Untitled

Abstract

Our previous observation that induction of low density lipoprotein (LDL) receptor expression by a variety of extracellular signals is blocked by PD98059, a specific mitogen-activated protein kinase kinase inhibitor, led to the suggestion that the growth-responsive p42/44(MAPK) cascade plays a critical role in regulating LDL receptor transcription. To analyze the specific contribution of the p42/44(MAPK) cascade in regulating cell growth and LDL receptor induction, we established a HepG2-derived cell line that stably expresses an inducible form of oncogenic human Raf-1 kinase. Using this system, we provide direct evidence that specific activation of this cascade alone is not only required but is sufficient to fully induce LDL receptor expression. Interestingly, degree of p42/44(MAPK) activation determines the extent of LDL receptor induction. However, activation of p42/44(MAPK) in the above cells led to the inhibition of DNA synthesis, caused growth arrest, decrease in cyclin A and upregulation of p21(Cip) expression in a time-dependent manner. These results suggest that each of these two processes can be regulated independently of each other in response to p42/44(MAPK) activation. Thus, extent of p42/44(MAPK) activation may be important in transducing divergent cellular responses in human cells with implications for altered signaling resulting in hypercholesterolemia.

Myc-interacting protein 1 target gene profile: A link to microtubules, extracellular signal-regulated kinase, and cell growth

“…Our initial rationale for using the ERK pathway inhibitor, PD98059, derived from the finding that microtubule disrupting drugs like nocodazole and colchicine not only activate MIZ-1, but also induce ERK (7). In addition, LDLR gene expression has been reported to respond to stimulation of the ERK cascade (12). We therefore sought to determine whether ERK activation in some way modulates MIZ-1 subcellular localization in response to microtubule depolymerization by T113242.…”

PKCβ: Expanding role in hepatic adaptation of cholesterol homeostasis to dietary fat/cholesterol

“…Both mechanisms may cooperate to maximize the effectiveness of PKC␤ deficiency on hepatic ERK1/2 activation. It is interesting to note that ERK1/2 plays an important role in controlling hepatic LDL receptor expression (20,50,53,54,91), SREBP-2 expression (52), VLDL assembly (96), and cholesterol efflux (68,104). The net effect of systemic PKC␤ deficiency would be to promote hepatic cholesterol accumulation, as was observed for PKC␤ Ϫ/Ϫ mice.…”

“…PKC␤ is also shown to mediate insulin-induced liver SREBP-1c expression (102), and downstream mitogen/extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) is reported to modulate SREBP-2 transactivation (3, 52) ( Table 1) and cholesterol-7␣-hydroxylase (Cyp7a1) expression via small heterodimer partner modification (65). Moreover, PKCs are shown to inhibit glycogen synthase kinase-3␤ (30), which is shown to modulate SREBP degradation (96); Finally, ERK1/2 inhibition promotes very low-density lipoprotein (VLDL) assembly/secretion (97), whereas we have reported earlier that ERK1/2 also controls hepatic low-density lipoprotein (LDL) receptor expression levels (20,50,53,54,91). Despite the implications of PKCs in cellular mechanisms critically important to regulation of cholesterol and bile acid homeostasis by in vitro studies and cell culture models, no in vivo evidence existed to suggest a physiologically relevant role for specific PKC isoforms in this process.…”