The lower risk of coronary artery disease in premenopausal women than in men and postmenopausal women implicates sex steroids in cardioprotective processes. beta-Estradiol upregulates liver low-density lipoprotein receptor (LDLR), which, in turn, decreases circulating levels of low-density lipoprotein, which is a risk factor for coronary artery disease. Conversely, LDLR protein is negatively regulated by proprotein convertase subtilisin/kexin type 9 (PCSK9).
METHODS AND RESULTS:
Herein, we investigated PCSK9 regulation by beta-Estradiol and its impact on LDLR in human hepatocarcinoma HuH7 cells grown in the presence or absence of β-estradiol. Immunoblot analysis showed upregulation of LDLR at 3 μm beta-Estradiol (140%), and the upregulation reached 220% at 10 μm beta-Estradiol; only at the latter dose was an increase in LDLR mRNA detected by qPCR, suggesting post-translational regulation of LDLR. No changes in PCSK9 mRNA or secreted protein levels were detected by qPCR or ELISA, respectively. beta-Estradiol-conditioned medium devoid of PCSK9 failed to upregulate LDLR. Similarly, PCSK9 knockdown cells showed no upregulation of LDLR by beta-Estradiol. Together, these results indicate a requirement for PCSK9 in the beta-Estradiol-induced upregulation of LDLR. A radiolabeling assay showed a significant, dose-dependent decrease in the ratio of secreted phosphoPCSK9 to total secreted PCSK9 with increasing beta-Estradiol levels, suggesting a change in the functional state of PCSK9 in the presence of beta-Estradiol.
CONCLUSIONS:
Our results indicate that the protein upregulation of LDLR at subtranscriptionally effective doses of beta-Estradiol, and its supratranscriptional upregulation at 10 μm beta-Estradiol, occur through an extracellular PCSK9-dependent mechanism. |