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Keyword: colorectal cancer

Primary colorectal tumors secrete VEGF-A, inducing CXCL1 and CXCR2-positive myeloid-derived suppressor cell (MDSC) recruitment at distant sites and establishing niches for future metastases, report Medical University of South Carolina (MUSC) investigators in an article published online ahead of print on April 28, 2017 by Cancer Research. Liver-infiltrating MDSCs help bypass immune responses and facilitate tumor cell survival in the new location. This research illuminates mechanisms by which primary tumors contribute to premetastatic niche formation and suggests CXCR2 antagonists may reduce metastasis.

Recent cancer research shows that premetastatic 'niches' form at sites far from the original tumor before new tumors occur. In colorectal cancer (CRC), these supportive microenvironments form in preferred secondary organs, such as the liver and lung, and facilitate the colonization, survival, and growth of metastasizing tumor cells. However, the mechanisms responsible for the formation of these premetastatic 'niches,' including what role(s) the primary tumor may play, are not well understood. It is critical to better understand the mechanics of CRC metastasis, as it is the second leading cause of cancer deaths in the US and patients with advanced cases often die because current treatments for widely metastasized disease are not effective.

MUSC investDr. Raymond N. DuBois, dean of the Medical University of South Carolina College of Medicineigators led by Raymond N. DuBois, M.D., Ph.D., dean of the MUSC College of Medicine and professor of Biochemistry and Molecular Biology, have now illuminated how primary CRC tumors contribute to premetastatic 'niche' formation.

"The idea that some sort of 'priming' needs to take place for metastasis to occur in distant organs - that there is some sort of activity in the future tumor location - is not new. But most research has focused on growth factors, chemokines and pro-inflammatory cytokines. There hasn't been much work looking at immune cell activity in distant organs prior to metastasis," explains DuBois. "We knew that the type and density of immune cells in the primary tumor plays a role in progression. For example, when more immature myeloid cells are present in the tumor, it becomes resistant to immune attack. But we didn't know what to expect in a metastatic model."

To explore this area, the team first evaluated whether the presence of a primary tumor affected immune cell profiles in premetastatic liver and lung tissues of mice. They found that the presence of a primary cecal tumor caused MDSCs to begin infiltrating the liver before metastasis began. Working backward from this finding, they used a series of experiments to reveal the chain of events that led up to MDSC infiltration.

Because CXCR2 is essential for drawing MDSCs out of the bloodstream and toward CRC tumors and colonic mucosa, the team began looking for CXCR2 and its ligands (CXCL1, CXCL2, and CXCL5) in mouse liver tissue. The team not only found that the ligand, CXCL1, attracted MDSCs from the bloodstream into premetastatic liver tissue, but also that administering a CXCR2 antagonist inhibited CXCL1 chemotaxis. This demonstrated that CXCR2 is required for CXCL1 to induce MDSC liver infiltration. In other words, the CXCL1-CXCR2 axis is required to recruit MDSCs to the liver. Importantly, they also found that liver- infiltrating MDSCs secrete factors that promote cancer cell survival and metastatic tumor formation without invoking the innate and adaptive immune responses.

Next, because VEGF is known to induce CXCL1 expression in lung cancer, the research team examined whether VEGF secreted by CRC tumors also regulated CXCL1 expression. Their results demonstrated that VEGF-A secretion by primary CRC tumor cells stimulates macrophages to produce CXCL1. Interestingly, although VEGF-A knockdown inhibited liver metastasis, it did not affect the growth of the primary tumor.

"We did not expect to find that a primary tumor could affect a distant organ before any of the cancer cells arrived on site," says DuBois. "We were surprised to see these changes before a single metastatic cell took up residence."

Together, these studies reveal that VEGF-A secreted by the primary CRC tumor stimulates macrophages to produce CXCR1, which recruits CXCR2-expressing MDSCs from the bloodstream into healthy liver tissue. The MDSCs then create a premetastatic 'niche' or micro-environment where cancer cells can grow to form new tumors. These results demonstrate for the first time that cells in the primary tumor contribute to forming distant pre-metastatic 'niches' which facilitate the spread of disease.

"Now that we know the primary tumor puts things in motion remotely prior to metastasis, we should be able to inhibit this process and have a positive impact on survival," explains DuBois. "We now know which molecules and immune cells are involved and that if we disrupt the CXCL1-CXCR2 axis we can possibly reduce the spread of disease. Both antibodies and small molecules can inhibit this pathway, but they have not yet been optimized. I hope these findings will speed up the development of inhibitors of the CXCR2 pathway."

Summary: Medical University of South Carolina (MUSC) investigators report preclinical research showing that Krüppel-like factor 12 (KLF12) promotes colorectal cancer (CRC) cell growth by activating early growth response protein 1 (EGR1), in the July 2016 issue of PLOS One. Data also reveal that levels of KLF12 and EGR1 correlate synergistically with a poor CRC prognosis. Results indicate that KLF12 plays an important role in CRC progression and provides a potential novel prognostic marker and therapeutic target.

