Sterol carrier protein-2/sterol carrier protein-x expression differentially alters fatty acid metabolism in L cell fibroblasts

Abstract

Sterol carrier protein-2 (SCP-2) and SCP-x are ubiquitous proteins found in all mammalian tissues. Although both proteins interact with fatty acids, their relative contributions to the uptake, oxidation, and esterification of straight-chain (palmitic) and branched-chain (phytanic) fatty acids in living cells has not been resolved. Therefore, the effects of each gene product on fatty acid metabolism was individually examined. Based on the following, SCP-2 and SCP-x did not enhance the uptake/translocation of fatty acids across the plasma membrane into the cell: i ) a 2-fold increase in phytanic and palmitic acid uptake was observed at long incubation times in SCP-2-and SCP-x-expressing cells, but no differences were observed at initial time points; ii ) uptake of 2-bromo-palmitate, a nonoxidizable, poorly metabolizable fatty acid analog, was unaffected by SCP-2 or SCP-x overexpression; and iii ) SCP-2 and SCP-x expression did not increase targeting of radiolabeled phytanic and palmitic acid to the unesterified fatty acid pool. Moreover, SCP-2 and SCP-x expression enhanced fatty acid uptake by stimulating the intracellular metabolism via fatty acid oxidation and esterification. In summary, these data showed for the first time that SCP-2 and SCP-x stimulate oxidation and esterification of branched-chain as well as straight-chain fatty acids in intact cells. 1)]. More recent studies indicate that SCP-2 also functions in the uptake and esterification of straight-chain (e.g., oleic acid) fatty acids (FAs) (2), while SCP-x participates in the oxidation of branchedchain (e.g., phytanic acid) FAs (3, 4). However, almost nothing is known about the function of SCP-x in FA uptake or esterification. Likewise, the relative roles of SCP-2 versus SCP-x in straight-chain or branched-chain FA oxidation remain unresolved. Despite the lack of conclusive evidence, several observations point to an important role for SCP-2 and SCP-x in functioning in these capacities, including FA binding affinities where SCP-2 binds straightchain FAs (5-11) and their metabolically active fatty acyl CoA derivatives (9, 12) to a high degree, as exhibited by K d s in the submicromolar and nM range, respectively. In addition, SCP-2 also binds branched-chain FAs such as phytanic or pristanic acid (9), as well as their respective acyl CoAs with high affinity (7, 9).The intracellular localization of SCP-x and SCP-2 also indicate a role in FA metabolism. The highest concentration of SCP-2 and SCP-x is in peroxisomes (13), where ␣ -oxidation of branched-chain FAs as well as the ␤ -oxidation of some straight chain FAs occurs (14-16). Additionally, it has been shown that half or more of total SCP-2 is extraperoxisomal (13, 17), where part of the SCP-2 may arise from the partial posttranslational cleavage of the SCP-x protein (18,19). Extraperoxisomal SCP-2 locations include: i ) plasma membrane in caveolae, wherein several FA translocase/transporter proteins reside (20); ii ) cytoplasm, where SCP-2 enhances diffusion of fluorescent fatty acid (21); ii...

Sexually dimorphic metabolism of branched-chain lipids in C57BL/6J mice

“…In addition, because SCP-x, SCP-2, and L-FABP have high affinity for phytanic acid (44,52), these results are consistent with these proteins being regulated by their fatty acid substrates in a manner similar to many enzymes involved in peroxisomal fatty acid oxidation (52). Moreover, because SCP-x, SCP-2, L-FABP, and FAT/CD36 enhance fatty acid uptake (28,(53)(54)(55) and SCP-x, SCP-2, and L-FABP enhance intracellular transport (26,56) and peroxisomal fatty acid oxidation (28,57), increased expression may reduce the toxic threshold of phytol by relieving the fatty acid load in the cell. However, the decreased liver SCP-x level of female versus male mice was shown to result in reduced capacity to handle high doses of phytanic acid despite the upregulation of SCP-2 and L-FABP.…”

“…Studies with intact cells overexpressing SCP-2 show that SCP-2 accelerates the intracellular transfer of fatty acids (26,27). The responses of both SCP-x and SCP-2 overexpression to supplementation with phytanic acid was shown to be highly dose dependent: i ) at low phytanic acid levels, SCP-x and SCP-2 stimulate phytanic acid oxidation severalfold more than that of straight-chain fatty acids (SCP-x ϾϾ SCP-2), with concomitant enhancement of the esterification of both branched-and straight-chain fatty acids (28); ii ) at high phytanic acid levels, phytanic acid oxidation is inhibited (SCP-x ϾϾ SCP-2) and cells exhibit signs of toxicity (29). Both SCP-x and SCP-2 contain a peroxisomal targeting signal at the C terminus, suggesting peroxisomal localization [reviewed in ref.…”

Subcellular localization of the mosquito sterol carrier protein-2 and sterol carrier protein-x

“…Vertebrate SCP-x and SCP-2 have been shown to be functionally different in fatty acid metabolism, although amino acid sequences of the C-terminal portion of SCP-x and SCP-2 are exactly same. SCP-2 enhances palmitic acid esterification, but SCP-x does not (7). Vertebrate SCP-x locates exclusively in peroxisomes; SCP-2 is found in both peroxisomes and cytosol (1).…”

Three-dimensional structure/function analysis of SCP-2-like2 reveals differences among SCP-2 family members

“…The vertebrate SCP-2 has been shown to facilitate lipid transfer between membranous vesicles in in vitro assays (2)(3)(4). SCP-2 also enhances lipid uptake and redistribution in cultured cells (5)(6)(7)(8)(9)(10). However, the biological function of SCP-2 is subject to debate as a result of its ability to bind diverse ligands (11)(12)(13).…”