Induction of
low-density lipoprotein (
LDL) receptor transcription in response to depletion of cellular
sterols in animal cells is well established. The intracellular signal or signals involved in regulating this process, however, remain unknown. Using a specific inhibitor of
protein kinase C (PKC),
calphostin C, we show the requirement of this
kinase in the induction process in human
hepatoma HepG2 cells. Overexpression of
PKC epsilon, but not PKC alpha, -gamma, -delta, or -zeta was found to dramatically induce (approximately 18-fold)
LDL receptor promoter activity. Interestingly,
PKC epsilon-mediated induction was found to be
sterol resistant. To further establish that
PKC epsilon is involved in the
sterol regulation of
LDL receptor gene transcription, endogenous
PKC epsilon was specifically inhibited by transfection with antisense
PKC epsilon phosphorothionate
oligonucleotides. Antisense treatment decreased endogenous
PKC epsilon protein levels and completely blocked induction of
LDL receptor transcription following
sterol depletion.
PKC epsilon-induced
LDL receptor transcription is independent of the
extracellular signal-regulated kinase 1 and 2 (p42/44(MAPK)) cascade, because the
MEK-1/2 inhibitor,
PD98059 did not inhibit, even though it blocked p42/44(MAPK) activation. Finally, photoaffinity labeling studies showed an
isoform-specific interaction between
PKC epsilon and
sterols, suggesting that
sterols may directly modulate its function by hampering binding of activators. This was confirmed by PKC activity assays. Altogether, these results define a novel signaling pathway leading to induction of
LDL receptor transcription following
sterol depletion, and a model is proposed to account for a new function for
PKC epsilon as part of a
sterol-sensitive signal transduction pathway in hepatic cells.