Recently, we reported that caMEK signaling is highly oncogenic and induces cellular transformation in rat intestinal epithelial cells [8
]. We now show,, through the use of microarray analysis, that many genes associated with cellular transformation have altered expression levels following constitutive MEK activation. MEK-ERK signaling is associated with cell migration, invasion, and metastasis [18
]. Our array results indicate that 10 transcripts associated with cell migration (e.g. MMP10, MMP13, etc) and adhesion (e.g. PTHLH, OPN, etc) have altered expression levels following MEK activation. Additionally, 7 genes known to possess tumor suppressor function (e.g. GPC3/OCI-5, LOT1/PLAGL1/ZAC1, etc) were down-regulated by MEK-activation. Furthermore, several genes that possess anti-apoptotic or chemo-resistant properties were over-expressed in caMEK expressing clones. The altered expression of transcripts was also seen in genes that are involved in growth and proliferation, transcription, signal transduction, biosynthesis, and the cytoskeleton. Together, this data supports our finding that MEK signaling positively regulates transformation in intestinal epithelial cells.
Among the most interesting findings, surface antigen CD44, complement resistance factor CD55/Daf, and secreted phosphoglycoprotein OPN, all of which are known to be implicated in colorectal cancer [25
], were also up-regulated by caMEK. All of these results suggest the importance of MEK signaling in the intestinal tumorigenesis. Oncogenic transformation of rat intestinal epithelial cells following MEK-activation may depend on the balance between increased transcription of tumor-promoting genes and reduced levels of tumor suppressor genes.
We also have shown three transcripts that may be involved in human colorectal cancer. Of particular interest are the up-regulation of tryptase-ε/PRSS22 and AMPD3, and the down-regulation of CRTL1. Tryptase-ε/PRSS22 is a member of the chromosome 16p13.3 family of human serine proteases that is preferentially expressed by epithelial cells [36
]. The tryptase-ε/PRSS22 gene is expressed in the airways in a developmentally regulated manner and is a major product of several different transformed epithelial cell lines [36
]. Malignant cells require a range of proteolytic activities to enable growth, survival, and expansion [37
]. Tryptase-ε/PRSS22 may play a role in this process. AMPD3 is one of the isoforms of the AMP deaminase family, which converts AMP to IMP and is a diverse and highly regulated enzyme that is a key component of the adenylate catabolic pathway [38
]. This enzyme serves to protect the cell against sharp decreases in the adenylate energy charge by removing AMP generated when the rate of utilization of ATP is suddenly increased [39
]. In cancer cells, a marked imbalance in the enzymic pattern of purine metabolism is linked with transformation and/or tumor progression [40
]. This enzymatic change of purine metabolism seems to be present in transformed intestinal epithelial cells. CRTL1 (also known as a link protein) is a small glycoprotein of the extracellular matrix that was originally identified for its role in stabilizing aggregates of aggrecan and hyaluronan in cartilage [41
]. In addition to being expressed in cartilage, CRTL1 is also immunolocalized in several noncartilaginous tissues [41
]. A recent study has suggested that CRTL1 may be a down-stream target of β-catenin in intestinal epithelial cells, which has been implicated early in the progression of colorectal epithelial cells to cancer [42
]. Therefore, this gene may also serve a role in preventing tumor formation of intestinal cells. This is the first report which indicates the involvement of these three genes in colorectal cancer.