Impact of Fabp1/Scp-2/Scp-x gene ablation (TKO) on hepatic phytol metabolism in mice

Abstract

Studies in vitro have suggested that both sterol carrier protein-2/sterol carrier protein-x () and liver fatty acid binding protein [ (L-FABP)] gene products facilitate hepatic uptake and metabolism of lipotoxic dietary phytol. However, interpretation of physiological function in mice singly gene ablated in the has been complicated by concomitant upregulation of FABP1. The work presented herein provides several novel insights:) An 8-anilino-1-naphthalenesulfonic acid displacement assay showed that neither SCP-2 nor L-FABP bound phytol, but both had high affinity for its metabolite, phytanic acid; ) GC-MS studies with phytol-fed WT and gene ablated [triple KO (TKO)] mice showed that TKO exacerbated hepatic accumulation of phytol metabolites in vivo in females and less so in males. Concomitantly, dietary phytol increased hepatic levels of total long-chain fatty acids (LCFAs) in both male and female WT and TKO mice. Moreover, in both WT and TKO female mice, dietary phytol increased hepatic ratios of saturated/unsaturated and polyunsaturated/monounsaturated LCFAs, while decreasing the peroxidizability index. However, in male mice, dietary phytol selectively increased the saturated/unsaturated ratio only in TKO mice, while decreasing the peroxidizability index in both WT and TKO mice. These findings suggested that: ) SCP-2 and FABP1 both facilitated phytol metabolism after its conversion to phytanic acid; and) SCP-2/SCP-x had a greater impact on hepatic phytol metabolism than FABP1.

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

“…7. Taken together with findings of an earlier study on phytol metabolism in mice ablated in both Fabp1 and Scp-2/Scp-x genes (TKO) [43], the data presented herein help to resolve the relative contributions of SCP-2 vs FABP1 in hepatic phytol metabolism. As shown herein, ablation of the Scp-2/Scp-x gene (DKO) in phytol fed mice significantly increased hepatic accumulation of branched-chain fatty acids about 20% in females (to 500 nmol/mg protein) and nearly 7-fold in males (to about 380 nmol/mg protein).…”

“…Although ANS may bind to a different site in SCP-2 than NBD-stearic acid, an earlier ANS displacement study similarly showed that SCP2 binds phytanic acid but has negligible affinity for phytol [43]. This pattern was also observed with the nuclear receptor RXRα which has 15-fold less affinity of phytol than for phytanic acid [16, 17].…”

“…cholesterol [34, 44, 45], and facilitating its metabolism by enhancing HDL-cholesterol uptake [46, 47] and cytoplasmic transport/targeting to and for esterification (endoplasmic reticulum) and oxidation (mitochondria and/or peroxisomes) [48]. While the identity of the hepatic cytosolic protein(s) that might function in phytol binding, uptake, and transport is not known, several possibilities may be considered: i) The even more highly prevalent liver fatty acid binding protein (FABP1) shares overlapping ligand specificity with SCP2, but like SCP2 does not bind phytol [43]; ii) The major hepatic cellular retinol binding protein (CRBP1) binds another lipidic alcohol (retinol) with high affinity K d of 16 nM [49]. While retinol differs from phytol in having five double bonds and a cyclic methyl terminus, nevertheless retinol shares significant structural similarity with phytol since both are straight-chain, methyl branched, lipidic alcohols.…”

“…By transporting bound phytanic acid and phytenic acid from the ER to the peroxisome, the increased FABP1 may indirectly increase uptake of phytol. This issue was further resolved in phytol-fed mice ablated in both the Fabp1 and Scp-2/Scp-x genes (TKO) [43]. Ablating both the Fabp1 and Scp-2/Scp-x genes (TKO) ablated mice increased hepatic accumulation of branched-chain fatty acids similarly as in DKO mice—only about 20% in females (to about 400 nmol/mg protein) and about 7-fold in males (to about 300 nmol/mg protein) fed the same phytol diet.…”

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.…”