Results of preclinical studies by MUSC investigators reported in the July 2016 issue of PLOS One (doi:10.1371/journal.pone.0159899) demonstrate for the first time that the transcription factor Krüppel-like factor 12 (KLF12) promotes poor colorectal cancer (CRC) cell growth, in part, by activating EGR1. Furthermore, data demonstrate that KLF12 and early growth response protein 1 (EGR1) levels synergistically correlate with  CRC prognoses.

CRC is the third most common and third deadliest cancer in the US. Like most cancers, CRC development is spurred by a series of genetic mutations and epigenetic changes that alter gene expression. In turn, this altered gene expression initiates tumors and supports their progression. Thus, transcription factors that regulate gene expression and signaling pathways during carcinogenesis have long been studied as potential therapeutic targets.

Dr. Raymond DuBois, Dean of the MUSC College of Medicine, Professor of Biochemistry and Molecular Biology, and senior author on the article is focused on understanding the role of inflammation in cancer. "We've been studying the connections between inflammation and cancer in my lab for some time now and have determined that some inflammatory mediators stimulate the progression of cancer,” DuBois said. “We found that KLF12 was increased dramatically in the presence of inflammation in certain cancers, so we were trying to determine the specific molecular mechanisms responsible for these effects."

Other researchers who were studying kidney development previously identified transcription factor KLF12 as a transcriptional repressor of the AP-2? gene. It was then discovered that AP-2? expression is also reduced in advanced CRC tumor tissue compared to matched normal tissue and that loss of AP-2? promoted CRC invasion. This connection illuminated a potential link between KLF12 and CRC. In vitro studies show that KLF12 promotes gastric cancer (GC) cell proliferation and invasion, and that KLF12 levels are elevated in about 40% of poorly differentiated GCs and correlate with tumor size. Furthermore, recent genome-wide analyses find high KLF12 levels in approximately 40% of esophageal adenocarcinomas and in 45% of salivary tumors. Until now, however, the role of KLF12 in CRC remained unclear.

The MUSC research team designed a series of in vitro and in vivo experiments to clarify the role of KLF12 in CRC. The first set of studies examined KLF12 expression in seven human CRC cell lines. They found not only that KLF12 was expressed in six of the seven cell lines, but also that its overexpression led to increased cell numbers and KLF12 knockdown led to reduced cell numbers. In addition, they also found that overexpression of KLF12 led to the formation of larger cecal tumors while KLF12 knockdown led to formation of smaller cecal tumors, compared to controls. Thus, this set of experiments indicates that KLF12 promotes CRC growth by enhancing cancer cell proliferation and/or survival.

The next set of experiments focused on clarifying which KLF12 target genes may be involved in regulating CRC growth. Using microarray assays, the researchers found that KLF12 overexpression altered multiple genes including EGR1. It has been previously reported that KLF12 regulates expression of some target genes by binding to the CACCC motif.  They found that the EGR1 promoter contains two possible KLF12 DNA-binding motifs located at -1488bp (motif 1) and -808bp (motif 2) relative to the transcription start site. Using ChIP assay, the MUSC researcher team found that KLF12 does, indeed, bind strongly to the EGR1 promoter motif 2 but not to motif 1. In vitro experiments demonstrated that, at both the mRNA and protein levels, CRC cells with undetectable levels of KLF12 expressed the lowest levels of EGR1 compared to cells expressing high levels of KLF12. In vivo studies using mice implanted with CRC tumor cells that overexpressed KLF12 showed that EGR1 expression was up-regulated compared to mice implanted with control cells. Furthermore, staining of human CRC tissue specimens produced the same pattern. Taken together, these results indicate that KLF12 directly activates EGR1 in CRC.

The third set of experiments looked at whether EGR1 mediated the effects of KLF12 on tumor cell growth. Results showed that EGR1 knockdown reduced KLF12-induced tumor cell growth, whereas EGR1 overexpression promoted CRC cell growth in vitro as well as tumor growth in the mouse model. The results of this set of studies, thus, indicate that KLF12 enhances CRC cell growth by activating EGR1.

The final set of experiments evaluated whether KLF12 and EGR1 levels correlate with CRC patients' prognoses. Using gene expression data from publicly available microarray databases (Moffitt [n = 177]; Vanderbilt Medical Center [n = 55]), CRC patients were stratified by level of KLF12 and/or EGR1 expression. These data showed that patients with high levels of either KLF12 or EGR1 had worse outcomes compared to those with low levels of these genes, and that those with high levels of both KLF12 and EGR1 had the lowest survival rates.

This is the first study to clarify the role of KLF12 in CRC tumor growth and progression, which appears to occur, at least in part, through EGR1 activation. The finding that synergistic contributions of KLF12 and EGR1 produce the worst outcomes among CRC patients illuminates their potential in developing novel therapies. More studies are needed to further clarify the role of KLF12 in CRC progression and its potential as a novel prognostic marker and therapeutic target. 

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