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J Biol Chem, Vol. 274, Issue 27, 18857-18863, July 2, 1999
From the Departamento de Bioquimica y Biologia Molecular II,
Facultad de Farmacia, Universidad Complutense,
28040 Madrid, Spain
Serum deprivation of
Ha-ras-transformed brown adipocyte cell line resulted in a
dramatic apoptotic cell death, as detected either by DNA laddering or
by an increase in the percentage of hypodiploid cells or by nuclei
condensation and fragmentation, as compared with immortalized cell line
or primary fetal brown adipocytes. Moreover, transient transfection of
immortalized brown adipocytes with a constitutively active
ras gene (Ha-raslys12) mimics the
high rate of apoptosis detected in the transformed cell line. On the
other hand, transient transfection of the dominant-negative construct
of raf-1 rescued serum-deprived
Ha-ras-transformed brown adipocytes from apoptosis,
decreasing the percentage of hypodiploid cells, the external display of
phosphatidylserine, and the DNA laddering. However, inhibition of
mitogen-activated protein kinase with PD098059 did not preclude
apoptosis and in fact increased the rate of apoptosis observed in
serum-deprived Ha-ras-transformed cells, indicating that
the Ras/Raf-1 pathway induced apoptosis throughout a mitogen-activated
protein kinase kinase 1 (MEK-1)-independent pathway. Furthermore,
apoptosis in Ha-ras-transformed brown adipocytes is
concurrent with an up-regulation in the expression of the pro-apoptotic protein Bcl-xS, the expression of the anti-apoptotic protein Bcl-2 being down-regulated. Finally, an association of Ras and Raf with phosphorylated Bcl-2 protein was demonstrated in immunoprecipitates from apoptotic cells. Thus, we propose a mechanism of apoptosis in
Ha-ras-transformed adipocytes under serum deprivation
involving Raf-1 association with phosphorylated Bcl-2, down-regulation
of Bcl-2 expression, and up-regulation of Bcl-xS expression.
Activated Ha-ras Induces Apoptosis by Association
with Phosphorylated Bcl-2 in a Mitogen-activated Protein
Kinase-independent Manner
Copyright © 1999 by The American Society for Biochemistry and Molecular Biology, Inc.
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