Biology and Biomedical Sciences: Developmental, Regenerative and Stem Cell Biology
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
Jason C Mills
Almost nothing is known about the identity of the epithelial stem cell of the gastric corpus, either during normal turnover or in response to injury. Our lab has shown that injection of the selective estrogen receptor modulator tamoxifen leads to near complete atrophy of parietal cells by 3 days and induces expansion of an undifferentiated cell population within the normal stem cell niche in the isthmus of the gastric unit. Here we show that CD44 labels the membranes of such undifferentiated isthmal cells, both in the normal gastric epithelium and when those cells expand fourfold upon injury with tamoxifen. Loss of CD44, either in knockout mice or by blocking its interaction with its ligand, leads to reduced proliferation. We found CD44 regulates proliferation by binding to active STAT3 and occupying the CyclinD1 promoter; accordingly, blocking STAT3 activity completely abrogates atrophy induced proliferation. We screened for signaling kinases potentially responsible for increased CD44 and/or proliferation and found only ERK MAPK was activated during early stages following injury: as few as 6 hours following tamoxifen injection). This burst of ERK activation is localized to non-differentiated cells of the isthmus, and blocking ERK activation with the inhibitor U0126 blocked the expansion of CD44-positive cells.
To determine which cytokines induced ERK in progenitor cells, we assayed sera of mice treated with tamoxifen for 6h. Compared to control injected mice, tamoxifen treated mice have a significant increase in the STAT3-inducing cytokine IL-6 levels, correlating with increased F4/80+ macrophages in the gastric mesenchyme. Isolated peritoneal macrophages treated ex vivo with tamoxifen showed significantly increased IL-6 expression, and depletion of bone-marrow derived macrophages in vivo blocks tamoxifen induced metaplasia and increased progenitor cell proliferation. Depletion of macrophages also blocks activation of ERK and expression of the stress signal, iNOS, in parietal cells. Inhibition of iNOS and scavenging of nitric oxide blocks parietal cell atrophy and stem cell expansion.
Taken together, our data suggest that CD44 marks a population of undifferentiated epithelial cells within the stem-cell niche of the gastric unit, which greatly expands on injury and is regulated by ERK-MAPK signaling. ERK, in turn, is potentially regulated by cytokines like IL-6 secreted by peritoneal and resident macrophages. Once induced, CD44 associates with pSTAT3 to increase Cyclin D1 expression and consequent stem/progenitor cell proliferation. In conclusion, this thesis identifies a marker and pathway for the presumptive stem cell of the gastric epithelium during response to atrophy and during normal homeostasis.
Khurana, Shradha Sachin, "The Mechanism of the Gastric Epithelial Stem Cell Response to Metaplastic Injury" (2013). All Theses and Dissertations (ETDs). 1199.