Retinoid-Induced Apoptosis and Sp1 Cleavage Occur Independently of Transcription and Require Caspase Activation

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Abstract
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.