Enzyme eliminated by cancer cells holds promise for cancer treatment

Cell lines used for all types of  research appear to support his hypothesis. Many are actually cancer cells because of their proclivity to keep producing;  Dr. Browning and others have shown PKG is lost in these cells. "You split them  once or twice and they kind of lose their character," he says.

The same appears true for tumors in  people, says Dr. Browning, whose lab has found dramatic differences in PKG  levels in tumors compared to even nearby, healthy tissue removed in surgery to  ensure a cancer-free margin.

The findings made him wonder if the change in PKG level was just an  artifact or was critical to cancer survival. "A lot of proteins are lost by  cancer cells, so we asked, 'What happens if we put PKG back into the cancer  cells?'"

He took metastatic colon cancer  cells, created a system for reintroducing PKG, then put the cells into mice  without an immune system. He admits he was disappointed that the PKG-enhanced  cells grew but became very interested in how  they grew.

Cancer cells without PKG created hard,  solid tumors that spread. PKG-enhanced cells created a soft, non-invasive tumor  that literally fell apart on contact and seemed to grow in little islands. After  consultation with pathologists and others, he realized the PKG-enhanced cells  were congregating around the few blood vessels. "We know that cancer cells,  particularly colon cancer cells, are very aggressive at bringing blood vessels  into the tumor," he says. Cells poor at recruiting blood vessels don't grow  well, which seems to be the case for PKG-enhanced colon cancer cells.

Now he wants to know how PKG  nullifies aggressive metastatic cancer cells. "We think PKG inhibits cancer by  getting rid of a cancer-promoting gene called beta-catenin, which slows growth  and blocks the tumor's ability to recruit blood vessels that are needed to grow  bigger," says Dr. Browning, who recently received a $720,000 American Cancer  Society grant to pursue his hypothesis. His proposal was ranked number one by  the ACS Cell Structure and Metastasis Study Section.

He's already shown that PKG can reduce vascular endothelial growth factor,  or VEGF; anti-VEGF drugs are the focus of numerous anti-cancer trials underway  in the country because of VEGF's critical role in development of new blood  vessels. "Maybe by activating PKG or increasing PKG expression in tumors, we  are going to reduce the amount of VEGF they produce," he says. "We don't know  whether PKG has a role in going from normal tissue to the initiation of a  tumor, but we think it's important to the tumor both in terms of angiogenesis  and blocking metastasis." He points to one of his studies in which colon  cancer's spread to the lungs – a common path for metastatic colon cancer – was  completely blocked by PKG expression.

A big part of the magic of PKG may be its impact on a gene called  beta-catenin, which enables many stem cells, including those in the skin, bone  marrow and colon, to proliferate throughout life. Little pits called crypts in  the wall of the colon contain Wnt hormone which stimulate nearby stem cells, causing  an increase in beta-catenin. The net effect is the colon makes new cells to  replace cells lost to the ongoing grind of absorbing water and minerals from  food and forming and eliminating waste.

As cells start moving out of the  crypt, away from the Wnt hormone, beta-catenin levels go down so cells should  stop dividing and start maturing. Essentially all colon cancers have an aberration  in this beta-catenin system that prevents normal degradation and allows cell to  keep proliferating.

"In the normal cells that line the colon, you don't see very much  beta-catenin. We think PKG in these cells keeps it that way to keep the cells  from continuing to proliferate and spread," says Dr. Browning, who has already  shown that in the test tube at least, adding PKG lowers beta-catenin levels. Interestingly,  beta-catenin also is known to regulate VEGF expression in colon cancer.

"In a nutshell, the first and most  important genetic lesions leading to colon cancer  cause increased beta-catenin levels," says Dr.  Browning. "We found PKG can knock down beta-catenin levels by up to 80 percent  in some colon cancer cells and we think that is part of the mechanism by which  PKG is able to block tumor angiogenesis and metastasis."

  He's excited by the implications and is involved in extensive  collaborations to understand how PKG regulates beta-catenin and how it might be  used in cancer therapies.

Evidence of PKG's effectiveness in fighting colon cancer in humans may  already be available. Colon and rectal cancer is  the third most common cancer in men and women in the United States but it's rare in  developing countries where residents eat less processed food and ingest more bacteria.  Some of these bacteria make a protein, STa, which appears to prevent and even  kill colon cancer cells. Dr. Browning believes that PKG is responsible for  STa's anti-cancer effects.