Surface Ig receptor-induced nuclear AP-1-dependent gene expression in B lymphocytes.

The relationship between signals generated via the sIgR complex of B lymphocytes and subsequent changes in gene expression is poorly understood at the molecular level. To illuminate mechanisms that may couple these events, we examined the expression and function of tetradecanoyl phorbol acetate-response element (TRE)-binding proteins (i.e., activator protein 1, (AP-1)) in the murine B lymphoma cell line BAL-17.7.1 (BAL-17), which models primary B lymphocyte responses in a number of respects. Cross-linking of sIgR led to substantial induction of nuclear AP-1, in BAL-17 B cells, that bound the TRE, as detected by electrophoretic mobility shift assay. The sIgR-induced TRE-binding activity consisted of both Jun and Fos proteins, on the basis of immunoreactivity of nucleoprotein complexes with specific antisera. In addition, immunoprecipitation with specific antisera showed that de novo synthesis of Jun-B and c-Jun proteins, accompanied by c-Fos, was stimulated after cross-linking of sIgR on BAL-17 B cells. Transient transfection of BAL-17 B cells with reporter gene constructs showed that B cell AP-1 failed to trans-activate the TRE-containing human collagenase gene promoter, for which activity is dependent upon functional expression of cellular c-Jun. In contrast, sIg-induced AP-1 trans-activated a HSV-tk promoter that contained three TRE; this pattern of gene expression is consistent with the presence of functional Jun-B-containing AP-1 in B lymphocytes. These results are the first to attribute a functional role to sIgR-mediated AP-1 in B lymphoid cells and suggest that AP-1 functions to couple the sIgR complex to changes in nuclear gene expression.     

Induction of human methionine adenosyltransferase 2A expression by tumor necrosis factor alpha. Role of NF-kappa B and AP-1

Two genes (MAT1A and MAT2A) encode for methionine adenosyltransferase (MAT), an essential cellular enzyme responsible for S-adenosylmethionine biosynthesis. MAT1A is expressed mostly in the liver, whereas MAT2A is widely distributed. We showed a switch from MAT1A to MAT2A expression in human hepatocellular carcinoma (HCC), which facilitates cancer cell growth. Using DNase I footprinting analysis, we previously identified a region in the MAT2A promoter protected from DNase I digestion in HCC. This region contains NF-kappa B and AP-1 elements, and the present study examined whether they regulate MAT2A promoter activity. We found nuclear binding of NF-kappa B and AP-1 to the MAT2A promoter increased in HCC. Tumor necrosis factor alpha (TNFalpha), which activates both NF-kappa B and AP-1, increased MAT2A expression in a dose- and time-dependent manner, binding of both NF-kappa B and AP-1 to the MAT2A promoter and MAT2A promoter activity, with the latter effect blocked by site-directed mutagenesis of the NF-kappa B and AP-1 binding sites. Blocking NF-kappa B with I kappa B super-repressor or AP-1 with dominant-negative c-Jun led to decreased basal MAT2A expression and prevented the TNF alpha-induced increase in MAT2A expression. Although blocking NF-kappa B had no influence on the ability of TNF alpha to increase AP-1 nuclear binding, blocking AP-1 with dominant-negative c-Jun prevented the TNF alpha-mediated increase in NF-kappa B binding. In conclusion, both NF-kappa B and AP-1 are required for basal MAT2A expression in HepG2 cells and mediate the increase in MAT2A expression in response to TNF alpha treatment. Increased trans-activation of these two sites also contributes to MAT2A up-regulation in HCC.