Nitric oxide can kick-start the gut lining to fight inflammation

Experimentally inducing the gut’s ability to make, adjust, and localize nitric oxide decreases colitis severity and could help fight inflammation-induced colon cancer, according to a new study published in Cell Reports.

“Nitric oxide (NO) plays an established role in numerous physiological and pathological processes, but the specific cellular sources of NO in disease pathogenesis remain unclear, preventing the implementation of NO-related therapy,” wrote lead author Ayelet Erez, MD, PhD, Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel, and colleagues.

NO is a key regulatory molecule in gut integrity, and increased NO signaling has been implicated in inflammatory bowel disease (IBD). But, the origins and precise functions of NO are complex and paradoxical, and have yet to be fully elucidated.

Experts have had difficulty parsing the homeostatic roles of NO, in part because it comes in three flavors, or isoforms, which are co-expressed in multiple cell types lending to redundancy. These three isoforms include endothelial nitric oxide synthase (eNOS), neuronal oxide synthase (nNOS), and inducible nitric oxide synthase (iNOS).

All three isoforms use the semi-essential amino acid arginine to synthesize NO. In turn, argininosuccinate lyase (ASL), which is a urea cycle enzyme, is the only enzyme in mammals that can create arginine. In arginine-deficient states, such as intestinal inflammation, experts suggest that ASL is integral in sustaining arginine homeostasis at the tissue level.

In the current study, Dr. Erez and colleagues employed mouse models with cell-specific loss of ASL (ASL knockout mice), along with genetic and chemical colitis models, to unravel the role that NO plays in IBD.

“Our hope was that if we prompted the organism to make ASL and strengthen its activity, it would in turn manufacture NO only where it’s needed and not where it can do harm,” said study author Noa Stettner, a PhD student in Dr. Erez’s lab.

To this end, the researchers promoted NO synthesis in enterocytes by upregulating ASL and complementing its substrates using citrulline and fisetin nutraceutical supplements. (Citrulline is naturally found in watermelon, spinach, and beets; fisetin is found in apples, persimmons, and strawberries.)

Consequently, this induction of endogenous NO production by enterocytes fortified epithelial integrity of the gut lining and decreased inflammation secondary to colitis. Moreover, because inflammation aggravates colon cancer, colon tumors in the mouse models shrank in size and number.

On a related note, the team found that NO derived from immune cells is linked to activation of macrophages, and thus more inflammation. By contrast, NO derived from enterocytes keeps these activated macrophages at bay, and inhibits macrophage infiltration and tissue damage.

“We show here that boosting the metabolic ability of the enterocytes to synthesize and adjust the level and localization of NO to the exact amount and place where it is needed is the most beneficial treatment for reducing colitis severity,” the researchers concluded. “Because the duration, severity, and extent of colitis have all been shown to increase the risk for colon cancer development, not surprisingly, our combination of citrulline/fisetin also decreased inflammation-associated colon cancer.”

Additionally, the investigators showed the greater metabolic benefit of supplementing substrates and upregulating metabolic enzymes versus that of administering therapy straight-up with molecules that are deficient.

“We trusted the body to make use of its own healing mechanisms,” reflected Dr. Stettner. “We provided it with the building blocks and let it manufacture NO in the right amount and exactly in the right place.”

Because both citrulline and fisetin are currently available as nutraceutical supplements, the findings from this study could quickly pave the way to human trials, the authors predicted.

This research was supported by the Adelis Foundation, the Rising Tide Foundation, the Comisaroff Family Trust, the Irving B. Harris Fund for New Directions in Brain Research, and the European Research Council.