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

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Abstract
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.