Figure 3.

CRTC1 directly regulates COX-2 and predicts a positive feedback loop deregulated following LKB1 loss. A) Protein immunoblot showing fold induction of COX-2 following ectopic expression of wt or constitutively activated S151A CRTC1. B) COX-2 promoter luciferase induction following ectopic expression of wt or S151A CRTC1. C) ChIP assay of CRTC1 binding to the 5’ COX2 promoter in lung cancer cells. P = .0016 vs IgG control for the real-time polymerase chain reaction analysis using the primers flanking the COX-2 promoter (two-tailed t test). The conserved CREB binding site (CRE) upstream of the TATA element in the COX-2 promoter is depicted. D) Illustration showing a new CRTC1 → COX-2 → PGE2 → cAMP → CRTC1 signaling loop that is normally restrained by wildtype LKB1 → AMPK/SIK1/2/MARK2 signaling. LKB1 is the master upstream regulator for AMPK-related kinases that phosphorylate and repress CRTC1 transcriptional activation via cytoplasmic sequestration. PGE2 (synthesized by COX-2), forskolin, and ß-adrenergic receptor agonists are known cAMP/CRTC1 activators and are depicted in red.