Sex-dependent impact of Scp-2/Scp-x gene ablation on hepatic phytol metabolism

Abstract

While prior studies focusing on male mice suggest a role for sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x; DKO) on hepatic phytol metabolism, its role in females is unresolved. This issue was addressed using female and male wild-type (WT) and DKO mice fed a phytoestrogen-free diet without or with 0.5% phytol. GC/MS showed that hepatic: i) phytol was absent and its branched-chain fatty acid (BCFA) metabolites were barely detectable in WT control-fed mice; ii) accumulation of phytol as well as its peroxisomal metabolite BCFAs (phytanic acid >> pristanic and 2,3-pristenic acids) was increased by dietary phytol in WT females, but only slightly in WT males; iii) accumulation of phytol and BCFA was further increased by DKO in phytol-fed females, but much more markedly in males. Livers of phytolfed WT female mice as well as phytol-fed DKO female and male mice also accumulated increased proportion of saturated straight-chain fatty acids (LCFA) at the expense of unsaturated LCFA. Liver phytol accumulation was not due to increased SCP-2 binding/transport of phytol since SCP-2 bound phytanic acid, but not its precursor phytol. Thus, the loss of Scp-2/Scp-x contributed to a sex-dependent hepatic accumulation of dietary phytol and BCFA.

Ablating both Fabp1 and Scp2/Scpx (TKO) induces hepatic phospholipid and cholesterol accumulation in high fat-fed mice

“…For example, phytol (a normal dietary constituent derived from the side chain of chlorophyll) markedly decreases body weight gain and increases weight loss more so in female than male WT mice [8,57,79]. This difference has been attributed to the lower expression of the protein products of the Fabp1 and/or Scp2/Scp-x genes, both of which are involved in phytol metabolism [5,8,75,107,113]. With regards to human disease, increased FABP1 level and human genetic variation in the Fabp1 gene have been associated with development of NAFLD as indicated above.…”

“…With regards to human disease, increased FABP1 level and human genetic variation in the Fabp1 gene have been associated with development of NAFLD as indicated above. While Scp-2/Scp-x gene ablation in mice is associated with altered phytol, lipid metabolism, and neurotoxicity [5,8,75,107,113], human genetic variation in Scp-2/Scp-x also results in neurotoxity exhibited as neurodegeneration, brain iron accumulation, leukencephalopathy, dystonia, and motor neuropathy [25,41]. …”

Binding properties of sterol carrier protein 2 (SCP2) characterized using Laurdan

“…First, in several cases reported in the literature, incomplete displacement of the probes in the presence of excess of competing ligands was observed. [7,18,20,[22][23][24][25][26][27]. Simple equilibrium calculations demonstrate that partial displacement is difficult to explain by a single-site binding competition model; whereas partial displacement is accounted for by the more complex binding model proposed here.…”

“…The spectral properties of SCP2-bound Laurdan, encouraged us to design an improved competition assay for generic non fluorescent SCP2 ligands. We aimed to better the widely used displacement assays [7,18,20,23,24,26] in several aspects: (a) using canonical equilibrium constants instead of empirically defined inhibition constants; (b) keeping constant the probe and ligand concentrations to minimize micellization; and (c) treating incomplete displacement of the probe consistently with equilibrium theory.…”

“…Most importantly, despite the molar excess of the competing ligands, on several occasions, competition assays showed incomplete displacement of the probe [7,18,20,[22][23][24][25][26][27]. This suggests the inadequacy of the simplest binding model and the need of more evolved analysis of the data.…”