Novel non-tumorigenic cell variants showing potentially different susceptibility to v-src-induced metastasis

Abrogation of G1 arrest after DNA damage is associated with constitutive overexpression of Mdm2, Cdk4, and Irf1 mRNAs in the BALB/c 3T3 A31 variant 1-1 clone

A search of cell lines showing aberrant cell-cycle checkpoints revealed the lack of gamma-irradiation-induced G1 arrest in the BALB/c 3T3 A31 variant 1-1 (A31-1-1) clone. This clone is well-known for its hypersensitivity to transformation by DNA damaging agents. p53 stabilization and p21 mRNA induction after 8 Gy irradiation were observed, suggesting that the abrogation of G1 arrest occurred in spite of normal p53 and p21 responses by abnormal regulation of other cellular factors. Constitutive overexpression of Mdm2 and Cdk4 mRNAs was found, which might have contributed to the loss of G1 arrest. In addition, overexpression of a growth-suppressive transcription factor, Irf1, a putative key molecule in the p53-independent pathway after DNA damage, was also observed, although the relation to the loss of G1 arrest could not be elucidated.

Different metastatic potentials of ras- and src-transformed BALB/c 3T3 A31 variant cells

The metastatic phenotype of tumor cells is thought to be induced by an aberrant signaling cascade or cascades that are different from those required for tumorigenicity. Oncogene-transfected cells with different tumorigenicities and metastatic potentials have been used to identify such pathways and responsible molecules. However, oncogenes that can induce tumorigenicity in recipient cells also frequently induce the metastatic phenotype at the same time. The difficulty in obtaining cell lines that are tumorigenic but not metastatic has hampered such studies. In this report, we transfected the activated c-Ha-ras oncogene into BALB/c 3T3 A31 variant cells and found that the transfectants were tumorigenic but they did not form metastatic lung modules in the experimental metastasis assay. The phenotype was very stable and was maintained during cultivation. On the other hand, the metastatic potentials of either the transfected cells or the original variant cells could be induced by transfection of the v-src oncogene. The src transfectants formed extensive nodules in lung when injected into the tail veins of congeneric mice. The cell motility of the metastatic src transfectants on Matrigel-coated dishes was greater than that of the ras transfectants. The src transfectants were also invasive in Matrigel when analyzed on a filter. These variant cells transformed by the ras and src oncogenes will be a useful system for identifying the signaling cascades responsible for the metastatic potential of tumors.