Functional Analysis of the Mouse ICER (Inducible cAMP Early Repressor) Promoter: Evidence for a Protein That Blocks Calcium Responsiveness of the CAREs (cAMP Autoregulatory Elements)

• Published in: Molecular endocrinology (Baltimore, Md.) Vol. 13; no. 7; pp. 1207 - 1217
• Main Authors: Krueger, Darcy A, Mao, Dailing, Warner, Elizabeth A, Dowd, Diane R
• Format: Journal Article
• Language: English
• Published: Endocrine Society 01-07-1999
• ISSN: 0888-8809; 1944-9917
• DOI: 10.1210/mend.13.7.0319

Although Ca2+ and cAMP
mediate their effects through distinct pathways, both signals converge
upon the phosphorylation of the cAMP response element (CRE) binding
protein, CREB, thereby activating transcription of CRE-regulated genes.
In WEHI7.2 thymocytes, cAMP increases the expression of the inducible
cAMP early repressor (ICER) gene through CRE-like elements, known as
cAMP autoregulatory elements (CAREs). Because
Ca2+- and cAMP-mediated transcription converge
in WEHI7.2 thymocytes, we examined the effect of
Ca2+ fluxes on the expression of the ICER gene
in these cells. Despite the presence of multiple CAREs within its
promoter, ICER gene transcription was not activated by
Ca2+. Moreover, Ca2+
attenuated the stimulatory effect of cAMP on ICER expression. Transient
expression of reporter constructs demonstrated that when these CAREs
were placed in a different DNA promoter context, the elements became
responsive to Ca2+. Detailed studies using
chimeric promoter constructs to map the region responsible for blocking
the transcriptional response to Ca2+ indicated
that a small portion of the ICER promoter was necessary for the effect.
Southwestern blot analysis identified a 83-kDa nuclear protein that
bound specifically to that region. The relative binding activity of the
factor to the ICER promoter and mutant promoter sequences correlated
with an inhibition of Ca2+-activated gene
expression in WEHI7.2 cells. These data suggest that the factor
functions as a putative Ca2+-activated
repressor of CREB/CRE-mediated transcription. Thus, depending on the
surrounding context in which the CRE is located, CREs of individual
genes can be regulated separately by Ca2+ and
cAMP despite the convergence of these two signaling pathways.