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Activation of Raf-1/MEK-1/2/p42/44(MAPK) cascade alone is sufficient to uncouple LDL receptor expression from cell
Gurpreet S Kapoor1, Brett A Atkins, Kamal D Mehta
1Department of Molecular and Cellular Biochemistry, The Ohio State University College of Medicine, Columbus 43210, USA.
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.
Insights
The p42/44 mitogen-activated protein kinase (MAPK) cascade is crucial for regulating low-density lipoprotein (LDL) receptor expression. Its activation is sufficient to induce LDL receptor expression and can independently control cell growth and DNA synthesis.
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Biochemistry
Background:
- Low-density lipoprotein (LDL) receptor expression is regulated by extracellular signals.
- Mitogen-activated protein kinase (MAPK) cascade, specifically p42/44, is implicated in LDL receptor transcription.
- PD98059, a MAPK kinase inhibitor, blocks LDL receptor induction.
Purpose of the Study:
- To investigate the specific role of the p42/44(MAPK) cascade in regulating cell growth and LDL receptor induction.
- To determine if p42/44(MAPK) activation is sufficient to induce LDL receptor expression.
- To analyze the independent effects of p42/44(MAPK) activation on cell growth and DNA synthesis.
Main Methods:
- Established a HepG2-derived cell line with inducible expression of oncogenic human Raf-1 kinase.
- Utilized this system to specifically activate the p42/44(MAPK) cascade.
- Monitored LDL receptor expression, DNA synthesis, cell growth, cyclin A, and p21(Cip) expression.
Main Results:
- Specific activation of the p42/44(MAPK) cascade was sufficient to fully induce LDL receptor expression.
- The extent of p42/44(MAPK) activation correlated with the degree of LDL receptor induction.
- p42/44(MAPK) activation led to growth arrest, inhibited DNA synthesis, decreased cyclin A, and upregulated p21(Cip) expression in a time-dependent manner.
Conclusions:
- The p42/44(MAPK) cascade plays a critical and sufficient role in regulating LDL receptor transcription.
- Cell growth and LDL receptor induction can be independently regulated by p42/44(MAPK) activation.
- Differential activation of p42/44(MAPK) may lead to divergent cellular responses, potentially impacting hypercholesterolemia.