ONTOGENY AND REGULATION OF PLATELET-DERIVED GROWTH-FACTOR GENE-EXPRESSION IN DISTAL FETAL-RAT LUNG EPITHELIAL-CELLS

Using flow cytometry, thymidine uptake into DNA, and expression of two growth-related genes, histone 3 and c-myc, we found an increase in th e proportion of distal lung epithelial cells in the G(0)/G(1) phase of the cell cycle with advancing gestation. Since our previous studies h ad demonstrated that platelet-derived growth factor (PDGF) is essentia l for the progression of these cells from the G(0)/G(1) to the S phase of cell cycle, we investigated the gene and protein expression of PDG F-related genes (PDGF-A, PDGF-B, alpha-receptor, and beta-receptor) in distal fetal lung epithelial cells. The cells transcribed all the PDG F-related genes and translated the PDGF-A and PDGF-B mRNAs into protei n, as demonstrated by immunocytochemistry and immunoprecipitation. To explore an autocrine role for PDGF in distal fetal lung epithelial cel ls, intervention studies using PDGF-A and -B chain-specific antisense oligodeoxynucleotides (ODN) were carried out. Antisense PDGF-B ODN, bu t not antisense PDGF-A ODN, significantly reduced the DNA synthesis of these cells. The inhibitory effect of antisense PDGF-B ODN on DNA syn thesis was reversed by the addition of exogenous PDGF-BB, which suppor ts an autocrine role in the DNA synthesis of these cells. We also exam ined the expression of PDGF genes in distal fetal lung epithelial cell s during late gestation. PDGF-A chain and beta-receptor gene expressio ns declined with advancing gestation, whereas expression of message fo r PDGF-B chain and alpha-receptor increased. The increases in message for PDGF-B chain and alpha-receptor with advancing gestation were due to a greater rate of transcription, whereas the developmental decrease of PDGF-A chain and beta-receptor mRNAs was caused by a decrease in R NA stability. Taken together with the ODN data, these results suggest that the G(0)/G(1) cell cycle arrest of distal lung epithelial cells d uring late fetal gestation is due to a decrease in PDGF beta-receptor expression by the cells.