CHOLERA-TOXIN STIMULATES TYPE-II PNEUMOCYTE PROLIFERATION BY A CYCLICAMP-INDEPENDENT MECHANISM

Cholera toxin (CT) stimulated DNA synthesis by low-density primary cul tures of adult rat type II pneumocytes (T2P) in a dose-dependent manne r, either in the presence or the absence of serum. In the presence of 1% rat serum, 1 mu g/ml CT also stimulated a 50% increase in cell numb er over 8 days of incubation (P < 0.01); this was in addition to a 2-f old increase in cell number induced by the serum alone (P < 0.05). The same dose of CT also elevated intracellular cAMP and the total activi ty of protein kinase A (both P < 0.01), suggesting toxin stimulation o f T2P proliferation by a cAMP-dependent mechanism. However, the effect of CT on DNA synthesis could not be mimicked by 8-bromoadenosine 3':5 '-cyclic monophosphate (8-bromo-cAMP), nor by N-6,2'-O-dibutyryladenos ine 3':5'-cyclic monophosphate (dibutyryl-cAMP), each tested over a wi de range of concentrations. L-Isoproterenol stimulated surfactant secr etion by over 5-fold (P < 0.01), but neither the P-agonist, forskolin nor 3-isobutyl-1-methylxanthine had any significant effect on DNA synt hesis. The purified B-subunit of CT stimulated DNA synthesis to the sa me degree as did the holotoxin, either in the presence or the absence of rat serum. In contrast, the purified A-subunit had no significant e ffect. These data suggest that cholera toxin stimulates type II pneumo cyte proliferation through a mechanism that is independent of cAMP, pr otein kinase A and toxin-catalyzed ADP-ribosylation. (C) 1998 Publishe d by Elsevier Science B.V. All rights reserved.