Notch signaling dictates cell fate and critically influences cell prolifera
tion, differentiation, and apoptosis in metazoans. Ligand binding initiates
the signal through regulated intramembrane proteolysis of a transmembrane
Notch receptor which releases the signal-transducing Notch intracellular do
main (NICD). The HES/E(spl) gene family is a primary target of Notch and th
us far the only known Notch effector. A newly isolated HER-P family, a HES-
related basic helix-loop-helix protein family, has been proposed as a poten
tial target of Notch, based on its induction following NICD overexpression.
However, NICD is physiologically maintained at an extremely low level that
typically escapes detection, and therefore, nonregulated overexpression of
NICD-as in transient transfection-has the potential of generating cellular
responses of little physiological relevance. Indeed, a constitutively acti
ve NICD indiscriminately up-regulates expression of both HERP1 and HERP2 mR
NAs. However, physiological Notch stimulation through ligand binding result
s in the selective induction of HERP2 but not HERP1 mRNA and causes only ma
rginal up-regulation of HES1 mRNA. Importantly, HERP2 is an immediate targe
t gene of Notch signaling since HERP2 mRNA expression is induced even in th
e absence of de novo protein synthesis. HERP2 mRNA induction is accompanied
by specific expression of HERP2 protein in the nucleus. Furthermore, using
RBP-jk-deficient cells, we show that an RBP-Jk protein, a transcription fa
ctor that directly activates HES/E(spl) transcription, also is essential fo
r HERP2 mRNA expression and that expression of exogenous RBP-jk is sufficie
nt to rescue HERP2 mRNA expression. These data establish that HERP2 is a no
vel primary target gene of Notch that, together with HES, may effect divers
e biological activities of Notch.