P53 and Egr-1 additively suppress transformed growth in HT1080 cells but Egr-1 counteracts p53-dependent apoptosis

The human fibrosarcoma cell line, HT1080, clone H-4, was used to determine if the transformation suppressive functions of p53 and Egr-1 have the same underlying mechanism. This cell Line expresses only mutant p53 and no detec table Egr-1, H4 clones stably expressing Egr-1 are less transformed in prop ortion to the level of Egr-1 expressed, acting through the induction of the TGF beta 1 gene. Here, H4 cells and the highest Egr-1 expressing clone mer e transfected with a vector expressing normal human p53 to derive stable cl ones expressing p53. The expression of p53 in H4 cells inhibited transforme d growth and reduced tumorigenicity. The effect of coexpression of both p53 and Egr-1 was additive, producing cell lines with 30% of normal growth rat e and sevenfold reduced tumorigenicity compared with control lines. These r esults indicated that each factor may act independently by different pathwa ys, although each additively increased the level of p21(WAF1) cell cycle in hibitor. However, exposure of the H4-derived cells to UV-C irradiation prod uced contrasting effects. Cell cycle analyses shelved that the presence of p53 was associated with loss of the G1 and S cells to apoptosis after irrad iation. In contrast, the expression of Egr-1 increased entry into S/G2 phas e of the cell cycle with Little apoptosis via a mechanism involving elevate d FAK and low caspase activities. Apoptosis was observed only in the cell l ines that expressed no Egr-1, especially those expressing wt-p53, and was p receded by high caspase activity. In summary, Egr-1 suppressed transformati on and counteracted apoptosis by the coordinated activation of TGF beta 1, FN, p21 and FAK, leading to enhanced cell attachment and reduced caspase ac tivity. In the doubly expressing cell line, the survival effect of Egr-1 wa s dominant over the apoptotic effect of p53.