| Chronic inflammation promotes development and/or progression of various tumors. Macrophages are a major component of the leukocyte infiltrates in various tumor stroma and macrophages would be "re-educated" to become alternative activated tumor-associated macrophages (M2 TAMs) by tumor cells. Proinflammatory factors from activated TAMs regulate preneoplastic development during inflammation-associated tumorigenesis. p53 (Trp53, Transformation related protein 53) is a major tumor-suppressor gene, mutationally inactivated in about half of all human cancers, and with its pathway disrupted in most of the other half of cases. In normal, unstressed cells, p53 is largely kept at very low levels through interactions with its two major negative regulators, MDM2 and MDM4. Genotoxic insults, activated oncogenes and a variety of other stresses including inflammation upregulate p53, usually through disruption of the p53-MDM2 interaction. Following its activation, the p53 protein induces cell-cycle arrest, apoptosis or cellular senescence, thus preventing the proliferation of cells that might spawn cancer. Whereas past work has focused on its cell-autonomous functions, p53 also possesses non-cell autonomous functions on tumor suppression. Activation of p53 can affect the growth and viability of neighboring cells through secreted factors from fibroblasts and endothelial cells. On the other hand, activated p53 and its target genes p21 and Mdm2 were detected in many inflammatory cells and chronic inflammatory samples. Wild-type p53 suppressed inflammation from different systems, probably resulting from its direct repression of promoter activity of NF-kB subunit p65, as well as negatively regulating the transcription of NF-KB-dependent genes, including 11-6, Cox-2 and others. However, it is unclear whether the role of p53 in suppression of inflammation links to its tumor suppression function.In this thesis, we established genetic mouse models of p53 deletion specifically in myeloid linage, to study its role in inflammation related tumorigenesis. With recombination of p53Flox allele by LysM-cre, myeloid cells exhibited an efficient deletion of p53. In a spontaneous ApcMin/+ intestinal tumor mouse model, mice with p53 insufficiency in myeloid cells exhibited significantly higher tumor burdens, associated with enhanced inflammatory cytokines, chemokines and other tumorigenic mediators, indicating a critical role of p53-dependent signaling in myeloid cells in protection against inflammation associated tumorigenesis.We next established genetic mouse models with myeloid-specific recombination of the Mdm2FM allele in the Mdm4+/- background. Myeloid cells from LysM; Mdm2FM/+; Mdm4+/-(LysM-MM) mice exhibited p53 activation, evidenced by both elevated protein level of p53 and mRNA levels of its target genes. This activation of p53 in myeloid lineage was moderate without affecting hemogram and tissue histology. Similar quantifications of both total peritoneal elicited cells and mature macrophages indicated normal survival and differentiation of myeloid lineage under activation of p53.Activation of p53 in myeloid cells efficiently dampened their responses to inflammatory stimuli in vitro. Importantly, p53 could be activated specifically in myeloid cells at inflamed stromal regions of colon. Furthermore, LysM-MM mice were more resistant to DSS-induced colitis within a less-inflammatory microenvironment. This resistance to inflammation from LysM-MM mice was probably due to the decreased induction of proinflammatory cytokines by dampened NF-kB and STAT-3 pathways. Accordingly, these findings indicated a protective role of p53 during inflammation, and prompted us to further investigate the role of p53 in myeloid lineage during inflammation-associated tumorigenesis.We took advantage of three mouse models of inflammation-associated intestinal cancer (ApcMin/+ mouse model, ApcMin+ /DSS colorectal cancer model and AOM/DSS- induced colorectal cancer model), to study the role of p53 in myeloid lineage on tumorigenesis. ApcMin/+; LysM-MM mice displayed a dramatic suppression of pro-tumorigenic inflammatory factors, resulting in significantly reduced tumor numbers. We also observed suppressed the invasion of DSS-induced inflammation-related colon tumors in ApcMin/+; LysM-MM mice. Lastly, LysM-MM mice protected against tumorigenesis and tumor invasion in AOM/DSS-induced colorectal cancer, associated with a limited STAT3 activation. Therefore, in models involved inflammation as a tumor promoter, activation of p53 attenuated tumor development by altering tumor microenvironment in a non-cell autonomous manner.In addition, activation of p53 in myeloid linage was able to suppress the alternative activation status of macrophages (M2), both in tumors in vivo and in peritoneal macrophages in vitro. This was consistent with the elevated M2 genes in ApcMin/+ tumors insufficiency of p53 in myeloid lineage. Specific pharmacological activation of p53 by Nutlin-3 could mimic the effects on the suppression of M2 polarization. This regulation of M2 polarization by activation of p53 was associated with elevated miR-34s and suppression of c-Myc. Importantly, suppression of M2 polarization was associated with the reduced pulmonary metastasis of MMTV-PyMT breast tumors, without affecting tumor initiation or tumor growth rate. Therefore, p53 status could modulate tumor microenvironment through affecting the polarization of M2 TAMs.In conclusion, our findings established for the first time that specific activation of p53 in myeloid lineage attenuated the formation and expansion of inflammation-associated intestinal tumors, as well as inhibited metastasis of primary breast tumors. These results provided a novel non-cell autonomous tumor suppressive function of p53. More importantly, our study suggested a possible development of novel therapeutic strategies in inflammation associated cancer through pharmacological activation of p53 in tumor microenvironment. |
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