Subcellular localization of soybean 7S globulin in HepG2 cells and LDL receptor up-regulation by its alpha' constituent subunit

The aims of this work were to monitor the subcellular localization of soybean 7S globulin in HepG2 cells and determine its interaction with cell protein components, by using laser-induced fluorescence capillary electrophoresis (LIF-CE). Furthermore, we evaluated in the same cell line the involvement of the alpha' constituent subunit from 7S globulin in the modulation of LDL catabolism. The results indicated a main fluorescein isothiocyanate-tagged 7S globulin (FITC-7S) component in the cytosolic fraction, that was not present in the nuclear compartment. The electrophoretic mobility of this tagged component suggested either a dissociation of the 7S oligomer or its partial intracellular degradation. Interactions of soybean 7S globulin with FITC-thioredoxin 1 and FITC-cyclophilin B, HepG2 cell membrane proteins, were demonstrated in in vitro assays. In a separate experiment with HepG2 cells, the ability of the alpha' subunit purified from soybean 7S globulin to modulate the activity of the LDL receptors was evaluated by tracking the uptake and degradation of labeled LDL. The up-regulation of LDL receptors by the alpha' subunit, as further confirmed by a LDL receptor promoter assay, was significantly greater than that found in the control cells. In conclusion, this study, while confirming our previous indirect evidence of the key role of alpha' subunit on the cell cholesterol homeostasis, reveals a potentially interesting association of soybean 7S globulin with proteins, such as thioredoxin 1 and cyclophilin B, that are involved in cell protection against oxidative stress.