A sensitive and specific in vitro bioassay for activin using a mouse plasmacytoma cell line, MPC-11

A new in vitro bioassay for activin was developed using the mouse plasmacyt oma cell line, MPC-11. Human recombinant (hr) activin A dose-dependently in hibited the proliferation of these cells, whereas a range of other factors, including inhibin, follistatin and transforming growth factor-beta 1, -bet a 2 and -beta 3 had no effect. Conditioned medium containing activin B indu ced an inhibition similar to hr-activin A. The inhibitory influence of acti vin A could be blocked by follistatin, but not by hr-inhibin A. This bioass ay had a sensitivity for activin A of around 0.4 ng/ml, an ED50 response of 35 ng/ml, and an intraassay coefficient of variation of <11%. It offers su bstantial advantages over existing in vitro activin bioassays in terms of e ase of use, specificity and throughput. The utility of the MPC-11 bioassay was demonstrated in the purification of activin from amniotic fluid, where an almost identical profile of bioactive activin A was detected compared wi th the pituitary cell bioassay of activin. Bioactive activin could also be detected in unpurified ovine allantoic and amniotic fluids and bovine folli cular fluid. Measuring activin in untreated and heat-treated human sera or seminal plasma was hampered by a non-specific inhibitory effect, so that se veral serum samples did not run parallel with the hr-activin A standard. Th is inhibitory effect by serum could not be overcome by addition of follista tin, suggesting it is not activin-like bioactivity. This new bioassay for a ctivin demonstrates widespread applicability for monitoring of purified or partially purified samples during purification procedures, bioactivity meas urements, receptor-binding studies and assays of cell culture medium.