Regulation of AP-1 and NF-κB activation
The transcription factors NF-κB and AP-1 play key roles in the initiation of an inflammatory response by inducing the expression and secretion of chemokines and cytokines that attract and activate immune cells. However, the signal transduction pathways and subsequent inflammatory cytokine induction by these transcription factors is not fully elucidated. The present study is aimed at determining the involvement of AP-1 and NF-κB in cytokine induction and regulation. PMA treatment resulted in an up-regulation of AP-1 after 2 h exposure and continued to increase throughout the analysis period (figure 1a). HK E. coli treatment did not affect AP-1 activation in Jurkat T-cells (figure 1b). To determine the involvement of associated pathways, we exposed cells to Ca2+ ionophore with or without PMA and observed a modest involvement of Ca2+ in PMA-dependent AP-1 activation (figure 1c) while Ca2+ alone did not alter AP-1 activity (data not shown). Furthermore, AP-1 activity decreased in a TCR-deficient Jurkat cell line when exposed to PMA compared to the parent cell line indicating that regulation of AP-1 was only partially T-cell receptor dependent (figure 1d).
Figure 1  AP-1 activation following long-term exposure of Jurkat T-cells to PMA. Jurkat T-cells were transfected with luciferase reporter plasmids containing the AP-1 cis-elements. (A) Time-dependent AP-1 activation in response to PMA. (B) HK E. coli does not activate AP-1. (C) Dose-dependent activation of AP-1 were performed using PMA alone (grey bars) and in combination with calcium ionophore (CaI 955 μM, black bars). (D) TCR-/- deficient cells responded to PMA (162 nM) by up-regulating AP-1 activity. Statistical significance from the control was determined using Student's t-test. (n = 4). Controls were arbitrarily set to 1.   NF-κB levels showed a transient increase at 1 min after exposure to PMA (figure 2a). However, 1 h after exposure the NF-κB levels began to drop reaching the lowest levels by 6 h, after which they increased again by 24 h. Exposure of Jurkat T-cells to HK E. coli resulted in a dose-dependent NF-κB activation, with the highest activity observed at a relative concentration of 5 × 107 CFU/ml (figure 2b). The time-dependent activation of NF-κB by HK E. coli was assessed further using the optimal concentration obtained from figure 2b and showed that the NF-κB activity increased after 3 h of exposure (figure 2c). Furthermore, increased intracellular Ca2+ reversed the PMA dependent NF-κB inhibition (figure 2d) and reduced the HK E. coli -dependent NF-κB activation (figure 2e).
Figure 2  NF-κB down-regulation by PMA and up-regulation by heat killed E. coli MG1655 following long-term stimulation. Transfection of Jurkat T-cells was performed using luciferase reporter plasmids containing NF-κB cis-elements. (A) Time-dependent stimulation of Jurkat T-cells using PMA. NF-κB activation was evaluated using HK E. coli in a (B) dose- and (C) time-dependant manner. Calcium ionophore increased NF-κB activity following PMA exposure (E) and resulted in a negative regulation in response to HK E. coli stimulation (D). Statistical significance from the control was determined using Student's t-test. (n = 4). Controls were arbitrarily set to 1.

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