Proteinase inhibitors I and II from potatoes block UVB-induced AP-1 activity by regulating the AP-1 protein compositional patterns in JB6 cells

Abstract

P lants typically respond to environmental stress such as insect herbivory, mechanical damage, and ultraviolet (UV) irradiation by inducing defense-related proteins (1, 2). Among these proteins, proteinase inhibitors I (Inh I) and II (Inh II) isolated from potato leaves are two well-characterized chymotrypsin inhibitors. Both proteins accumulate in potato and tomato leaves and are involved in signal transduction pathways in the plant's protective response against environmental herbivores and pathogens (3-5). In addition, these inhibitors have also been reported to have an inhibitory effect on irradiation-induced cell transformation in mammalian cells (6). We reported previously that both Inh I and Inh II block UVB-or UVC-induced transcription activator protein 1 (AP-1) activity in mouse JB6 cells (7). Considering the critical role that AP-1 activation plays in malignant cellular transformation and tumorigenesis (8-15), the inhibitory effects of these compounds on AP-1 activation may explain their reported antitumor effects. The precise mechanism explaining the inhibition is, however, unclear.AP-1 is an inducible eukaryotic transcription factor composed of products of the jun and fos oncogene families that form Jun-Jun or Jun-Fos dimers (16,17). When stimulated, AP-1 binds to specific transactivation promoter regions or TREs (12-O-tetradecanoylphorbol 13-acetate response elements) on DNA to induce transcription of genes involved in cell proliferation, metastasis, and metabolism (18). Many positive and negative components are involved in the regulation of AP-1 activity. Among these regulators, mitogen-activated protein kinase (MAP kinase) signaling pathways are common mediators of AP-1 function (19). However, we have previously observed that both Inh I and Inh II fail to block extracellular signalregulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), or p38 kinase, three members of the MAP kinase family (7). This observation suggested that inhibition of AP-1 activity by Inh I and Inh II is mediated by a mechanism independent of the MAP kinase pathways.Nuclear factor B (NF B) is another eukaryotic transcriptional factor that appears to be critically involved in regulating the expression of a variety of genes that participate in the inflammatory response and suppression of apoptosis, as well as cell proliferation (20,21). Furthermore, subunits of AP-1 and NF B are able to ''cross-talk,'' and thus both transcription factors may play a role in cellular transformation (22,23). Recent reports have indicated that both AP-1 and NF B are involved in tumor promoter-induced progression in the human keratinocyte transformation model (24,25).To elucidate how the proteinase inhibitors suppress AP-1 activity and whether NF B is involved in this mechanism, we investigated changes in both AP-1 and NF B activities and DNA-binding capacities in response to cellular treatment with Inh I and Inh II and exposure to UVB, respectively. We also determined changes in the AP-1 protein composition after treatment with the Inh I and Inh I...

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Signal Transduction Pathways: Targets for Green and Black Tea Polyphenols

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