Cleavage of sterol regulatory element binding proteins (SREBPs) by CPP32 during apoptosis.

Abstract

Cellular cholesterol homeostasis is controlled by sterol‐regulated proteolysis of membrane‐bound transcription factors called sterol‐regulatory element binding proteins (SREBPs). CPP32, a cysteine protease, was shown previously to cleave SREBP‐1 and SREBP‐2 in vitro at an aspartic acid between the basic helix‐loop‐helix leucine zipper domain and the first trans‐membrane domain, liberating a transcriptionally active fragment. Here, we show that CPP32 exists in an inactive 32 kDa form in Chinese hamster ovary (CHO) cells. When apoptosis was induced with the protein kinase inhibitor staurosporine, CPP32 was cleaved to subunits of 20 and 10 kDa to form the active protease. Under these conditions membrane‐bound SREBP‐1 and SREBP‐2 were both cleaved, and the transcriptionally active N‐terminal fragments were found in nuclear extracts. Similar results were obtained in human U937 cells induced to undergo apoptosis by anti‐Fas and etoposide. The apoptosis‐induced cleavage of SREBPs was not suppressed by sterols, indicating that apoptosis‐induced cleavage and sterol‐regulated cleavage are mediated by different proteases. CHO cells expressing a mutant SREBP‐2 with an Asp–> Ala mutation at the CPP32 cleavage site showed sterol‐regulated cleavage but no apoptosis‐induced cleavage. These data are consistent with the emerging concept that CPP32 is a central mediator in apoptosis. They also indicate that SREBPs, like poly (ADP) ribose polymerase, are cleaved by CPP32 during programmed cell death.

Requirement of protein kinase (Krs/MST) activation for MT-21-induced apoptosis

“…Moreover, we showed that apoptosis induced by MT-21 is dependent on caspase activation. Caspase-3 normally exists in the cytosolic fraction of cells as an inactive precursor, and it is activated proteolytically when cells are signaled to undergo apoptosis (Schlegel et al, 1996;Wang et al, 1996). The 32 kDa precursor form of caspase-3 is cleaved at two aspartic residues and yield an active caspase-3 consisting of two proteolytic fragments, 20 kDa and a 12 kDa (Nicholson et al, 1995;Wang et al, 1996).…”

Caspases: the executioners of apoptosis

“…The biological significance of these proteolytic cleavages and their relationship with the ensuing apoptotic morphology is often not known. Caspase-3 is responsible, either wholly or in part, for the proteolytic cleavage of a large number of substrates during apoptosis, including PARP, DNA-dependent protein kinase (DNA-PK), U1-70 kDa, heteronuclear ribonucleoproteins C1 and C2, sterol regulatory binding proteins, D4-GDP dissociation inhibitor, huntingtin, and almost certainly retinoblastoma protein (Rb) [104][105][106][107][108][109][110][111] (Table 3). A common feature of all these substrates is the presence of a DXXD motif (Table 3), similar to that originally described in PARP [68].…”

Hsp27 functions as a negative regulator of cytochrome c-dependent activation of procaspase-3

“…Activation of the Apaf-1/cas-9 complex appears to be a common pathway employed by diverse stimuli that induce apoptosis in mammalian cells (Li et al, 1997). Cas-3 usually exists in the cytoplasm as an inactive precursor that becomes proteolytically activated in cells undergoing apoptosis (Schlegel et al, 1996;Wang et al, 1996). Cas-3 is activated by multiple cleavages of its 32 kDa precursor form to generate the 17/11 kDa or 20/11 kDa active forms (Nicholson et al, 1995;Wang et al, 1996).…”

“…Cas-3 usually exists in the cytoplasm as an inactive precursor that becomes proteolytically activated in cells undergoing apoptosis (Schlegel et al, 1996;Wang et al, 1996). Cas-3 is activated by multiple cleavages of its 32 kDa precursor form to generate the 17/11 kDa or 20/11 kDa active forms (Nicholson et al, 1995;Wang et al, 1996). Other ®ndings that active cas-3 cleaves cas-3 has indicated that cas-3 can be activated through autocatalysis .…”