Mechanism of the growth-related activation of the low density lipoprotein receptor pathway

Growth activation of quiescent cells leads to enhanced low density lipoprotein (LDL) receptor expression at the cell surface. To determine the basis for this stimulated receptor activity, we measured LDL receptor activity, changes in receptor protein mass, and mRNA abundance in quiescent and growth-activated cultured human skin fibroblasts. Growth activation, using insulin or platelet-derived growth factor, led to dose-dependent increases in cellular LDL receptor mRNA level (average 5.2-fold increase at 10 ng/ml platelet-derived growth factor, 4.1-fold increase at 58 ng/ml insulin) and cell surface expression (average 3.5-fold increase at 10 ng/ml platelet-derived growth factor, 2.5-fold increase at 58 ng/ml insulin). Increased LDL receptor mRNA levels could be detected as early as 2 h after addition of growth factor (3.2-fold), whereas increased levels of LDL receptor binding and mass were not detected until after 4-8 h. Growth activation led to induction of LDL receptor gene transcription, led to induction of LDL receptor gene transcription, and the increase of LDL receptor mRNA produced by addition of growth factor was completely prevented by actinomycin D. These observations indicate that growth-related activation of the LDL receptor pathway is accounted for, primarily, by growth-activated enhancement of LDL receptor gene transcription.