Continuous maintenance of transformed fibroblasts under reduced serum conditions: Utility as a model system for investigating growth factor‐specific effects in nonquiescent cells

We report the continuous growth maintenance of untransformed and chemically transformed fibroblasts (AKR‐2B, AKR‐MCA cells) in low concentrations of serum (0.1% FBS). The cell lines established (AKR‐0.1F, MCA‐0.1F) proliferated at rates comparable to cells maintained under high serum conditions (10% FBS). Complete removal of serum from the cells did not induce quiescence. The MCA‐0.1F cells were more similar to the untransformed AKR‐2B fibroblasts in their morphology, saturation density, inability to form colonies under anchorage‐independent conditions, steady‐state level of c‐myc expression, and kinetics of induction of c‐myc in response to specific growth factors. This report demonstrates the utility of this cell line as anonquiescent model system for investigating growth factor‐specific effects in serum‐free, cycling cells. Addition of transforming growth factor‐β (TGF‐β) (5 ng/ml) to proliferating MCA‐0.IF cells, in the absence of any serum, induced a multilayered growth pattern at confluency, similar to that of AKR‐MCA cells maintained in 10% FBS. Other growth factors tested did not elicit this effect. The induction of this growth pattern by TGF‐β was associated with a sustained induction of the c‐myc proto‐oncogene at con‐fluency, but not with a restoration of anchorage‐independent growth. The data suggest that TGF‐β may play a role in the up‐regulation of c‐myc at confluency previously described for AKR‐MCA cells maintained in 10% serum.