Mouse Balb/c3T3 cell mutant with low epidermal growth factor receptor activity: Induction of stable anchorage-independent growth by transforming growth factor β

A mutant clone (Mo‐5) was originally isolated as a clone resistant to Na+/K+ ionophoric antibotic monensin from mouse Balb/c3T3 cells. MO‐5 was found to show low receptor‐endocytosis activity for epidermal growth factor (EGF): binding activity for EGF in MO‐5 was less than one tenth of that in Balb/c3T3. Anchorage‐independent growth of MO‐5 was compared to that of Balb/c3T3 when assayed by colony formation capacity in soft agar. Coadministration of EGF and TGF‐β efficiently enhanced anchorage‐independent growth of normal rat kidney (NRK) cells, but neither factor alone was competent to promote the anchorage‐independent growth. The frequency of colonies appearing in soft agar of MO‐5 or Balb/c3T3 was significantly enhance by TGF‐β while EGF did not further enchance tha tof MO‐5 or Balb/c3T3. Colonies of Balb/c3T3 formed in soft agar in the presence of TGF‐β showed low colony formation capacity in soft agar in the absence of TGF‐β. Colonies of MO‐5 formed by TGF‐β in soft agar, however, shoed high colony formation capacity in soft agar in the absence of TGF‐β. Pretreatment of MO‐5 with TGF‐β induced secretion of TGF‐β‐like activity from the cells, while the treatment of Balb/c3T3 did not induce the secretion of a significant amount of TGF‐β‐like activity. The loss of EGF‐receptor activity in the stable expression and maintenance of the “transformed” phenotype in MO‐5 is discussed.