Epithelial cell proliferation contributes to airway remodeling in severe asthma.

RATIONALE: Despite long-term therapy with corticosteroids, patients with severe asthma develop irreversible airway obstruction. OBJECTIVES: To evaluate if there are structural and functional differences in the airway epithelium in severe asthma associated with airway remodeling. METHODS: In bronchial biopsies from 21 normal subjects, 11 subjects with chronic bronchitis, 9 subjects with mild asthma, and 31 subjects with severe asthma, we evaluated epithelial cell morphology: epithelial thickness, lamina reticularis (LR) thickness, and epithelial desquamation. Levels of retinoblastoma protein (Rb), Ki67, and Bcl-2 were measured, reflecting cellular proliferation and death. Terminal deoxynucleotidyl-mediated dUTP nick end labeling (TUNEL) was used to study cellular apoptosis. MEASUREMENTS AND MAIN RESULTS: Airway epithelial and LR thickness was greater in subjects with severe asthma compared with those with mild asthma, normal subjects, and diseased control subjects (p=0.009 and 0.033, respectively). There was no significant difference in epithelial desquamation between groups. Active, hypophosphorylated Rb expression was decreased (p=0.002) and Ki67 was increased (p<0.01) in the epithelium of subjects with severe asthma as compared with normal subjects, indicating increased cellular proliferation. Bcl-2 expression was decreased (p<0.001), indicating decreased cell death suppression. There was a greater level of apoptotic activity in the airway biopsy in subjects with severe asthma as compared with the normal subjects using the TUNEL assay (p=0.002), suggesting increased cell death. CONCLUSIONS: In subjects with severe asthma, as compared with subjects with mild asthma, normal subjects, and diseased control subjects, we found novel evidence of increased cellular proliferation in the airway contributing to a thickened epithelium and LR. These changes may contribute to the progressive decline in lung function and airway remodeling in patients with severe asthma.

Analysis of human cellular retinol-binding protein II promoter during enterocyte differentiation.

Cellular retinol binding protein II (CRBP II) is a vitamin A-binding protein that is expressed specifically in small intestinal villus absorptive cells. Previous studies have shown that retinoic acid upregulates endogenous human CRBP II gene expression in differentiated Caco-2 cells. To better characterize the regulation of human CRBP II expression, we analyzed the ability of receptor-selective agonists to enhance transcription from the 5'-upstream flanking region of the human CRBP II gene. Stable transfection experiments showed that the proximal 2.8-kb region of the human CRBP II gene is sufficient for retinoic acid inducibility in differentiated Caco-2 cells. However, direct sequence analysis and transient transfection experiments indicate that, unlike the rat CRBP II promoter, the human CRBP II promoter is not a direct retinoid X receptor target. The results indicate that the retinoic acid responsiveness of the human CRBP II promoter is mediated by an indirect mechanism and that this mechanism is associated with enterocyte differentiation.

Increased neonatal mortality in mice lacking cellular retinol-binding protein II.

Cellular retinol-binding protein II (CRBP II) is a member of the cellular retinol-binding protein family, which is expressed primarily in the small intestine. To investigate the physiological role of CRBP II, the gene encoding CRBP II was inactivated. The saturable component of intestinal retinol uptake is impaired in CRBP II(-/-) mice. The knockout mice, while maintained on a vitamin A-enriched diet, have reduced (40%) hepatic vitamin A stores but grow and reproduce normally. However, reducing maternal dietary vitamin A to marginal levels during the latter half of gestation results in 100% mortality/litter within 24 h after birth in the CRBP II(-/-) line but no mortality in the wild type line. The neonatal mortality in heterozygote offspring of CRBP II(-/-) dams (79 +/- 21% deaths/litter) was increased as compared with the neonatal mortality in heterozygote offspring of wild type dams (29 +/- 25% deaths per litter, p < 0.05). Maternal CRBP II was localized by immunostaining in the placenta at 18 days postcoitum as well as in the small intestine. These studies suggest that both fetal as well as maternal CRBP II are required to ensure adequate delivery of vitamin A to the developing fetus when dietary vitamin A is limiting.