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Transcription of the low density lipoprotein receptor (LDL-R) and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase genes was rapidly and transiently induced (8.5- and 2.3-fold, respectively) early during phorbol 12-myristate 13-acetate (PMA)-induced macrophage differentiation of the human monocytic leukemia cell line THP-1. The levels of mRNA coding for LDL-R and HMG-CoA reductase increased soon after induction, reached a maximum (12- and 7-fold increase, respectively) in 2-3 hr, and then rapidly returned to the low constitutive levels observed before induction. The stability of LDL-R mRNA did not change significantly during differentiation, whereas that of HMG-CoA reductase mRNA decreased by about 5-fold 6 hr after the addition of PMA. Transcriptional induction of both LDL-R and HMG-CoA reductase genes (5.6- and 2-fold, respectively) was also observed when undifferentiated cells were treated with cycloheximide (CHX), resulting in a transient increase in steady-state mRNA (7- and 3-fold, respectively). These results suggest that expression of the two genes is maintained at low constitutive levels in uninduced THP-1 cells by a protein with a short half-life. Superinduction of both genes occurred when PMA and CHX were added simultaneously. The induction of LDL-R and HMG-CoA reductase mRNAs during early macrophage differentiation is mediated by protein kinase C. It is hypothesized that protein kinase C acts directly or indirectly to inactivate the labile negative regulatory protein. Induction of LDL-R mRNA was also observed when the human hepatocarcinoma cell line Hep G2 was treated with PMA and CHX, suggesting that this mechanism of regulation may exist in several cell types.

proceedings of the national academy of sciences of the united states of america

Regulation of the low density lipoprotein receptor and hydroxymethylglutaryl coenzyme A reductase genes by protein kinase C and a putative negative regulatory protein.