Growth factors induce c‐fos transcription by stimulating phosphorylation of transcription factor TCF/Elk‐1, which binds to the serum response element (SRE). Under such conditions Elk‐1 could be phosphorylated by the mitogen‐activated protein kinases (MAPKs) ERK1 and ERK2. However, c‐fos transcription and SRE activity are also induced by stimuli, such as UV irradiation and activation of the protein kinase MEKK1, that cause only an insignificant increase in ERK1/2 activity. However, both of these stimuli strongly activate two other MAPKs, JNK1 and JNK2, and stimulate Elk‐1 transcriptional activity and phosphorylation. We find that the JNKs are the predominant Elk‐1 activation domain kinases in extracts of UV‐irradiated cells and that immunopurified JNK1/2 phosphorylate Elk‐1 on the same major sites recognized by ERK1/2, that potentiate its transcriptional activity. Finally, we show that UV irradiation, but not serum or phorbol esters, stimulate translocation of JNK1 to the nucleus. As Elk‐1 is most likely phosphorylated while bound to the c‐fos promoter, these results suggest that UV irradiation and MEKK1 activation stimulate TCF/Elk‐1 activity through JNK activation, while growth factors induce c‐fos through ERK activation.

Trivalent arsenic (As3+) is highly carcinogenic, but devoid of known mutagenic activity. Therefore, it is likely to act as a tumor promoter. To understand the molecular basis for the tumor‐promoting activity of As3+, we examined its effect on transcription factor AP‐1, whose activity is stimulated by several other tumor promoters. We found that As3+, but not As5+, which is toxic but not carcinogenic, is a potent stimulator of AP‐1 transcriptional activity and an efficient inducer of c‐fos and c‐jun gene expression. Induction of c‐jun and c‐fos transcription by As3+ correlates with activation of Jun kinases (JNKs) and p38/Mpk2, which phosphorylate transcription factors that activate these immediate early genes. No effect on ERK activity was observed. As5+, on the other hand, had a negligible effect on JNK or p38/Mpk2 activity. Biochemical analysis and co‐transfection experiments strongly suggest that the primary mechanism by which As3+ stimulates JNK activity involves the inhibition of a constitutive dual‐specificity JNK phosphatase. This phosphatase activity appears to be responsible for maintaining low basal JNK activity in non‐stimulated cells and its inhibition may lead to tumor promotion through induction of proto‐oncogenes such as c‐jun and c‐fos, and stimulation of AP‐1 activity. The same phosphatase may also regulate p38/Mpk2 activity.

Induction of phase 2 detoxification enzymes by phenolic antioxidants can account for prevention of tumor initiation but cannot explain why these compounds inhibit tumor promotion. Phase 2 genes are induced through an antioxidant response element (ARE). Although the ARE resembles an AP-1 binding site, we show that the major ARE binding and activating protein is not AP-1. Interestingly,

The basal amounts of metallothionein (MT) and its rates of biosynthesis were compared in resting and proliferating Chang liver (CCl-13) cells. In resting cells the total amounts of the detectable isoforms MT-2 and MT-1e were approx. 1.6x10(6) and 4x10(5) molecules per cell respectively. In exponentially growing cultures the cellular contents of both isoforms increased co-ordinately approx. 4-fold and decreased again to the initial values within 48 h after entering density-mediated growth arrest. As documented for MT-2 its transient accretion was attributable to a 10-fold rise in the rate of biosynthesis of this protein during the growth phase. Measurements of the relative amounts of MT-2 mRNA indicated the occurrence of a more than 50% increase within the first 12 h after subculturing of the cells, followed by a return to basal levels thereafter. These results denote a direct link between the programming of MT synthesis and proliferation and thus attest to a central housekeeping function of the MTs.

The synthesis of metallothionein (MT) was investigated in three different human epithelial cell lines, each derived from one of the embryonic germ layers. The accretion of different isoforms of the protein was monitored using a sensitive neutral-pH h.p.l.c. method. Induction of MT synthesis by zinc ions and dexamethasone revealed differences between the three cell lines, both with respect to the number and the amounts of the different isoMTs formed. Dose-response experiments showed that an increase in dexamethasone concentration enhances MT accretion asymptotically to a limit, whereas within the concentration range explored zinc produces an exponential augmentation.