The intestinal enterochromaffin cell (EC) compartment expands in response to acute injury, microbial infection and colitis to produce serotonin (5-hydroxytryptamine, 5HT). Elevated levels of plasma serotonin stimulate fluid secretion and gut motility to expel the infectious or toxic irritants. Although an essential component of the innate immune response, these gut functions contribute to patient discomfort and if sustained become a source of significant morbidity and possible mortality. Despite its central role, it is not understood what regulates EC cell function. During the prior funding cycle, we found that the zinc finger transcription factor ZBP-89 interacts directly with the tumor suppressor protein ataxia telangiectasia mutated (ATM) in response to histone deacetylase inhibition (HDACi), e.g., butyrate or trichostatin A (TSA). Butyrate induces ZBP-89 expression and binding to GC-rich DNA elements in several promoters including the cyclin-dependent kinase inhibitor (CDKI) p21. Butyrate also triggers auto-phosphorylation of ATM at serine 1981 (pATMS1981) that subsequently complexes with ZBP-89 to activate p21. In the current proposal, pATMS1981 positive expression in the gut was found to occur exclusively in the cytoplasm of intestinal EC cells suggesting that pATMS1981 participates in an essential function of these cells. Mice conditionally null for ZBP-89 in the colon exhibited reduced numbers of EC cells and reduced tryptophan hydroxylase 1 gene (TPH1) transcripts on a microarray. Indeed, we found GC-rich DNA elements in the proximal promoter of the TPH1 gene, the rate-limiting synthetic enzyme for 5HT biosynthesis. In addition, pATMS1981-expressing cells in APCmin polyps were absent suggesting that excess Wnt signaling prevents EC cell differentiation. In the current application, three aims are proposed to test the overarching hypothesis that ZBP-89 stimulates serotonin production in gut EC cells by regulating TPH1 gene expression while pATM in the cytoplasm regulates 5HT release. Moreover, we hypothesize that inflammation perturbs their function setting the stage for neoplastic transformation.
In Aim 1, we will determine how ZBP-89- regulates TPH1 gene expression.
In Aim 2, we will dissect the role of pATMS1981 in butyrate-mediated secretion of 5HT.
In Aim 3, we will study the role of these two proteins in 5HT biosynthesis during chronic inflammation and colonic transformation. Collectively, these studies will establish a link between ZBP-89 and pATMS1981, in the biosynthesis and function of 5HT in response to luminal butyrate.
Enterochromaffin (EC) cells secrete serotonin into the circulation to exert a number of clinical effects including increased gut motility and diarrhea. Both the transcription factor ZBP-89 and the tumor suppressor protein ATM are regulated by luminal butyrate from commensal bacteria and are important in maintaining 5HT biosynthesis and secretion from EC cells.
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