The search for metabolic syndrome targets has strongly supported SCD1 inhibitors as an attractive option for preventing obesity, insulin resistance, hypertriglyceridemia, and hepatic steatosis.4–16
Unfortunately, there are severe side effects associated with diminished SCD1 activity in mice, including skin pathology17–20
and accelerated atherosclerosis.21,22
These warning signs have unfortunately impeded many SCD1 inhibitor programs,42
without complete understanding of the etiology of these complex side effects. It is logical to assume many of the side effects seen with SCD1 inhibition stem from the abnormal accumulation of SFAs in multiple tissues. Indeed, SFAs are potent proinflammatory molecules24–34
, and have been linked to innate immunity. 24,28,29,31–34
Therefore, SCD1 may indirectly suppress inflammation by preventing SFA-induced activation of TLR4. The concept of fatty acids regulating inflammation is not unique to SFAs. In fact, long chain ω-3 PUFA from fish oil have been shown to inhibit inflammation, and more importantly, counteract SFA-induced TLR4 activation in cultured cells. 28,30–33
This study provides new evidence that this reciprocal relationship between SFA and ω-3 PUFA in modulating inflammation also holds true in vivo,
and can be exploited to protect against multiple metabolic diseases.
The question now becomes: how does the combination of SCD1 ASO treatment and dietary fish oil synergize to comprehensively prevent the development of obesity, insulin resistance, hyperlipidemia, hepatic steatosis, and atherosclerosis? There are likely both shared and independent mechanisms by which these treatments mediate their effects. When given as a monotherapy, SCD1 ASO treatment results in striking protection against diet-induced obesity7,21
, insulin resistance7,10,21
, and hepatic steatosis.7,21
Unfortunately, SCD1 ASO treatment promotes severe atherosclerosis in hyperlipidemic mice fed either a SFA- or MUFA-rich diet.21
We believe that dietary fish oil supplementation is able to prevent SCD1 ASO-driven atherosclerosis through at least three independent mechanisms: 1) lowering LDLc, 2) enriching the remaining LDL-CE in atheroprotective ω-3 PUFAs, and 3) counteracting SFA-driven inflammation. In support of this, dietary fish oil lowered LDLc by 47–53%, compared to SFA-fed mice regardless of ASO treatment (). However, the LDL-CE remaining in fish oil fed mice treated with SCD1 ASO was highly enriched in ω-3 PUFAs, yet enriched in SFA to the same extent as SFA-fed SCD1 ASO-treated mice (). These results indicate that dietary fish oil does not diminish SCD1 ASO-mediated SFA enrichment of plasma lipids, but rather shifts the fatty acid composition to be more polyunsaturated, which has the potential to diminish the production of VLDLc (,45
). In this regard, it has been previously demonstrated that long chain fatty acids such as docosahexaenoic acid (DHA) can inhibit VLDL secretion by promoting post-ER presecretory proteolysis (PERPP)-mediated degradation of apolipoprotein B46
or endoplasmic reticulum (ER) stress related-degradation of apoB47
. Although we did not directly measure oxidant stress or PERPP, we saw no indication that either fish oil or SCD1 ASO promoted hepatic ER stress (Supplemental Figure 4
It is important to note that SCD1 ASO treatment unexpectedly results in a dramatic HDLc reduction in hyperlipidemic mice fed a SFA-rich diet.21
Based on this, it has recently been speculated that SCD1 ASO-driven HDLc lowering may be the cause of accelerated atherosclerosis seen under these conditions.22
However, our data suggests that HDLc lowering plays little, if any, role in SCD1 ASO-driven accelerated atherosclerosis. In support of this, dietary fish oil alone actually decreased HDLc, compared to SFA fed mice (), yet atherosclerosis was also decreased in fish oil fed mice. Most importantly, fish oil supplementation did not prevent SCD1 ASO-mediated reductions in HDLc seen in SFA-fed mice (). In fact, the mice treated with SCD1 ASO and fed dietary fish oil had the lowest HDLc of any group with the rank order of the four groups being: SFA-fed/Control ASO (79 mg/dl) > Fish oil-fed/Control ASO (49 mg/dl) > SFA-fed/SCD1 ASO (35 mg/dl) > Fish oil-fed/SCD1 ASO (30 mg/dl). Collectively, these data suggest that HDLc modulation is not the primary mechanism by which dietary fish oil protects against SCD1 ASO-driven atherosclerosis.
In addition to reducing plasma lipoprotein levels, dietary fish oil prevents SCD1 ASO-driven TLR4 hypersensitivity (). This may be due, in part, to the enrichment of macrophage membranes with long chain ω-3 PUFAs (), which are known to prevent SFA-driven TLR4 activation.28,30–33
Importantly, SCD1 ASO treatment results in marked accumulation of SFA in plasma, multiple tissues, and isolated macrophages (, Supplemental Figure 3D
, and ,21
). However, this SFA enrichment is not prevented by dietary fish oil supplementation (, Supplemental Figure 3D
, and ). Hence, even in the face of massive SCD1 ASO-driven SFA accumulation, moderate dietary ω-3 PUFA supplementation can prevent SFA-driven inflammation () and atherosclerosis (). This makes it tempting to speculate that the other diverse pathologies associated with genetic deletion of SCD1,11,17–20,23
including alopecia, might likewise be ameliorated by the anti-inflammatory effects of dietary fish oil. Interestingly, during the preparation of this manuscript, a recent study warned that previous work29–33
describing SFA-mediated activation of TLR4 or TLR2 may have been confounded by contamination of the fatty acid vehicle (BSA) with LPS and/or bacterial lipoproteins.48
Importantly, since in vivo
dietary feeding of long-chain fatty acids does not require a BSA vehicle, this is likely not the only explanation for SFA-induced TLR4 activation. Rather, our results provide in vivo
evidence that saturated fatty acids do indeed promote TLR4-dependent signaling and that n-3 PUFAs can antagonize SFA-driven TLR4 hypersensitivity (). However, whether fatty acids exert their effects through direct TLR4 agonism or by modulating membrane organization is still a matter of debate, and requires further work.
It is important to point out that ASO-mediated inhibition of SCD1 does not alter SCD1 protein expression in the skin or result in alopecia.21
This tissue specific pattern of inhibition seen with in vivo
ASO administration has been documented previously,43,44
and is ideal for SCD1 inhibition where tissue specificity is required to avoid unwanted side effects. In a recent study by MacDonald and colleagues,22
it was speculated that the accelerated atherosclerosis seen with genetic SCD1 deficiency was in part due to dermal inflammation.22
This is unlikely to be the primary mechanism for the accelerated atherosclerosis, since SCD1 ASO treatment also results in severe atherosclerosis, without affecting skin SCD1 expression or alopecia.21
Therefore, ASO-mediated inhibition may provide a unique tissue-specific therapeutic strategy to avoid the skin pathology17–20
that would likely accompany small molecule SCD1 inhibitors without tissue specificity.
In summary, we have demonstrated that SCD1 ASO treatment protects against development of the metabolic syndrome, but unfortunately promotes atherosclerosis in mice fed diets enriched in either SFA or MUFA.6
However, SCD1 ASO-driven atherosclerosis can be completely prevented by dietary fish oil (). Importantly, the pro-inflammatory effects of SCD1 ASO treatment can be overcome by dietary ω-3 PUFA supplementation, and the dual therapy provides dramatic protection against atherogenic hyperlipidemia. Therefore, this synergistic dual therapy may provide a novel therapeutic approach for the metabolic syndrome and atherosclerosis.