Differential response to exogenous and endogenous activin in a human ovarian teratocarcinoma-derived cell line (PA-1): Regulation by cell surface follistatin

The activin/follistatin system is implicated in growth and differentiation of various cell types. Follistatin (FS), through binding and neutralizing activin, plays a major role in the regulation of activin bioavailability. We previously reported that ovarian PA1 cells constitutively secrete FS and show a decreased proliferation rate in response to exogenous activin only if cell surface associated FS is first removed by heparin treatment. These observations suggest that cell-associated FS prevents exogenous activin from accessing its receptor. We hypothesized that cell surface FS would differentially regulate the bioavailability of endogenous and exogenous activin in these cells. To examine the effect of endogenous activin, PA1 cells were stably transfected with an activin beta(A)-subunit complementary DNA (cDNA). The proliferation rate of five activin-secreting clones was measured by [H-3]thymidine incorporation and compared with the proliferation rate of untransfected cells. In clones secreting levels of activin ranging from 22.6 +/- 7.1 to 42.4 +/- 9.9 ng/ml, proliferation was decreased by 31-72% at 96 h of culture, whereas one cell line secreting lower levels of activin (0.4 +/- 0.1 ng/ml) proliferated similarly to the untransfected cells, in which activin was not detectable. To further assess activin signaling, wild-type PA1 cells and activin-secreting clones were transiently transfected with an activin response element-luciferase re-porter construct. Basal luciferase activity was 6-fold higher in activin-secreting clones than in wild-type PA1 cells. Exogenous activin (100 ng/ml) increased the transcriptional response of wild-type PA1 cells by 3-fold but did not increase reporter activity in activin secreting clones. Interestingly, the transcriptional response in activin secreting clones was always greater than the basal or activin-stimulated response in wild-type cells. Furthermore, we found that FS was removed from the cell surface by lipofectamine used for these transfections. Therefore, these results show that activation of the luciferase reporter gene occurs under conditions in which proliferation is affected, suggesting that the antiproliferative effect of activin could be due to direct stimulation of activin signaling pathways. In summary, as opposed to exogenous activin, endogenous activin decreased proliferation of PA1 cells even in the presence of cell surface associated FS. These results are consistent with a model in which FS acts as a barrier for exogenous (endocrine-paracrine) but not for endogenous (autocrine) activin. In addition, the higher PA1 cell responsiveness to endogenous compared with exogenous activin, suggests that activin overexpression in PA1 cells may up-regulate an activin signaling component, or down-regulate an activin signaling inhibitor.