Growth promoting effect of thioredoxin on intestinal epithelial cells

Paneth cells are located at the bases of intestinal crypts, and their cytoplasmic granules contain large amounts of zinc. We previously showed that administration of diphenylthiocarbazone (dithizone), a zinc chelater, to rats induced the selective death of Paneth cells. This was followed by a transient wave of epithelial cell proliferation in the entire crypts. In the study described here, we again applied this experimental model in an attempt to identify novel growth-promoting factors in the small intestine. Male Wistar rats were injected with dithizone and killed 6 hr later. Messenger RNAs (mRNAs) were extracted from the terminal ileum for the construction of a cDNA library. This library was then transfected into the human intestinal cell line Caco-2, and the cells that continued to grow in the medium containing only 1% FBS were cloned. One of the cDNA sequences identified from those transfection experiments was the full-length rat thioredoxin (TRX) gene. To confirm the growth-promoting effect of this cDNA, we transfected it into Caco-2 cells again. These clones proliferated in the medium containing only 1% FBS, while the control clones failed to show any growth. Transient oxidative stress exerted by the addition of oxidative reagents diamide and hydrogen peroxide partially suppressed the growth of TRX-transfected cells. Northern hybridization analysis revealed that TRX expression in rat ileum after dithizone treatment was altered in accordance with intestinal epithelial regeneration in the crypts. Single-cell RT-PCR also showed TRX mRNA expression in Paneth cells. These studies identify rat thioredoxin as a growth-promoting factor for intestinal epithelial cells.