The induction of apoptosis of rat liver parenchymal cells (PC) by transforming growth factor β (TGF-β)-expressing transforming fat-storing cells (FSC), i.e., myofibroblasts (MFB), was studied under culture conditions and compared with the apoptotic effect of human recombinant TGF-β1. MFB were obtained by subculture of FSC. The TGF-β concentration in the conditioned medium of myofibroblast (MFBcM) determined with the Mink cell proliferation inhibition assay was <0.25 ng/mL/24 in the native medium, but 1.9 ng/mL24 h after transient acidification. MFBcM added in various dilutions and for different times to PC monolayers induced progressive cell detachment from the plastic support and increase of lactate dehydrogenase (LDH) activity in the medium. The reduction of mitochondrial dehydrogenase activity in PC (XTT or WST-1 test) was an early sign of MFBcM-induced functional impairment of PC. Short term exposure of PC with MFBcM for 3 hours was sufficient to induce the deleterious effects on PC, but neither native (nonactivated) MFBcM nor conditioned medium of untransformed FSC (FSCcM), in which TGF-β was not detectable, were able to impair function and viability of PC. Activated MFBcM increased strongly (up to 21-fold) the concentration of oligonucleosomal DNA fragments both in the adherent and detached fraction of PC. Internucleosomal DNA fragments (DNA ladder) were demonstrated by electrophoresis of extracted DNA on agarose gels and by in situ end-labeling of DNA breaks (TUNEL reaction) only in MFBcM-exposed PC. MFBcM-treated PC exhibited intense fluorescence after staining with DNA-binding dye Hoechst 33342 and an increased number of cells with fragmented nuclei. All these criteria point to MFBcM-generated apoptosis of cultured PC, which were found to be very similar to those induced by human recombinant TGF-β1. The exclusive role of active TGF-β in MFBcM as mediator of the apoptotic effects of MFB was proven by preincubation of the conditioned medium with human recombinant latency-associated peptide, which reversed completely MFBcM induced reduction of the XTT-test and the MFBcM-generated increase of oligonucleosomal DNA fragments. Partial reversibility was reached by preincubation of the medium with recombinant soluble type II TGF-β receptor. The data let us conclude that transformed FSC, i.e., MFB in damaged liver, could participate in the mechanisms of PC apoptosis by paracrine loops involving TGF-β.