Acinus-provoked protein kinase C δ isoform activation is essential for apoptotic chromatin condensation

Abstract

Histone H2B phosphorylation tightly correlates with chromatin condensation during apoptosis. The caspase-cleaved acinus (apoptotic chromatin condensation inducer in the nucleus) provokes chromatin condensation in the nucleus, but the molecular mechanism accounting for this effect remains elusive. Here, we report that the active acinus p17 fragment initiates H2B phosphorylation and chromatin condensation by activating protein kinase C d isoform (PKC-d). We show that p17 binds to both Mst1 and PKC-d, which is upregulated by apoptotic stimuli, enhancing their kinase activities. Acinus mutant susceptible to degradation elicits stronger chromatin condensation and higher H2B phosphorylation than wild-type acinus. Dominant-negative PKC-d but not Mst1 robustly blocks acinus-initiated H2B phosphorylation. Surprisingly, depletion of Mst1 triggers caspase-3 activation, provoking H2B phosphorylation through activating PKC-d. Chromatin condensation, a hallmark of apoptosis, is associated with histone H2B phosphorylation. Phosphorylation of H2B is usually negligible in both quiescent and growth states, but it is markedly stimulated when chromatin condensation and nucleosomal DNA fragmentation is initiated.1 Allis and coworkers demonstrated that phosphorylation of H2B at serine 14 by caspase-cleaved Mst1 tightly correlates with cells undergoing programmed cell death.2 Histone 2B serine 14 (H2BS14) phosphorylation has also been consistently accumulated at DNA double-strand breaks, indicative of its important role in apoptosis.3 Chromatin condensation can be induced by a nuclear protein called acinus (apoptotic chromatin condensation inducer in the nucleus), which is cleaved during apoptosis. 4 Acinus is cleaved by caspases on both its N-and C-termini, producing a p17 active form (amino acids (a.a.) 987-1093 in -L isoform), which elicits chromatin condensation in the absence of caspase-3. Recently, we showed that Akt phosphorylates acinus on serine 422 and 573, resulting in its resistance to caspase cleavage in the nucleus and the inhibition of acinus-dependent chromatin condensation. Thus, Akt inhibits chromatin condensation during apoptosis by phosphorylating acinus in the nucleus, revealing a specific mechanism by which nuclear Akt promotes cell survival. 5 However, the molecular mechanism as to how p17 couples to chromatin condensation remains elusive.MST1 (mammalian Ste20-like kinase 1) is ubiquitously expressed. MST1 contains a Ste20-related kinase catalytic domain in the amino-terminal segment (a.a. 30-270), followed by a noncatalytic tail that contains successively an autoinhibitory domain (a.a. 331-394), a dimerization domain (after a.a. 431), and a nuclear localization signal at the COOH terminus.6-8 MST1 is predominantly cytoplasmic but cycles continuously through the nucleus. Mst1 is activated and cleaved by a broad range of stimuli in various cell types, which suggests that it is a common component in the diverse signaling pathways leading to apoptosis.9 Overexpression of Mst1 in some cell types induces apoptotic fe...

Protein kinases of the Hippo pathway: Regulation and substrates

“…Many early studies of recombinant Mst1/Mst2 demonstrated that overexpression resulted in activation of the stress kinases p38 and JNK/SAPK[60,88,133,134], however no evidence supports the activation of these pathways as physiologic outputs of Mst1/Mst2 rather than indirect responses to a stress state engendered by Mst1/Mst2 overexpression. Similarly, the evidence is weak that Mst1[135], as opposed to PKC[136] is the kinase responsible for Histone H2B[Ser14] phoshorylation during apoptosis.…”

Chromatin Structure Following UV-Induced DNA Damage—Repair or Death?

“…Histone H2B phosphorylation at serine-14 (S14) is a hallmark of apoptosis and has been associated with the condensation of chromatin [66,67]. The phosphorylation of H2B at S14 is mediated by mammalian sterile twenty (MST1) kinase, following caspase-3 mediated cleavage of MST1 [67].…”

“…The phosphorylation of H2B at S14 is mediated by mammalian sterile twenty (MST1) kinase, following caspase-3 mediated cleavage of MST1 [67]. Recently, it was also shown that H2BS14 phosphorylation occurred following UVB irradiation and this was mediated by activated MAPKs (such as ERK1/2, JNK1/2 and P38), showing that H2BS14 phosphorylation is mediated by both MAPKs and caspase-3/MST1 pathways [68].…”

Serine/Arginine Protein–Specific Kinase 2 Promotes Leukemia Cell Proliferation by Phosphorylating Acinus and Regulating Cyclin A1

“…Recently, we show that acinus is a physiologic substrate of nuclear Akt, which phosphorylates acinus on serine 422 and 573 and leads to its resistance to caspase cleavage and the inhibition of acinus-dependent chromatin condensation (17). Moreover, we found that the active fragment of p17 binds PKC-y and enhances its apoptotic kinase activity, triggering histone H2BS14 phosphorylation and chromatin condensation (18). Most recently, we show that zyxin binds acinus, which is regulated by Akt, and diminishes acinus proteolytic cleavage and chromatin condensation (19).…”