Previous reports have presented three pathways by which exposure to the common food additive carrageenan stimulates inflammation in human colonic epithelial cells () [4
]. These include pathways initiated by interaction of carrageenan with the toll-like receptor (TLR)-4, requiring MyD88, IRAK, and BCL10. The canonical pathway of NF-κB activation involves BCL10 interaction with IKKγ
, the regulatory subunit of the IKK signalosome, leading to IKKγ ubiquitination, IKKβ phosphorylation of IκBα, and nuclear translocation of NF-κB RelA (p65) and p50. Alternatively, as in the non-canonical pathway of NF-κB activation, phospho-BCL10 interacts with NIK and NIK is phosphorylated, leading to the phosphorylation of IKKα and the nuclear translocation of RelB and p52. In addition to the TLR4-BCL10 mediated pathways, carrageenan exposure stimulates a reactive oxygen species (ROS)-mediated pathway, in which phospho-Hsp27 interacts with the IKK signalosome, leading to the phosphorylation of IκBα and the nuclear translocation of NF-κB. To these pathways, we now add a feedback loop, by which NF-κB binding to the BCL10 promoter leads to increased production of BCL10, which can thereby continue activation of the inflammatory process.
The caspase recruitment domain (CARD)-containing protein BCL10 has previously been implicated in sustained inflammatory processes in lymphocytes, since translocations involving BCL10 are of etiologic significance in development of mucosa-associated lymphoid tissue (MALT) lymphomas [11
]. The MALT lymphomas, first described in 1983, are the third most common subtype of non-Hodgkin’s Lymphoma. They arise in a wide range of sites, including most commonly stomach (~70%), lung (~14%), then ocular adnexa, thyroid, and small intestine [23
]. The majority of the gastric MALT lymphomas are attributable to Helicobacter pylori
infections, and are amenable to cure by eradication of the infection. The association of reciprocal chromosomal translocations involving BCL10 [t(1;14)(p22;q32)] or MALT1 [t(11;18)(q21;q21) and t(14;18)(q32;q21)] with MALT lymphomas that are not attributable to H. pylori
indicates an overlap between the molecular mechanisms that are manifestations of infection- and inflammation-associated malignancies.
Carrageenan due to its unique chemical structure stimulates innate immune responses and evokes sustained activation of NF-κB and BCL10. Previously, we reported that NF-κB activation in response to DSS treatment was mediated via reactive oxygen species and Hsp27, and did not involve TLR4 or BCL10, since BCL10 silencing had no impact on the responses to DSS, DSS exposure did not increase BCL10, and MyD88 and TLR4 were not required for DSS effects [26
]. In the current study, inflammatory responses are not prolonged following withdrawal of DSS, consistent with observed effects in animal models [27
]. The difference in the inflammatory mechanisms between carrageenan and DSS, both of which are highly sulfated polysaccharides, but vary in their specific hexoses and in their glycosidic bonds, supports the essential role of the unique chemical structure of carrageenan, involving its unusual alpha-1,3-galactosidic bond, in activation of an innate immune response [28
]. In contrast, DSS, which is not used as a food additive, but predictably causes inflammation, is composed of glucose residues, linked predominantly in alpha-1,6-bonds [30
The post-translational modifications of BCL10 may modulate the effects of specific inflammatory signals, and BCL10 may serve a critical function in the downstream regulation of different inputs, such as those from carrageenan, lipopolysaccharide, platelet-activating factor, angiotensin II, and lysophosphatidic acid [6
]. The phosphorylation of BCL10 Ser138 is required for phosphorylation of NIK in the non-canonical pathway of NF-κB activation, and was not affected by proteasomal inhibition by MG-132 or by CAPE, indicating that BCL10 phosphorylation occurs independently of NF-κB nuclear translocation, in contrast to BCL10 expression [6
]. The study finding that the RelB activation is sustained for a shorter interval than the RelA activation suggests that another process, likely involving the phosphorylation of NIK, BCL10, or IKKα, is also required to prolong the activation of the non-canonical pathway.
Exposure of cells to inflammatory stimuli results in rapid phosphorylation and degradation of IκBα and subsequent translocation of NF-κB to the nucleus. These events enable prompt cellular response in the absence of de novo
protein synthesis [31
]. Given the central role of NF-κB in the pathogenesis and maintenance of chronic inflammatory states, examination of mechanisms that contribute to the prolonged activation of NF-κB is of considerable relevance to human disease. In the current report, carrageenan exposure for 12 hours caused sustained RelA activation, IL-8 production, and BCL10 protein level for 48 hours. Since upregulation of BCL10 expression results from NF-κB activation, and NF-κB activation following carrageenan exposure involves BCL10, a positive feedback loop involving BCL10 and NF-κB that causes prolonged activation of NF-κB in response to carrageenan may have clinical significance, leading to extended inflammation in intestinal epithelial cells. The relationship between infectious or inflammatory processes and malignancy, such as H. pylori
and gastric cancer, human papillomavirus and cervical cancer, or ulcerative colitis and colorectal carcinoma, is also of interest, since therapeutic interventions with anti-microbial and anti-inflammatory agents that retard inflammation may impact upon neoplastic transformation. Identification of specific transcriptional regulatory mechanisms, such as this NF-κB-BCL10 loop that is set in motion by carrageenan exposure, may contribute to improved understanding of the origins of malignancy, as well as of chronic inflammation.