Structure, hormonal regulation, and identification of the interleukin-6- and dexamethasone-responsive element of the rat haptoglobin gene.

Abstract

Hepatic expression of the haptoglobin (Hp) gene in mammalian species is stimulated severalfold during an acute-phase reaction. To identify the molecular mechanism responsible for this regulation, the single-copy rat Hp gene has been isolated. The genomic sequences showed a high degree of homology with the primate Hp gene. Activity of the rat Hp gene was increased in cultured liver cells by interleukin-1 (IL-1), IL-6, and glucocorticoids. The genomic Hp gene sequence spanning from -6500 to +6500, when transiently introduced into human hepatoma (HepG2) cells, directed IL-6- and dexamethasone-stimulated expression of rat Hp mRNA and protein. No response to IL-1 was detected, suggesting that the corresponding regulatory element(s) might lie outside of the tested gene sequences. An IL-6- and dexamethasone-responsive element has been localized to the promoter proximal region -146 to -55. Although the nucleotide sequences of this rat Hp gene region showed substantial divergence from that of the human gene, analysis of sequential 5' and 3' deletion constructs indicated an arrangement of functional IL-6 response elements in the rat Hp promoter sequence comparable to that of the human homolog. The magnitude of IL-6 regulation through the rat Hp gene promoter was severalfold lower than that of the human Hp gene. The reduced activity could be ascribed to a single-base difference in an otherwise conserved sequence corresponding to an active element in the human gene. The IL-6 response of the rat Hp element was improved severalfold by substituting that base with the human nucleotide.

Distinct regulation of the interleukin-1 and interleukin-6 response elements of the rat haptoglobin gene in rat and human hepatoma cells.

“…The results indicated the presence of one major regulatory element at position -146 to -56 which is responsive to IL-6, LIF, and dexa-methasone (11,37). This element appears to consist of two interacting regions which are similar in sequence to the two proximal regulatory regions B and C of the human gene (42).…”

“…To understand the molecular mechanism of hormone action, the functional Hp gene from humans (12) and rats (37) has been cloned. The cis-acting regulatory element of the human gene which is responsive to IL-6 in transiently transfected Hep3B cells has been localized to the promoter region at position -180 to -50 relative to the transcription start site (43).…”

Hypoxia-inducible Factor-1α Enhances Haptoglobin Gene Expression by Improving Binding of STAT3 to the Promoter

“…Glucocorticoid hormone acts synergistically with IL-6 in human hepatic Hp expression (31). Because no glucocorticoidresponsive element is found in the human Hp gene, it is likely that glucocorticoid-induced factors (rather than the activated hormone receptor) are associated in Hp transcription.…”

Synergistic action of interleukin-6 and glucocorticoids is mediated by the interleukin-6 response element of the rat alpha 2 macroglobulin gene.

“…The induction of the rat a2M gene and many other acute-phase genes by IL-6 is greatly facilitated by glucocorticoids, both in living animals (4,8) and in cultured hepatic cells (4,6,8,30,40). IL-6 alone is capable of inducing the transcription of the rat a2M gene promoter, but this induction is greatly amplified by glucocorticoids in rat heptoma cells (19).…”