Various stimulators of the cyclic AMP pathway fail to promote adipose conversion of porcine preadipocytes in primary culture

The cyclic adenosine-monophosphate (cAMP) pathway is generally recognized as one of the essential pathways for the adipose conversion of rodent preadipocytes in vitro. However, divergent effects of cAMP on adipocyte differentiation have also been reported. Since there is very little data on non-rodent preadipose cells, the aim of the present work was to analyze the effects of classic activators of the cAMP pathway on the proliferation and differentiation of porcine preadipocytes grown either in serum-free or in serum-containing medium. In both media, the addition of 10 mu M forskolin from day 1 after cell plating to day 3 or 7 did not affect cell proliferation. Such stimulations also failed to enhance preadipocyte differentiation, as assessed by the measurement of lipoprotein lipase (LPL) and glycerol 3-phosphate dehydrogenase (GPDH) activities, two markers of adipose conversion. Similar results were obtained when various concentrations of forskolin (0.1 nM-100 mu M) were added for 2 days either during the growth phase (days 1-3) or after confluence (days 5-7). Addition of methylisobutylxanthine (MIX) or 8-bromo-cAMP was also found inefficient to stimulate porcine preadipocytes differentiation clearly. By contrast, post-confluence treatment of the murine 3T3-L1 cell line with either forskolin or MIX markedly enhanced lipid accumulation and led to a dramatic increase in GPDH activity (up to 120 times). This indicates that similar culture conditions are adipogenic for the murine 3T3-L1 preadipocytes but not for porcine preadipose cells. In summary, this work clearly highlights the finding that porcine preadipocytes do not respond to classic activators of the cAMP pathway like rodent cells do. This calls in question again the general model proposed for the action of this pathway in adipose conversion and suggests that the mechanisms regulating adipocyte differentiation may differ among species.