INITIATION OF GROWTH-INHIBITION BY TGF-BETA-1 IS UNLIKELY TO OCCUR ING(1)

Type beta transforming growth factors represent a family of polypeptid es that modulate growth and differentiation. They exert their effect o n target cells through interaction with multiple cell surface receptor s, Transforming growth factor-beta 1 has a strong inhibitory action on cell division in mink lung CCI64 cells, a process that is initiated b y immediate induction of junB and phosphorylation of nuclear protein f ollowed by a reduced expression of cdk4. However, its signal transduct ion pathways are still unresolved, In this study we report a detailed analysis of cell kinetic events following addition of transforming gro wth factor-beta 1 to mink lung CCL64 cells, We show that transforming growth factor-beta 1 reduces [H-3]thymidine incorporation after a dela y of 8 hours, which reaches its nadir at 16 hours, The reduced growth rate is maintained for at least 48 hours as shown by flow cytometric a nalysis of DNA content, Using time-lapse-video microscopy it was shown that control cells double on average every 14.4 hours, whereas the tr ansforming growth factor-beta 1-treated cells have a doubling time of on average 20.3 hours, The difference in intermitotic time is a conseq uence of a prolonged G(1) phase (a shift from 7.5 to 13.5 hours on ave rage). However, changes in intermitotic times occur only after cells h ave undergone division in the presence of transforming growth factor-b eta 1 and treated cells finish the ongoing cell cycle exactly like con trol cells, From these findings we conclude that transforming growth f actor-beta 1 may change cell cycle parameters by interfering with cell ular events prior to G(1). Immunoprecipitation studies with antibodies against cyclin B demonstrate that transforming growth factor-beta 1 i nduces the association of a 20 kDa protein with the immune complex. Th is observation suggests that interference with the cell cycle may comm ence in late S or G(2).