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Mol. Cell. Biol., 11 1997, 6348-6358, Vol 17, No. 11
FJ Piedrafita and M Pfahl
Vitamin A and its derivatives, the retinoids, are essential regulators of
many important biological functions, including cell growth and
differentiation, development, homeostasis, and carcinogenesis. Natural
retinoids such as all-trans retinoic acid can induce cell differentiation
and inhibit growth of certain cancer cells. We recently identified a novel
class of synthetic retinoids with strong anti-cancer cell activities in
vitro and in vivo which can induce apoptosis in several cancer cell lines.
Using an electrophoretic mobility shift assay, we analyzed the DNA binding
activity of several transcription factors in T cells treated with apoptotic
retinoids. We found that the DNA binding activity of the general
transcription factor Sp1 is lost in retinoid-treated T cells undergoing
apoptosis. A truncated Sp1 protein is detected by immunoblot analysis, and
cytosolic protein extracts prepared from apoptotic cells contain a protease
activity which specifically cleaves purified Sp1 in vitro. This proteolysis
of Sp1 can be inhibited by N-ethylmaleimide and iodoacetamide, indicating
that a cysteine protease mediates cleavage of Sp1. Furthermore, inhibition
of Sp1 cleavage by ZVAD-fmk and ZDEVD-fmk suggests that caspases are
directly involved in this event. In fact, caspases 2 and 3 are activated in
T cells after treatment with apoptotic retinoids. The peptide inhibitors
also blocked retinoid-induced apoptosis, as well as processing of caspases
and proteolysis of Sp1 and poly(ADP-ribose) polymerase in intact cells.
Degradation of Sp1 occurs early during apoptosis and is therefore likely to
have profound effects on the basal transcription status of the cell.
Interestingly, retinoid-induced apoptosis does not require de novo mRNA and
protein synthesis, suggesting that a novel mechanism of retinoid signaling
is involved, triggering cell death in a transcriptional
activation-independent, caspase-dependent manner.
Copyright © 1997, American Society for Microbiology
Retinoid-induced apoptosis and Sp1 cleavage occur independently of transcription and require caspase activation
Sidney Kimmel Cancer Center, San Diego, California 92121, USA.
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