Maturation-dependent sequential expression of interferon regulatory factor-1 and inducible nitric oxide synthase in cultured human enterocytes

We tested the hypothesis that inducible nitric oxide (NO) synthase (iNOS) and interferon regulatory factor 1 (IRF-1), a trans-acting factor in iNOS transcriptional activation, are maturation-dependently expressed in cytokine-stimulated human enterocytes. Caco-2BBe cells, at varying stages of maturation, were stimulated with IL-1β and IFN-γ. Cytokine stimulation of 3-day-old undifferentiated Caco-2BBe cells induced low levels of NO production, iNOS and IRF-1 immunoreactivity, iNOS and IRF-1 mRNA expression, and iNOS activity, whereas 24-day-old mature cells responded with a large and prolonged activation of iNOS and IRF-1 expression. The basis for this difference was accounted in part by the relatively greater iNOS transcription rate in 24-vs. 3-day-old cells. Sequential expression of IRF-1 followed by iNOS mRNA occurred in both 3- and 24-day-old cells. We conclude that enterocyte maturation profoundly alters the magnitude and duration of human iNOS and IRF-1 expression in response to cytokine stimulation. The differences in iNOS mRNA levels between the immature and mature cells are only partially explained by difference in transcriptional rates, implying that post-transcriptional regulation may also be influenced by the state of enterocyte maturation. Induction of IRF-1 expression precedes and parallels the level of iNOS expression at all stages of maturation. We propose that IRF-1 may modulate the expression of cytokine-induced iNOS activity in differentiating enterocytes.