Resumption of cell cycle in Balb/c‐3T3 fibroblasts arrested by polyamine depletion: Relation with “Competence” gene expression

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Abstract
Serum deprivation arrests Balb/c-3T3 fibroblasts (clone A31) in G0 phase, where resumption of the cell division cycle can be induced by addition of serum or of specific growth factors in a defined sequence: PDGF (inducer of a state of “competence,” characterized by the expression of a family of genes including c-myc), epidermal growth factor EGF and IGF1 (Leof et al., 1982, 1983). When exponentially growing A31 cells are placed for ≥ 2 days in a medium containing the alpha-difluoromethylornithine (αDFMO), an irreversible inhibitor of ornithine decarboxylase, they become arrested in G1 phase as a consequence of polyamine depletion (Medrano et al., 1983). In the αDFMO-arrested cells, addition of putrescine (60 μM) in a culture medium containing 6% fetal calf serum (FCS), but not in serum-free medium, is sufficient to induce G1 progression and entry into S phase (as determined by 3H-thymidine incorporation). The level of “competence” mRNAs is high in αDFMO-arrested cells. After addition of putrescine in FCS-containing medium, these mRNAs continue to be present for at least 3 h. A large proportion of αDFMO-arrested cells can be induced to progress to S phase by insulin (1 μM, acting via IGF1 receptor) plus putrescine in a serum-free medium (≥ 50% of FCS effect). In this case, the levels of “competence” mRNAs become low or undetectable within 3 h, EGF (10 nM) plus insulin had only slightly greater effect than insulin alone on the progression of αDFMO-arrested cells. When the αDFMO-arrested cells are subsequently incubated during 3 days in a low-serum-containing medium (0.25% FCS), they do not replenish their supply of polyamines, and then continue to express the c-myc gene. The recruitment of the polyamine-depleted, serum-deprived cells into the cell division cycle does not require PDGF and can be induced by addition of EGF and insulin plus putrescine. These data indicate that αDFMO arrests majority of the cells at a point situated beyond the PDGF- and EGF-dependent portion of G1 phase. During the subsequent serum deprivation, the αDFMO-arrested cells remain “competent” (PDGF-independent), probably as a consequence of their continued expression of c-myc (and possibly other PDGF-inducible genes).