RAT SERUM INHIBITS PROGRESSION OF ALVEOLAR EPITHELIAL-CELLS TOWARD THE TYPE-I CELL PHENOTYPE IN-VITRO

Serum contains a number of polypeptide growth factors, hormones, and s oluble matrix components and may influence the state of differentiatio n of epithelial cells in general and of alveolar epithelial cells (AEC ) in particular. To evaluate the influence of sera: on the transition from the type II toward the type I cell phenotype, we compared the eff ects of newborn bovine serum (NBS) and rat serum (RS) on morphologic c hanges and expression of a type I cell-specific epitope in AEC monolay ers with time in primary culture. Rat type II AEC were harvested and c ultured in defined serum-free medium (MDSF), MDSF + RS (5%), or MDSF NBS (10%). Monolayer integrity was monitored by measuring transepithe lial resistance (similar to 2,000 Ohm.cm(2)) and short-circuit current (similar to 4 mu A/cm(2)). Binding of the type I cell-specific monocl onal antibody VIIIB2 was assessed between day 1 and day 11 by cell-bas ed enzyme-linked immunosorbent assay (ELISA) and by immunoelectron mic roscopy (IEM). By ELISA, in MDSF and MDSF + NBS, VIIIB2 binding increa sed markedly after day 2, rising approximately 4-fold by day 8 (compar ed with day 1). In dramatic contrast, there was essentially no increas e in VIIIB2 binding through day 11 in MDSF + RS. Results from IEM for apical surface binding of VIIIB2 were similar to those obtained by ELI SA. Some morphologic differences were also noted, with cells in MDSF RS being somewhat less spread at later times than those in MDSF or MD SF + NBS. These data indicate that the rate of rat type II AEC differe ntiation toward the type I cell phenotype is significantly modulated b y soluble factor(s) present in rat serum.