Several studies have revealed that hypoxia inducible factor (HIF-1α) plays an important role in the evolution and propagation of the inflammatory process [
40‐
42]. In order to check this property, in our present study, we have determined the HIF-1α level in the brain homogenate of rats under hypoxia. The results showed that not only the activation of cerebral NF-κB was inhibited by curcumin treatment but also showed stabilised HIF-1α protein expression. Furthermore, we also observed that HIF-1α regulated gene VEGF, which was increased under hypoxia, was significantly downregulated, rather maintained more or less similar to that of control in curcumin-treated rats. Curcumin is an in vivo inhibitor of VEGF upregulation, thereby controlling the angiogenesis [
43]. VEGF expression can be regulated through dual independent mechanisms involving HIF-1α directly (via HIF-1α-VEGF promoter) and also through NF-κB activation [
44]. It seems that evolutionary conserved HIF-1α is regulated by NF-κB [
40]. It was reported by Jordi et al. [
45] that Ikappa kinase (IKK) β in different cell types demonstrate that NF-κB is a critical activator of HIF-1α in macrophages, responding to bacterial infection in the brain and liver of hypoxic animals. IKKβ deficiency results in defective induction of various HIF-1α targets genes including VEGF in mice. Hence, IKKβ provides an important physiological link between the hypoxic response to innate immunity and inflammation, two ancient stress response systems. Moreover, it was reported by Xiong et al. [
46] that treatment with bacterial lipopolysaccharide (LPS) or with CD40 L stimulates VEGF production in human primary macrophages. This effect has been reported to be NF-κB dependent, because it is sensitive to the overexpression of the inhibitory protein Ikappa B (IKB) α [
47]. Whereas, Robert et al. [
48] have revealed an interesting observation that VEGF expression in macrophages is regulated by Liver X factor (LXR) which is independent of HIF-1 activation and also did not require the previously characterised hypoxia response element in the VEGF promoter. The VEGF is primarily known as the inducer of angiogenesis, but this cytokine has roles in vascular permeability and haematopoietic cell development and differentiation [
49]. VEGF is important in both inflammation and repair and is critical in resolution of primary process of wound healing. It is well known that higher expression of HIF-1α and its down regulatory gene expression of VEGF are necessary in maintaining the oxygen homeostasis in the cells during hypoxia, but excess is detrimental, as increased VEGF levels alter permeability markedly exacerbating the high permeability cerebral edema [
6]. Our study suggests that activation of NF-κB and HIF-1α in this context may also lead to increased production of VEGF. However, it was reported that inhibition of NF-κB correlates with downregulation of VEGF mRNA [
50] which may be mediated through HIF-1α. On the other hand, the enhanced expression of NF-κB levels seems to be positively correlated with increased paracellular permeability as these changes were associated with alterations in tight junction proteins. This indicates that inflammation plays a significant role in tight junction protein disruption under hypoxia. Recently, we demonstrated that NF-κB and oxidative stress contribute in transvascular leakage leading to fluid influx in the brain [
15]. Curcumin significantly downregulated the higher VEGF levels under hypoxia. Our results are in agreement with the results reported earlier [
43]. This perhaps helps in maintaining oxygen homeostasis by facilitating acclimatisation thereby abridging the fluid influx into the brain of rats exposed to hypoxia under current experimental conditions. A study by Witt et al. [
51] suggested that transcription factors such as HIF-1α and NF-κB are upstream mediators of TJ protein alterations during hypoxia and hypoxia re-oxygenation which may involve VEGF induction and expression. In this regard at high altitude, lack of HIF-1α response or excessive expression of HIF-1α results in the lack of ability to adapt to hypoxia. Therefore, these results clearly reveal the importance of HIF-1α stabilisation and downregulation of NF-κB, with enhanced anti-inflammatory molecule (IL-10) through curcumin preconditioning, leading to reduction in transvascular leakage in the brain of rats.
There is a major concern that the low serum concentrations of curcumin normally observed in rodents and humans may not reach a particular organ in sufficient concentrations to have an effect. Recent studies, however, have suggested a favourable tissue distribution of curcumin. At least two studies suggest that curcumin is a fluorescent compound that binds to amyloid deposits. Garcia-Alloza et al. [
52] were able to use multiphoton microscopy to demonstrate that curcumin administered systematically in mice crossed the blood brain barrier, bound to amyloid plaque in the brain and reversed the existing amyloid pathology using fluoropropyl substituted synthetic curcumin. Ryu et al. [
53] showed that curcumin was taken up by the brain. Importantly, curcumin does not block the pathway totally, but only, downregulate the overactive pathway to basal levels. In vitro and in vivo and human clinical studies have all established curcumin’s promise and revealed its therapeutic value. Cheng et al. [
54] reported that no treatment-related toxicity was observed up to 8 g daily in phase I clinical trials, but beyond this dose, the bulk volume of the drug was unacceptable to the patients. Similarly, several other clinical trials have reported use of curcumin on various disease conditions viz, cancer [
55], familial adenomatous polyposis [
56], tropical pancreatic cancer [
57], inflammatory bowel disease [
58,
59], gall bladder function [
60], psoriasis [
61] and helicobacter pylori infection [
62].