PCSK9 and obesity due to melanocortin 4 receptor deficiency: While reliable experimental data we initially applied to confirm that PQQ administration not only effectively mediated damaged lipid and cholesterol metabolism with an increased level of intracellular cholesterol, but also depressed the expression of PCSK9, up-regulating downstream target LDLR and the T-converting proteins in the LCs, therefore, which may be the potential mechanism where PQQ improves the obesity-incurred dysfunction of T synthesis.