No abstract

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces a profound suppression of the primary immunoglobulin-M (IgM) antibody response. The suppression of IgM production by TCDD can occur through direct interactions with the B cell, is aryl hydrocarbon receptor-dependent, and is mediated through alterations in the differentiation of B cells into plasma cells. The objective of the present investigation was to characterize the effects of TCDD on the regulation of Pax5, a crucial repressor of B-cell differentiation, and four downstream targets that are directly regulated by Pax5 and involved in immunoglobulin regulation, immunoglobulin heavy chain (IgH), kappa light chain (Igkappa), J chain, and X box protein-1 (XBP-1). Lipopolysaccharide (LPS) activation of aryl hydrocarbon receptor-expressing CH12.LX cells induced B cell differentiation and robust immunoglobulin secretion that was markedly (~50%) suppressed in the presence of 10 nM TCDD. Kinetic studies show that LPS-activation induced a time-dependent decrease in Pax5 mRNA levels, protein, and DNA binding activity during a 72-h culture period that was almost completely blocked in the presence of TCDD. Concomitant with the time-dependent down-regulation of Pax5 in LPS-activated control CH12.LX cells, a reciprocal induction of IgH, Igkappa, J chain mRNA levels, and cellular XBP-1 was observed. Conversely, and consistent with the absence of Pax5 down-regulation associated with TCDD treatment, IgH, Igkappa, J chain mRNA, and XBP-1 protein were persistently repressed in LPS-activated CH12.LX cells. Collectively, these studies demonstrate the involvement of altered Pax5 regulation in the suppression of the primary IgM antibody response by TCDD.

B cell differentiation and humoral immune responses are markedly suppressed by the persistent environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The suppression of humoral immune responses by TCDD occurs by direct actions on the B cell and involves activation of the aryl hydrocarbon receptor. Transcriptional regulation of paired box gene 5 (Pax5), an important regulator of B cell differentiation, is altered by TCDD in concordance with the suppression of B cell differentiation and humoral immunoglobulin M response. We hypothesized that TCDD treatment leads to dysregulation of Pax5 transcription by interfering with the basic B cell differentiation mechanisms and aimed to determine the effects of TCDD on upstream regulators of Pax5. A critical regulator of B cell differentiation, B lymphocyte-induced maturation protein-1 (Blimp-1) acts as a transcriptional repressor of Pax5. In lipopolysaccharide (LPS)-activated murine B cell lymphoma, CH12.LX, Blimp-1 messenger RNA, and DNA-binding activity within the Pax5 promoter were suppressed by TCDD. Furthermore, LPS activation of CH12.LX cells upregulated DNA-binding activity of activator protein 1 (AP-1) at three responsive element-like motifs within the Blimp-1 promoter. TCDD treatment of LPS-activated CH12.LX cells suppressed AP-1 binding to these motifs between 24 and 72 h, in concordance with the suppression of Blimp-1 by TCDD. A more comprehensive analysis at 72 h demonstrated that the suppression of AP-1 binding within the Blimp-1 promoter by TCDD was concentration dependent. In summary, our findings link the TCDD-mediated suppression of Blimp-1 through AP-1 to the dysregulation of Pax5, which ultimately leads to the suppression of B cell differentiation and humoral immune responses.

Propolis is a resinous material collected by honeybees from several plant sources. This research aimed at showing its protective effect against UVA-induced apoptosis of human keratinocyte HaCaT cells. Using Hoechst staining, it was demonstrated that propolis (5 and 10 μg/mL) significantly inhibited the apoptosis of HaCaT cells induced by UVA-irradiation. Propolis also showed the protective effect against loss of mitochondrial membrane potential induced by UVA-irradiaiton in HaCaT cells. Propolis also inhibited the expression of activated caspase-3 induced by UVA-irradiation. To investigate the role of ROS in UVA-induced apoptosis and protection by propolis, the generation of ROS was determined in cells. The results showed that the generation of ROS was markedly reduced in cells pretreated with propolis. Consequently, propolis protected human keratinocyte HaCaT cells against UVA-induced apoptosis, which might be related to the reduction of ROS generation by UVA-irradiation.

Propolis, a natural product derived from plant resins collected by honeybees, has been reported to exert a wide spectrum of biological functions. This research aimed at investigating the effect of propolis on the proliferation of human leukaemia HL-60 cells and whether propolis might induce apoptosis in HL-60 cells. The results showed dose- and time-dependent decreases in the proliferation of HL-60 cells treated with propolis (above 3 microg mL(-1) of propolis). Further studies revealed that the anti-proliferative effects of propolis were caused by inducing apoptosis. Agarose electrophoresis of genomic DNA of HL-60 cells treated with propolis showed the ladder pattern typical for apoptotic cells. Propolis induced the activation of caspase-3 and cleavage of poly (ADP-ribose) polymerase in HL-60 cells. Propolis also induced the release of cytochrome c from mitochondria to cytosol. Taken together, these findings demonstrate that the inhibitory effect of propolis on HL-60 cell proliferation is caused by inducing apoptosis through the mitochondrial pathway.