that increase HDL levels with regards to cholesterol related health outcomes, including the potential that more negative events become more probable. So what can be done about this issue?
Clearly if the above proposed theory regarding SR-BI as a limiting factor in the effectiveness of HDL is accurate then if one wants to raise HDL pharmaceutically to produce some form of health benefit, one must also increase SR-BI expression to properly manage the increased HDL and cholesterol associate concentrations. This process on its face should not be difficult as there are already existing pharmaceutical agents as well as natural agents that appear to increase SR-BI expression, but will demand proper study to identify its viability and safety over the long-term.
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