Colon cancer development and molecular events
Tackling colon cancer

Colon crypt cells 

Colon crypt cells stained to define P-bodies (green). 

Rather than its possible triggers, Dan Dixon, Ph.D., co-leader of Cancer Prevention, is far more interested in molecular events along the chain reaction of colon cancer development. One such event involves COX-2, a protein frequently found at out-of-whack levels in colon cancer cells and with a well-established role in promoting inflammation and pain. Essentially, Dixon is keen to figure out how to make COX-2 – a target of anti-inflammatory drugs like aspirin and Celebrex – pipe down. 

“While not normally present in cells, COX-2 gets turned on when inflammation occurs and is one of the first proteins on the scene,” he explains. “Then it’s supposed to go away, but the production brakes are somehow jammed in colon cancer.”

Like all proteins, COX-2 is expressed from its gene via an intermediary transcript called messenger RNA (mRNA). As a routine exercise to keep a lid on the levels of any one protein, a whole slew of molecular factors are responsible for “seeing” transcripts like COX-2 and thereby destining them for cellular destruction. The machinery at work here is highly complex, Dixon says. For example, a particular sequence rich in A and U bases along COX-2’s mRNA enables a protein called TTP to latch on. TTP then shuttles these marked transcripts over to P-bodies, small sites in the cell that house multiple enzymes involved in RNA decay – they are, in fact, trash disposal units. 

Similarly, short strings of bases called microRNAs recognize their own docking sites and help drag COX-2 transcripts off for destruction. Examples of these microRNAs include miR-16 and miR-542. So there’s more than one trash collector at work to ensure that COX-2 is only present when necessary; but wily cancer cells, in hanging on to any and all tumor promoting forces available, have found ways to interfere with this otherwise well-regulated system. 

“It’s thought, for instance, that the docking sites get blocked by other proteins known as RNA stability factors, which prevents the collectors from seeing their trash,” Dixon says. One likely culprit is a stability factor called HuR: frequently overexpressed in tumor cells, it goes for the same AU-rich sequence on COX-2 transcripts that TTP and certain microRNAs normally target. 

Alternatively, Dixon adds, tiny genetic tweaks to the docking sites themselves – called single nucleotide polymorphisms (SNPs) – can also affect the ability of microRNAs to head transcripts toward the landfill. In August 2012, he published a paper in Oncogene outlining one of these minute changes, from T to C, that effectively disrupted miR-542’s binding ability to COX-2 transcripts, in turn allowing for unfettered protein production. Epidemiological studies have shown that the distribution of this particular SNP varies across different ethnicities: 38 percent in people of African descent, 11 percent in Caucasians and less than one percent in Asians. 

More recently, Dixon has been examining the importance of regular trash pickup and disposal, molecularly speaking, from a slightly different angle. It involves TGFβ, a protein prominent in controlling cell proliferation so different cell types with specific functions can emerge in key organs like the colon. Predictably, TGFβ is often messed up in colon cancer. Curious as to whether this protein influences RNA stability, Dixon and his group tagged P-bodies in normal intestinal cells so they glowed fluorescent green, then counted their numbers with and without adding TGFβ to the mix. They observed that higher TGFβ levels corresponded with significantly more P-bodies – basically, more centers of RNA decay.

“It looks like TGFβ has its foot on the proliferation brake; it promotes the disposal of transcripts that would otherwise lead to enhanced cell growth signaling,” Dixon says. “Now we have a clearer picture of what goes wrong in the colon, leading to tumors, when TGFβ signaling is messed up.” 

Having identified some of the processes that go awry in colon cancer, Dixon and his crew are tackling these issues head-on. One potential fix involves using high-throughput drug screening assays to hunt for compounds capable of undercutting cancer cells’ attempts to keep as much COX-2 mRNA as possible around. Viable candidates might prevent HuR from hogging key docking sites; induce and ensure TTP’s activity – frequently repressed in colon cancer – so HuR faces stiff opposition; or modulate microRNAs like miR-542 so they can still make use of their favored binding sequences, SNPs notwithstanding. 

“We’re looking for ways to force interactions that the tumor cells don’t want, and block other interactions that they do want,” Dixon says. “The library of compounds we’re using contains a number of chemical backbone structures. If we’re able to identify one or more that fit the bill, we can then work with our colleagues in medicinal chemistry to “dress up” an otherwise simple structure so it’s more specifically tailored to our target.”

“These defects in RNA decay we’re studying seem to occur across the board with colon and other solid cancers,” he adds. “So the overall picture with our research is bigger than just figuring out how to keep COX-2 in line. We could apply this knowledge toward controlling other genes which are also often overexpressed in cancer and chronic inflammation.” 

Funding sources for this research

  • NIH RO1 CA134609: “Post-Transcriptional Regulation in Colorectal Cancer”
  • 2012–2013 Pilot Award, The University of Kansas Cancer Center: “Discovery of Drugs Stabilizing miR-542-3p: COX-2 mRNA Interaction”
  • 2014–2015 Pilot Award, NIH COBRE (Center for Cancer Experimental Therapeutics): “Discovering Drugs That Induce Oncogenic mRNA Degradation”

Related publications

  • Young, L.E., Sanduja, S., Bemis-Standoli, K., Pena, E.A., Price, R.L., Dixon, D.A., “The mRNA Binding Proteins HuR and Tristetraprolin Regulate Cyclooxygenase 2 Expression During Colon Carcinogenesis.” Gastroenterology (May 2009).  
  • Moore, A.E., Young, L.E., Dixon, D.A., “A Common Single-Nucleotide Polymorphism in Cyclooxygenase 2 Disrupts MicroRNA-Mediated Regulation.” Oncogene (Mar 2012). 
  • Blanco, F.F., Sanduja, S., Deane, N.G., Blackshear, P.J., Dixon, D.A., “TGFβ regulates P-body formation through induction of the mRNA decay factor tristetraprolin.” Mol. Cell. Biol. (Nov 2013).