Members of the HRT family of basic helix-loop-helix proteins act as transcriptional repressors downstream of Notch signaling

The Hairy-related transcription-factor (HRT) genes encode three related bas ic helix-loop-helix transcription factors that show sequence similarity to the Hairy and Enhancer of split family of transcriptional repressors. HRT p roteins are expressed in specific regions of the developing heart, vasculat ure. pharyngeal arches and somites, and the periodicity of their expression in semitic precursors mirrors that of Notch signaling-related molecules. I n the present study, we show that the intracellular domain of the Notch 1 r eceptor (Notch1 IC), which is constitutively active, up-regulates HRT2 expr ession in 10T(1)/(2) fibroblasts. Luciferase reporter assays using the regu latory regions of the mouse HRT genes revealed that transcription of all th ree genes is stimulated by Notch1 IC. The promoters of the HRT genes share homology in a binding site for Suppressor of Hairless [Su(H)], a transcript ional mediator of Notch signaling. A dominant-negative Su(H) mutant abolish ed Notch-activated HRT2 expression, and mutation of the conserved Su(H) con sensus site in the HRT2 promoter attenuated transcriptional activation by N otch. Ectopic expression of HRT proteins also blocked activation of HRT2 ex pression by Notch1 IC through a mechanism requiring the basic region, but n ot the conserved carboxyl-terminal YQPW-TEVGAF motif of HRT2. These finding s identify HRT genes as downstream targets for Notch signaling and reveal a negative autoregulatory loop whereby HRT proteins repress their own expres sion through interference with Notch signaling.