Caspase-3-resistant uncleavable form of acidic leucine-rich nuclear phosphoprotein 32B potentiates leukemic cell apoptosis.

ABSTRACT

One member of the highly conserved acidic leucine­rich nuclear phosphoprotein 32 kDa (ANP32) family of proteins, ANP32B, is critical for normal development, as demonstrated by a study in ANP32B­deficient mice. Another study indicated that ANP32B was a direct substrate of caspase­3, and was primarily cleaved at the sequence Ala­Glu­Val­Asp, following Asp­163. To investigate the significance of ANP32B cleavage in apoptosis, leukemic U937T cell lines were generated with inducible expression of ANP32B(wild type; WT), the uncleavable mutant ANP32B(D163A) and the N­terminal fragment ANP32B(1­163). Notably, overexpression of ANP32B(WT) and ANP32B(D163A) moderately increased and significantly enhanced etoposide­induced apoptosis and caspase­3 activation, whereas expression of ANP32B(1­163) produced no effect. Two hypotheses have been generated, which may explain the distinct roles of the various ANP32B forms i) ANP32B(WT) and ANP32B(D163A) localize in the nucleus while ANP32B(1­163) mainly resides in the cytosol; or ii) ANP32B(WT) and ANP32B(D163A), but not ANP32B(1­163), inhibit the expression of the anti­apoptotic protein Bcl­2. Based on these observations, caspase­3­resistant uncleavable ANP32B(D163A) is hypothesized to be pro­apoptotic in leukemic cells.