Elucidating the mechanism behind the lipid-raising effect of cafestol

The objective of this thesis was to identify genes that control the response of serum lipid levels to diet. To this end we used cafestol as model substance for a food component that affects serum lipids and therefore health. Cafestol is a cholesterol‑raising diterpene present in coffee beans and unfiltered coffee types.A possible explanation for the cholesterol-raising effect of cafestol is inhibition of bile acid synthesis. This is observed in APOE3Leiden mice upon treatment with cafestol. The nuclear receptors FXR and PXR are key regulators of genes involved in lipid and bile acid metabolism and detoxification.Both these nuclear receptorscan mediate inhibition of cholesterol 7a‑hydroxylase, the rate-limiting enzyme in bile acid synthesis. Therefore, we hypothesized that cafestol is able to activate FXR and/or PXR.We used promoter-reporter gene assays to show that cafestol interacts with FXR and PXR in vitro .This suggests that cafestol can regulate gene expression via these receptors. Indeed cafestol regulated several mRNA levels of target genes of FXR and PXR in livers of APOE3Leiden mice. For a number of target genes these effects were absent in livers of FXR and PXR knockout mice. This confirms that FXR and PXR are involved in the regulation of gene expression by cafestol. However, we could not confirm suppression of bile acid synthesis in humans. We measured plasma levels of7a‑hydroxy-4-cholesten-3-one, a marker for activity of cholesterol 7a‑hydroxylase, in volunteers that consumed coffee oil. Surprisingly, we observed an increase rather than a decrease in the level of 7a‑hydroxy‑4‑cholesten‑3‑one upon coffee oil treatment. In conclusion, it is likely that the interaction with FXR and PXR is at least partly responsible for the effect of cafestol on serum lipids in humans. However, the exact mechanism by which cafestol raises serum cholesterol remains to be elucidated. Elucidation of this mechanism will provide insight into how dietary components can affect serum lipid levels.