Hormonal regulation of surfactant production in cultured fetal lung cells

Namita Sen

Type II pneumocytes are responsible for the synthesis, storage and secretion of surfactant, a compound which lowers surface tension in the lung thus preventing alveolar collapse at end expiration. The rate of synthesis and secretion of the major surface-active component, disaturated phosphatidylcholine (DSPC), has been shown to be influenced by a number of agents. In this study it has been shown that epidermal growth factor (EGF) increases the rate of choline incorporation into DSPC by cultured fetal type II cells in a manner analogous to glucocorticoids. Whereas EGF has no effect on the rate of DSPC synthesis when added directly to type II pneumocytes, the growth factor is effective if it is present during preliminary conditioning of the media by lung fibroblasts. This effect is concentration dependent with a maximal effect at 20 ng. mL-1 and can be mimicked by the phorbol ester, phorbol 12-myristate 13-acetate (PMA), Although the calcium ionophore A23187 is without effect alone, it does enhance the stimulatory effect when added with PMA. These findings suggest that the response to EGF might well involve activation of protein kinase C. When lung fibroblasts are incubated with both glucocorticoids and EGF there is no significant effect of the growth factor over and above that seen with the steroid alone. This suggests that the two agents might act via a similar mechanism. This is supported by the observation that each inducer leads to the production by lung fibroblasts of a stimulatory factor which has a similar, if not identical, chromatographic elution profile. EGF has also been shown to interact directly with type II pneumocytes and, in a time- and concentration-dependent manner, enhances the rate of secretion of surfactant phospholipids. This effect was shown to be mimicked by either the calcium ionophore, A23187, or the protein kinase C activator, PMA. The positive response to A23187 on the rate of surfactant secretion is in contrast to the lack of response when surfactant synthesis was examined and suggests the elevated secretion rate in response to EGF probably involves both Ca2+ mobilization and protein kinase C activation. This study has also demonstrated that p-adrenergic receptors are present and functional in cultures enriched with fetal type II pneumocytes. Using the specific radioligand I-CYP, and both pi and P2 antagonists, it was established that the p2-subtype was the predominant p-adrenoceptor associated with fetal rat type II cells. Direct exposure of the type II cells to either fibroblast-conditioned media or glucocorticoids results in an elevation in the level of p-adrenoceptors. These conditions also lead to an enhanced response to the p-agonist, (—) -isoproterenol, as judged by a greater rate of phospholipid secretion, indicating a close link between the level of p-adrenoceptors and the extent to which p-agonists are able to stimulate surfactant secretion.

Hormonal regulation of surfactant production in cultured fetal lung cells

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