Lupin peptides induce hypocholesterolemic effects through the regulation of the post-translation HMGCoAR activity on HepG2 cells

State of art and aim: Lupin is a protein-rich grain legume, which has been domesticated long time ago and cultivated in different continents, either for animal or human nutrition. The seeds of these plants have some favorable features, in particular the protein percentage is comparable to that of soybean [1] and the content of indispensable amino acids is only slightly inferior. Besides these important nutritional features, lupin seed may also provide some health benefits, particularly in the area of dislipidemia prevention [2-4]. Our previous findings indicate that peptides obtained from the hydrolysis of lupin proteins are able to interfere with the HMGCoAR activity. Moreover, immunoblotting experiments show that lupin peptides are able to modulate the cholesterol metabolism inducing the SREBP2 activity, which in turn leads to an increase of LDLR and HMGCoAR protein levels. From a functional point of view, HepG2 cells treated with lupin peptides are able to increase their ability to uptake extracellular LDL with a final hypocholesterolemic effects [5]. With the objective of providing further evidences that peptides generated by the digestion of lupin proteins may be responsible of the hypocholesterolemic effects through the regulation of LDLR/SREBP2 pathway, HepG2 cells were treated with lupin peptides obtained either by pepsin (P) or trypsin (T) hydrolysis and molecular investigations of the AMPK pathway activation were performed. Results and discussion: HMGCoAR is among the most highly regulated known enzymes. In particular, it can be long-term regulated by the control of its synthesis and its degradation or short-term regulated through phosphorylation or dephosphorylation. In particular, HMGCoAR is the direct AMPK substrate. Our findings provide some evidence according to which lupin peptides are able to increase the phosphorylation level of AMPK at the Thr172 residue of the catalytic α subunit, indicating the activation of AMPK, which in turn produces an inhibitory effect on HMGCoAR activity. In fact, the AMPK activation mediated by P and T lupin peptides leads to a significant increase of the phosphorylation levels of the HMGCoAR at Ser872 residue, which is the phosphorylation site of AMPK. For this reason, lupin peptides are able not only to act as competitive inhibitors of the HMGCoAR, but also to inhibit HMGCoAR activity by enhancing AMPK activation.