Qp. Chen et al., SECRETAGOGUES INCREASE THE EXPRESSION OF SURFACTANT PROTEIN-A RECEPTORS ON LUNG TYPE-II CELLS, The Journal of biological chemistry, 271(41), 1996, pp. 25277-25283
Since secretagogues have been shown to increase the internalization of
surfactant phospholipid and protein by lung cells, we postulated that
their action occurred through a mechanism involving increased surfact
ant protein A (SP-A) receptor density, Therefore, we evaluated the inf
luence of secretagogues on the binding of iodinated SP-A to alveolar t
ype II cells, Type II cells were isolated from rat lung and maintained
in primary culture for 18 h on Transwell membranes. Upon exposure to
8-bromo-cyclic AMP (cAMP, 0.1 mM), phorbol 12-myristate 13-acetate (PM
A, 10 nM), terbutaline (0.1 mM), or ATP (1 mM), the binding of SP-A in
creased 1.5-2-fold. This stimulation was cell substrate-dependent sinc
e type II cells plated on plastic dishes did not show this effect, A t
ime course of the stimulation of SP-A binding due to secretagogues sho
wed that both cAMP and PMA increased SP A binding by a-fold after 20 m
in, With cAMP, binding remained elevated for 2 h, while binding in the
presence of PMA had returned to control values, The effects of submax
imal concentrations of cAMP and PMA on binding were additive, Inhibiti
on of cellular protein synthesis with cycloheximide did not alter the
increase of SP-A binding stimulated by the secretagogues, Type II cell
s pretreated with PMA responded to subsequent treatment with cAMP by i
ncreasing SP-A binding, while these cells were refractory to subsequen
t treatment with PMA, Both constitutive and regulated binding of SP-A
to type II cells were sensitive to trypsin, The binding of SP-A to typ
e TI cells showed saturation at a concentration of 1 mu g/ml SP-A unde
r control and secretagogue-stimulated conditions, with both total and
calcium-dependent binding showing a a-fold increase upon secretagogue
exposure, The data are consistent with the hypothesis that secretagogu
es stimulate surfactant uptake, at least in part, through recruitment
of SP-A receptors to the type II cell surface, resulting in an increas
e in the number of SP-A binding sites.