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Prepublished online as a Blood First Edition Paper on January 9, 2003; DOI 10.1182/blood-2002-07-2212.
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Blood, 15 May 2003, Vol. 101, No. 10, pp. 3885-3892
HEMATOPOIESIS
Reciprocal effects of C/EBP and PKC on JunB expression and
monocytic differentiation depend upon the C/EBP basic
region
Huaitian Liu,
Jeffrey R. Keefer,
Qian-fei Wang, and
Alan D. Friedman
From the Division of Pediatric Oncology, Johns Hopkins
University, Baltimore, MD.
Monocytic differentiation of 32DPKC cells in response to
activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) was inhibited by exogenous CCAAT/enhancer
binding protein  -estradiol receptor (C/EBP-ER), which
impeded morphologic maturation and induction of macrosialin mRNA.
Inhibition of monopoiesis was also evident in 32DPKC subclones
expressing C/EBPLeu12Val-ER, which cannot dimerize or bind
DNA because of mutation of the leucine zipper, C/EBPGZ-ER, in which
the leucine zipper has been replaced by the GCN4 zipper, or
C/EBP 3-8-ER, lacking the C/EBP transactivation domains. In
contrast, C/EBPBR3-ER, containing a mutant basic region, did not
inhibit monocytic differentiation. C/EBP-ER strongly inhibited
endogenous AP-1 DNA-binding. Supershift analysis revealed that the
major AP-1 complex contains JunB. Activation of C/EBP-ER specifically reduced endogenous JunB RNA and protein and exogenous JunB
levels without affecting endogenous or exogenous c-Jun. The stability
of PMA-induced JunB was not affected. Thus, C/EBP-ER suppresses both
JunB transcription and posttranscriptional protein generation or
induction. PU.1 levels and activity were increased. The Leu12Val, GZ,
and 3-8 mutants also inhibited JunB expression, whereas the BR3
mutant was ineffective, indicating that inhibition of JunB expression
and monocytic differentiation by C/EBP-ER depends upon an
interaction mediated by its basic region. Exogenous JunB restored AP-1
DNA-binding but did not prevent inhibition of macrosialin expression by
C/EBP-ER, indicating that JunB is not the only target relevant to
inhibition of monopoiesis by C/EBP.
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