ASBT transcription is downregulated by which mechanism?

• A. By vitamin D receptor signaling
• B. By insulin signaling
• C. By inflammatory cytokines via NF-kB
• D. By bile acids via farnesoid X receptor

Answer: (D)

Bile acids exercising feedback control over their own reabsorption is the key concept. In ileal enterocytes, bile acids activate the farnesoid X receptor (FXR), a nuclear receptor. Once FXR is activated by bile acids, it downregulates ASBT transcription, reducing the reuptake of bile acids from the intestinal lumen. This helps limit the enterohepatic bile acid pool when bile acid levels are high. FXR activation also promotes other elements of bile acid homeostasis, such as inducing FGF19 to signal the liver to curb bile acid synthesis, reinforcing the feedback.

Other mechanisms listed aren’t the primary physiologic means by which ASBT transcription is controlled in this context. Vitamin D receptor signaling targets different genes; insulin signaling isn’t the main regulator of ASBT; and while inflammatory cytokines can modulate transporter expression in disease, the canonical mechanism for downregulating ASBT in response to bile acids is via FXR activation by bile acids.