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  • Treatment with histamine had no effect on histamine


    Treatment with histamine had no effect on histamine H1 receptor expression in HepG2 LY294002 (Fig. 1), while knockdown of histamine H1 receptor expression prevented histamine from repressing apo A-I gene expression (Fig. 2). Overexpression of the histamine H1 receptor regulated apo A-I promoter activity in a biphasic manner (Fig. 3). Low amounts of the histamine H1 receptor expression plasmid actually decreased apo A-I promoter activity while higher amounts increased it. This relationship is typical of squelching and suggests that there are at least two limiting factors that are being sequestered by the transfected DNA [48]. This squelching effect has not been reported with regard to the histamine receptor, however it is quite prevalent with nuclear hormone receptors. Treatment of MCF7 cells with estradiol was found to rapidly induce some genes while repressing others [49]. Estradiol treatment caused a rapid redistribution of the co-activator and acetyltransferase p300 from many expressed genes to the estrogen receptor, leading to decreased expression of many genes [49]. Likewise, Simon et al. [50] showed that by simply varying the amount of generic transcriptional coregulatory proteins within the cells could alter both the efficacy and potency of the aryl hydrocarbon receptor in cells treated with 2,3,7,8-tetracholorodibenzodioxin. Competition for limiting signaling components may also be critical in G-protein receptor signaling in our cells and may explain our findings. Treatment with histamine significantly increased NF-κB expression and activity (Fig. 4), while knockdown of NF-κB expression using either SN50 or an siRNA targeting the p65 subunit of NF-κB prevented these responses as well as the ability of histamine to suppress apo A-I gene expression (Fig. 5, Fig. 6). Though we did not examine p65 phosphorylation in response to histamine treatment, we demonstrated that treatment with histamine increased NF-κB binding to the apo A-I gene promoter in HepG2 cells in vivo (Fig. 7). This response would require p65 phosphorylation. These results suggest that histamine may suppress apo A-I promoter activity and gene expression in hepatocytes via modulating NF-κB activity. It is well known that many pro-inflammatory stimuli, including histamine, stimulate NF-κB activity [30, 31], however, in HepG2 cells, histamine actually increased expression of both p50 and p65, the major subunits of NF-κB. Once activated, NF-κB has been shown to suppress apo A-I expression by binding to PPARα (and potentially other transcription factors) already bound to the apo A-I gene promoter. This effect, called trans-repression, may be utilized by NF-κB to silence the expression of various anti-inflammatory genes [51]. Once synthesized and secreted, the prepro-apo A-I protein is processed by peptidases to the mature form [52]. It is then lipidated by hepatic ABCA1, a process essential for the formation of discoidal HDL, forming the favored form of substrate for ABCA1-mediated cholesterol efflux from macrophage cells [53]. HDL and apo A-I have potent anti-inflammatory effects such as suppressing the activity of pro-inflammatory receptors on macrophage cells via up-regulation of activating transcription factor 3 (a potent suppressor of toll-like receptor stimulation) [54] as well as by binding and neutralizing inflammatory molecules such as lipopolysaccharide [55]. HDL has also been shown to deplete lipid rafts of their cholesterol content with negative effects on pro-inflammatory receptor activation [56, 57].
    Conclusions The following are the supplementary data related to this article.
    Funding This work was supported by a grant (70755) to MJH from the University of Florida Jacksonville College of Medicine.
    Conflict of interest statement
    Introduction Caffeine (1,3,7-trimethylxanthine) is known to exert various behavioral effects like an increase in vigilance, attention, arousal [1,2] and locomotor activity [3,4]. There is no doubt that caffeine is widely consumed by subjects who need to stay awake [5] by modulation of neuronal pathways in the central nervous system (CNS), indicative of its CNS stimulant profile [5,6]. Plethora of reports have demonstrated that caffeine similar to other drugs of abuse possess reinforcement action as evident from the observed sensitization to its locomotor stimulant activity [[7], [8]] and could be attributed to its effects on ventral striatum, comprised of the nucleus accumbens (NAc) (involved in expression to sensitization), which receives its dopaminergic input from the ventral tegmental area (VTA) (involved in induction of sensitization) and this projection constitutes the mesolimbic reward pathway [[9], [10]]. Therefore, these areas might be a target for caffeine induced behavioral sensitization or aversive effects. However, till date exact mechanism of caffeine-induced locomotor sensitization is obscure. Caffeine is an adenosine A1 and A2A receptors antagonist [6,11] and this action of caffeine could contribute to its locomotor sensitizing effect. Indeed, antagonist action on adenosine A1 and A2A the in brain regions involved in the modulation of locomotion such as the hippocampus, cortex, cerebellum with high A1 expression and striatum, nucleus accumbens (NA), olfactory tubercle with high levels of A2A receptors could be cardinal for its locomotor sensitization [12]. However, highly interconnected nature of the locomotor-regulatory circuitry of the brain with the dorsal striatum, thalamus, cortex linked by means of the globus pallidus, has resulted to ponder, which of the two adenosine receptors is principally involved in the caffeine induced motor stimulatory effects [6]. On the other hand, biphasic effects of caffeine on locomotion i.e. low dose induced increase and high doses induced decreased locomotor activity have been well reported [13,14]. Therefore, the exact mechanism of caffeine induced locomotor sensitizing effect is still a matter of investigation.