Cholesterol depletion enhances TGF-β Smad signaling by increasing c-Jun expression through a PKR-dependent mechanism

Area of Science:

• Cellular signaling pathways
• Molecular biology
• Cholesterol metabolism

Background:

• Transforming growth factor-β (TGF-β) signaling is crucial for physiological and pathological processes.
• Cellular cholesterol levels influence signaling pathways, but cross-talk with TGF-β signaling is unclear.
• Statins, used to lower cholesterol, can have side effects, potentially linked to cellular signaling alterations.

Purpose of the Study:

• To investigate the impact of reduced free cholesterol on TGF-β signaling components.
• To elucidate the molecular mechanisms linking cholesterol homeostasis to TGF-β pathway activation.
• To explore the potential contribution of these mechanisms to statin-associated side effects.

Main Methods:

• Studied the effects of mild cholesterol reduction on TGF-β signaling components in cells.
• Analyzed protein expression and phosphorylation levels, including Smad2/3, c-Jun, eIF2α, and PKR.
• Investigated downstream signaling events following receptor activation.

Main Results:

• Mild cholesterol reduction enhances TGF-β signaling downstream of receptor activation.
• Reduced cholesterol leads to PKR-dependent eIF2α phosphorylation and increased c-Jun translation.
• This results in elevated JNK-mediated c-Jun phosphorylation, enhancing Smad2/3 expression and phosphorylation.
• The phospho/total Smad2/3 ratio remained unchanged, suggesting the effect is not due to altered receptor activity.

Conclusions:

• Cholesterol depletion activates a novel pathway involving PKR, JNK, and Smad2/3, leading to enhanced TGF-β signaling sensitivity.
• This cholesterol-mediated regulation provides new insights into TGF-β pathway modulation.
• The findings suggest that cholesterol depletion-induced overactivation of PKR, JNK, and TGF-β signaling may contribute to the side effects of statins.