VITAMIN-D STIMULATES DNA-SYNTHESIS IN ALVEOLAR TYPE-II CELLS

Alveolar type-II cells are responsible for alveolar epithelial cell pr oliferation during growth and development and in response to lung inju ry. Based on the observation of abnormal lung development in rachitic rat pups and the expression of receptors for vitamin D by fetal alveol ar epithelial cells, the present study examined the influence of 1,25- dihydroxy vitamin D (DHD) on the proliferation of primary cultures of fetal, neonatal and adult alveolar epithelial cells. The ontogony of v itamin D responsiveness was examined, using fetal (days 18, 19 and 22 = term), neonatal (days 7 and 18) alveolar epithelial cells as well as adult alveolar type-II cells. Maximal stimulation of [H-3]thymidine i ncorporation occurred in neonatal d18 cells: (250 +/- 4.8%, n = 4, P < 0.05). Incubation of adult type-II cells, in the presence of 10(-9) M DHD increased thymidine incorporation into DNA (149.1 +/- 33.2%, mean +/- S.E., n = 3, P < 0.001) compared to control cells maintained in b asal medium. Exposure to DHD also increased thymidine incorporation af ter stimulation with a mixture of conventional progression factors (in sulin (10 mu g/ml) (I), cholera toxin (10 mu g/ml> (C) and EGF (20 ng/ ml) (E)) (349.4 +/- 42.9% vs. 213.5 +/- 23.6%, n = 6, P < 0.005). Auto radiographic labeling indices of adult type-II cells increased from 3. 1 +/- 0.6% for cells cultured in basal medium to 7.2 +/- 1.7% in cells exposed to DHD from the time of plating and I, C, E from 20-68 h in c ulture (n = 4, P < 0.05). Although no increase in the number of adult type-II cells was observed in these experiments, flow cytometric analy sis of nuclear DNA content revealed an increased proportion of cells i n the S and G2 phases of the cell cycle (basal: S = 2.6%, G2/M= 3.0%, DHD + GF: S = 4.7%, G2/M = 5.6%, P < 0.05 for each comparison). These data demonstrate that vitamin D3 is a growth factor for alveolar type- II cells and suggest the possibility that local elaboration of vitamin D may provide a novel mechanism of modulation of epithelial prolifera tion in the context of lung development and repair.