RESTRICTIN-P STROMAL ACTIVIN-A, KILLS ITS TARGET-CELLS VIA AN APOPTOTIC MECHANISM

We have recently found that the inhibitor of plasmacytoma cell growth, restrictin-P, is a stroma derived activin A and that it is an antagon ist of interleukin-6 and interleukin-ll. The present study was aimed a t determining the mode by which this cytokine kills its target cells. On addition of the cytokine there was little or no net increase in cel l number, depending on the specific target cells. All plasmacytoma cel l lines tested exhibited a similar time dependent inhibition of DNA sy nthesis and a G(0)/G(1) shift in the cell cycle. Electron microscope e xamination revealed classical apoptotic features i.e. chromatin conden sation and membrane blebbing. DNA fragmentation, measured qualitativel y and quantitatively, occurred in all cytokine treated plasmacytoma ce ll lines. Bovine activin A had an identical capacity to reduce cell vi ability, to induce G(0)/G(1) shift and to cause DNA fragmentation. X-r ay microanalysis of intracellular ions revealed an increase in calcium ions, following exposure of plasmacytoma cells to restrictin-P, accom panied by a decrease in phosphor ions. The cytotoxicity of the inhibit or was augmented in an additive manner by cycloheximide (CHX) indicati ng that the process did not require de novo protein synthesis. This st udy thus shows that restrictin-P/stromal activin A kills its target ce lls by inducing apoptosis. This effect was mediated by subnanogram con centrations and therefore may represent one physiological function of this pleiotropic cytokine.