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Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104
The protein kinase
C (PKC) family represents an important group of enzymes whose
activation is associated with their translocation from the cytosol to
different cellular membranes. In this study, the spatial distribution
of PKC-
, -
and -
in rat liver epithelial (WB) cells has been
examined by Western blot analysis after subcellular fractionation.
Cytosolic, membrane, nuclear, and cytoskeletal fractions were obtained
from cells stimulated with phorbol 12-myristate 13-acetate (PMA),
angiotensin II (ANG II), or epidermal growth factor (EGF). PMA caused
most of the PKC-, - and - initially present in the cytosol to
be transported to the membrane and nuclear fractions. In contrast, both
ANG II and EGF induced only a minor translocation of PKC- to the
membrane fraction but caused a statistically significant
membrane-directed movement of PKC- and -. Translocation of
PKC- and - to the nucleus induced by ANG II and EGF was transient and quantitatively smaller than that induced by PMA. PKC- and -
were present in the cytoskeleton of resting cells, but although PMA,
ANG II, and EGF caused some changes in their content, these were
variable, suggesting that the cytoskeleton fraction was heterogeneous. PKC depletion inhibited ANG II-induced mitogenesis and the sustained activation of Raf-1 and extracellular regulated protein kinase (ERK).
However, although PKC depletion inhibited EGF-induced mitogenesis, the
maximum EGF-induced activation of the ERK pathway was only slightly
retarded. We hypothesize that PKC- and - are involved in
mitogenesis via both ERK-dependent and ERK-independent mechanisms. These results support the notion that specific PKC isozymes exert spatially defined effects by virtue of their directed translocation to
distinct intracellular sites.
mitogen-activated protein kinase; extracellular regulated protein kinase; Raf-1; mitogenesis; angiotensin II; epidermal growth factor
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