Apoptotic-induced cleavage shifts HuR from being a promoter of survival to an activator of caspase-mediated apoptosis

Abstract

Little is known about the cellular mechanisms modulating the shift in balance from a state of survival to cell death by caspasemediated apoptosis in response to a lethal stress. Here we show that the RNA-binding protein HuR has an important function in mediating this switch. During caspase-mediated apoptosis, HuR is cleaved to generate two cleavage products (CPs). Our data demonstrate that the cleavage of HuR switches its function from being a prosurvival factor under normal conditions to becoming a promoter of apoptosis in response to a lethal stress. In the absence of an apoptotic stimuli, HuR associates with and promotes the expression of caspase-9 and prothymosin a (ProT) mRNAs, and pro-and antiapoptotic factors, respectively, both of which have been characterized as important players in determining cell fate. During the early steps of caspase-mediated apoptosis, however, the level of caspase-9 protein increases, while ProT remains unchanged. Under these conditions, the two HuR-CPs selectively bind to and stabilize caspase-9 mRNA, but do not bind to ProT. Hence, taken together, our data show that by maintaining a threshold of expression of proapoptotic factors such as caspase-9 in response to a lethal stress, the HuR-CPs help a cell to switch from resisting death to undergoing apoptosis.

The C-terminal RNA binding motif of HuR is a multi-functional domain leading to HuR oligomerization and binding to U-rich RNA targets

“…11 The RRM3-containing cleavage product may acquire new functions in triggering apoptosis as it selectively binds to and stabilizes caspase-9 mRNA in an ARE-dependent manner. 12 The NMR titration data herein reported and earlier findings with HuB, 59 demonstrate that isolated RRM3 can bind to AREs, although previous studies suggested a negligible role of HuR RRM3 ¡ as part of HuR FL ¡ in AREs recognition. 34,[44][45][46] Isolated RRM3 is in a monomer/dimer exchange on a time scale unfavorable for NMR observation of the residues at the dimerization site, notably those in the W261-T271 region at the C-end of helix a 1 .…”

“…In this sense, our data is consistent with RRM3 being easily removed from the HuR core by a caspasemediated cleavage, which is a regulatory event that enhances the apoptotic response. 11,12 Solution structure of HuR RRM3…”

“…46 Meriting particular interest, the RRM3-containing cleavage product specifically binds ARE1 of caspase-9 mRNA, which lacks of 5 0 -AUUUA-3 0 sequences but it contains a U-rich region. 12 HuR is regulated by post-translational modifications. Phosphorylation at RRM3 S318 residue by PKCd means an important mode in HuR regulation, with consequences in colon carcinoma cells due to HuR dysregulation.…”

Neuroprotection requires the functions of the RNA-binding protein HuR

“…58 In response to lethal stress, caspase-3 and -7 cleave HuR to release interacting apoptosome effectors and amplify proapoptotic loops. 59,60 Our current findings suggest for the first time that these differences may be because of the differential need for HuR in apoptosis versus necrosis, which includes several modes of programmed cell death (e.g. necroptosis, oxytosis, parthanatos).…”

Physiological regulation of the heat shock response by glutamine: implications for chronic low-grade inflammatory diseases in age-related conditions

“…During caspase-mediated apoptosis, HuR switches its function from pro-survival to pro-apoptotic [203]. Caspases can mediate cleavage of HuR under different situations [204].…”