AUTOCRINE TRANSFORMING-GROWTH-FACTOR-ALPHA PROVIDES A GROWTH ADVANTAGE TO MALIGNANT-CELLS BY FACILITATING REENTRY INTO THE CELL-CYCLE FROM SUBOPTIMAL GROWTH STATES

CBS human colon carcinoma cells are poorly tumorigenic in athymic nude mice, whereas FET colon carcinoma cells are non-tumorigenic, Both cel l lines have well differentiated properties in tissue culture. Transfo rming growth factor alpha (TGF-alpha) was ectopically expressed by sta ble transfection of a TGF-alpha cDNA under repressible tetracycline co ntrol. The TGF-alpha-transfected cells showed enhanced clonal initiati on and shortened lag phase growth in tissue culture without an alterat ion in doubling time in exponential phase relative to untransfected ce lls. Furthermore, the TGF-alpha transfectants showed increased indepen dence from exogenous growth factors in clonal growth assays and induct ion of DNA synthesis after release from quiescence. Growth factor inde pendence was associated with sustained epidermal growth factor recepto r activation in quiescent TGF-alpha-transfected cells and the requirem ent of exogenous insulin for stimulation of quiescent cells to reenter the cell cycle. Higher cloning, reduced lag time in tissue, and the a cquisition of growth factor independence for DNA synthesis without a c hange in doubling time of TGF-alpha-transfected cells indicate that au tocrine TGF-alpha functions by facilitating re-entry into the cell cyc le from sub-optimal growth states rather than promoting or controlling the proliferation of actively cycling cells, The modulation of growth regulation by autocrine TGF-alpha was associated with increased malig nant properties as TGF-alpha transfectants showed increased tumorigeni city in athymic nude mice. The administration of tetracycline reversed the effects of TGF-alpha expression in these cells both in vivo and i n vitro, indicating that the alterations of the biological properties were due to the expression of TGF-alpha. Since these cells are continu ously grown in a completely chemically defined medium without serum su pplementation, it was possible to assign the mechanism underlying the generation of growth factor independence to the replacement of a requi rement for exogenous insulin in parental cells by autocrine TGF-alpha.