Ct. Taylor et al., Critical role of cAMP response element binding protein expression in hypoxia-elicited induction of epithelial tumor necrosis factor-alpha, J BIOL CHEM, 274(27), 1999, pp. 19447-19454
Tissue hypoxia is intimately associated with a number of chronic inflammato
ry conditions of the intestine. In this study, we investigated the impact o
f hypoxia on the expression of a panel of inflammatory mediators by intesti
nal epithelia, Initial experiments revealed that epithelial (T84 cell) expo
sure to ambient hypoxia evoked a time-dependent induction of the proinflamm
atory markers tumor necrosis factor-alpha (TNF-alpha), interleukin-8 (IL-8)
, and major histocompatibility complex (MHC) class II (37 +/- 6.1-, 7 +/- 0
.8-, and 9 +/- 0.9-fold increase over normoxia, respectively, each p < 0.01
). Since the gene regulatory elements for each of these molecules contains
an NF-kappa B binding domain, we investigated the influence of hypoxia on N
F-kappa B activation. Cellular hypoxia induced a time dependent increase in
nuclear p65, suggesting a dominant role for NF-kappa B in hypoxia-elicited
induction of proinflammatory gene products. Further work, however, reveale
d that hypoxia does not influence epithelial intercellular adhesion molecul
e 1 (ICAM-1) or MHC class I, the promoters of which also contain NF-kappa B
binding domains, suggesting differential responses to hypoxia, Importantly
, the genes for TNF-alpha, IL-8, and MHC class II, but not ICAM-1 or MHC cl
ass I, contain cyclic AMP response element (CRE) consensus motifs, Thus, we
examined the role of cAMP in the hypoxia-elicited phenotype, Hypoxia dimin
ished CRE binding protein (CREB) expression. In parallel, T84 cell cAMP was
diminished by hypoxia (83 +/- 13.2% decrease, p < 0.001), and pharmacologi
c inhibition of protein kinase A induced TNF-alpha and protein release (9 /- 3.9-fold increase). Addback of cAMP resulted in reversal of hypoxia-elic
ited TNF-alpha release (86 +/- 3.2% inhibition with 3 mM 8-bromo-cAMP). Fur
thermore, overexpression of CREB but not mutated CREB by retroviral-mediate
d gene transfer reversed hypoxia-elicited induction of TNF-alpha defining a
causal relationship between hypoxia-elicited CREB reduction and TNF-alpha
induction. Such data indicate a prominent role for CREB in the hypoxia-elic
ited epithelial phenotype and implicate intracellular cAMP as an important
second messenger in differential induction of proinflammatory mediators.