326 LIPOMODULIN ANTIBODY PREVENTS THE GLUCOCORTICOIDINDUCED INCREASE ON PULMONARY B-ADRENERGIC RECEPTORS (β-AR)

Abstract

Glucocorticoids produce most of their effects by inducing the synthesis of endogenous, phospholipase A₂ inhibitory proteins, such as lipomodulin (J Biol Chem 256:7730). Glucocorticoids also increase the cellular numbers of β-AR in the lung (Bioch Biophys Res Commun 94:390), which may be the mechanism of the synergism between glucocorticoids and β-adrenergic agonists for increasing surfactant production, and of the sensitization of the lung to β-adrenergic bronchodilation. We studied the mechanism of the increase in β-AR by glucocorticoids in a human lung adenocarcinoma cell line (A549) frequently used as a model of alveolar type II cells. Cells were cultured to confluency in RPMI 164 medium containing 10% fetal calf serum. Cells were then incubated for a further 24 hours in serum-free Dulbecco MEM with or without 10 μM corticosterone, in the presence of 5 μl/ml of saline (S), a monoclonal antibody of lipomodulin (AB), or control ascites fluid (ASC). Cells detached in calcium-free medium were freeze-thawed, and β-AR binding was assayed using ¹²⁵I-cyanopindolol. CORT increased β-AR/cell from 680 to 1460 in S and from 240 to 1650 in ASC (P < 0.05 for both), but CORT was ineffective in the presence of AB (450 vs 480 β-AR/cell). These findings suggest that glucocorticoids do not directly induce the synthesis of lung β-AR, but increase their numbers indirectly through the induction of the phospholipase inhibitory protein, lipomodulin.