Regulation of H1° Gene Expression by Nuclear Receptors Through an Unusual Response Element: Implications for Regulation of Cell Proliferation

Cloning and sequence analysis of the 5′-flanking region of the human H1° histone gene, a differentiation-specific member of the H1 family, has revealed several potential regulatory elements. In this study, we have characterized the interactions of nuclear receptors with an unusual response element consisting of two half-sites arranged as a direct repeat with an 8-bp spacer (DR-8). Thyroid hormone receptors (TR) bind this DR-8 as homodimers and heterodimers with RXR. Retinoic acid receptors (RARs) also bind as heterodimers with RXR to the DR-8, and this binding is enhanced in the presence of retinoic acid (RA) and/or 9-cis RA. Reporter constructs containing the DR-8 allowed a several-fold induction by T3 in the presence of TRs. RARα and RARβ allowed RA-dependent transcriptional activation whereas RARγ mostly increased basal activity. 9-cis RA inhibited the T3 response, indicating a hormonal cross-talk among the subfamily of nuclear receptors. Two orphan receptors, COUP-TF and v-erbA, also bind the DR-8 sequence in the human H1° promoter. COUP-TF, which usually represses RAREs, enhances transcriptional activation through the DR-8 whereas v-erbA completely represses TR-RXR induction of the H1° gene. Thus, a number of signaling pathways that play important roles during development and differentiation are able to influence the transcription rate of this special H1 subtype directly through a DR-8 response element in its promoter. Because H1° expression levels inversely correlate with cell proliferation, our data suggest that several nuclear receptors and the v-erbA oncogene can influence cell proliferation via the regulation of H1° expression.