Sequential involvement of Cdk1, mTOR and p53 in apoptosis induced by the HIV-1 envelope

Abstract

Syncytia arising from the fusion of cells expressing the HIV‐1‐encoded Env gene with cells expressing the CD4/CXCR4 complex undergo apoptosis following the nuclear translocation of mammalian target of rapamycin (mTOR), mTOR‐mediated phosphorylation of p53 on Ser15 (p53S15), p53‐dependent upregulation of Bax and activation of the mitochondrial death pathway. p53S15 phosphorylation is only detected in syncytia in which nuclear fusion (karyogamy) has occurred. Karyogamy is secondary to a transient upregulation of cyclin B and a mitotic prophase‐like dismantling of the nuclear envelope. Inhibition of cyclin‐dependent kinase‐1 (Cdk1) prevents karyogamy, mTOR activation, p53S15 phosphorylation and apoptosis. Neutralization of p53 fails to prevent karyogamy, yet suppresses apoptosis. Peripheral blood mononuclear cells from HIV‐1‐infected patients exhibit an increase in cyclin B and mTOR expression, correlating with p53S15 phosphorylation and viral load. Cdk1 inhibition prevents the death of syncytia elicited by HIV‐1 infection of primary CD4 lymphoblasts. Thus, HIV‐1 elicits a pro‐apoptotic signal transduction pathway relying on the sequential action of cyclin B–Cdk1, mTOR and p53.

Mechanisms of apoptosis induction by the HIV-1 envelope

“…After a transient activation of the mitotic progression factor (MPF, composed of cyclin B and cyclin-dependent kinase 1, Cdk1) with activation and phosphorylation of the checkpoint kinase-2 (Chk2) 59 as well as activation of NF-kB, 61 cells undergo an abortive entry into the prophase of mitosis, where they arrest and manifest nuclear fusion (karyogamy) within the heterokaryon. 21 It is at that point (but not during the interphase) that syncytia manifest p53 phosphorylation both on serine 15 55,62 and serine 46. 61 Moreover, a series of p53 target genes including two proapoptotic Bcl-2 family numbers, namely Bax and Puma, are transcribed, 55,61,62 thus activating the mitochondrial pathway of apoptosis with Cyt c-dependent caspase activation.…”

“…21 It is at that point (but not during the interphase) that syncytia manifest p53 phosphorylation both on serine 15 55,62 and serine 46. 61 Moreover, a series of p53 target genes including two proapoptotic Bcl-2 family numbers, namely Bax and Puma, are transcribed, 55,61,62 thus activating the mitochondrial pathway of apoptosis with Cyt c-dependent caspase activation. 21,45,63,64 Thus, p53 emerges as a critical mediator of syncytial apoptosis (Figure 4).…”

“…55,62 Blockade of the aberrant advancement in cell cycle, by inhibition of Cdk1, Chk2, or NF-kB strongly reduces the phosphorylation of p53 on both S15 and S46. 58,59,61,62,65 However, inhibition of mTOR with rapamycin only partially reduced the phosphorylation of p53 on S15 (p53S15P), by about 50-60%.…”

“…55,62 Blockade of the aberrant advancement in cell cycle, by inhibition of Cdk1, Chk2, or NF-kB strongly reduces the phosphorylation of p53 on both S15 and S46. 58,59,61,62,65 However, inhibition of mTOR with rapamycin only partially reduced the phosphorylation of p53 on S15 (p53S15P), by about 50-60%. 55,61,62 Another p53 kinase that has been identified more recently, cytoplasmic p38 mitogen-activated protein kinase (MAPK), was found to undergo an activating phosphorylation (on T180/ Y182P) before karyogamy and to translocate into karyogamic nuclei, where p38T180/Y182P colocalized and coimmuno- Figure 4 Mechanisms of syncytial apoptosis.…”

A cellular machine generating apoptosis-prone aneuploid cells

“…CXCR4), thus forming syncytia. 1,2 We have used such Env-elicited syncytia to dissect a lethal p53-dependent signal transduction cascade [3][4][5] relevant to AIDS, [6][7][8] as well as 'mitotic catastrophe', an apoptosis-like cell death that occurs during the metaphase, after fusion of nonsynchronized cells and inactivation of the cell cycle checkpoint kinase Chk2. 9,10 One of the intrinsic advantages of a model of cell death affecting giant multinuclear cells is the ease with which the subcellular localization of apoptosis-regulatory proteins can be studied.…”