Inhibition of hepatic apolipoprotein A-I gene expression by histamine

In a recent high throughput analysis to identify drugs that alter hepatic apolipoprotein A-I (apo A-I) expression, histamine receptor one (H₁) antagonists emerged as potential apo A-1 inducing drugs. Thus the present study was undertaken to identify some of the underlying molecular mechanisms of the effect of antihistaminic drugs on apo AI production. Apo A-I levels were measured by enzyme immunoassay and Western blots. Apo A-I mRNA levels were measured by reverse transcription real-time PCR using glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA as the internal control. The effects of histamine and antihistamines on apo A-I gene were determined by transient transfection of plasmids containing the apo A-I gene promoter. Histamine repressed while (H₁) receptor antagonist azelastine increased apo A-I protein and mRNA levels within 48 h in a dose-dependent manner. Azelastine and histamine increased and suppressed, respectively, apo A-I gene promoter activity through a peroxisome proliferator activated receptor α response element. Treatment of HepG2 cells with other H₁ receptor antagonists including fexofenadine, cetirizine, and diphenhydramine increased apo A-I levels in a dose-dependent manner while treatment with H₂ receptor antagonists including cimetidine, famotidine, and ranitidine had no effect. We conclude that H₁ receptor signaling is a novel pathway of apo A1 gene expression and therefore could be an important therapeutic target for enhancing de-novo apo A-1 synthesis.