A moderate reduction of Bcl-xL expression protects against tumorigenesis; however, it also increases susceptibility to tissue injury

Bcl-xL deamidation is regulated by multiple ion transporters and is intramolecularly catalyzed

In susceptible tumor cells, DNA-damaging antineoplastic agents induce an increase in intracellular pH during the premitochondrial stage of apoptosis. The rate of nonenzymatic deamidation of two asparagines in the anti-apoptotic protein Bcl-x_L is accelerated by this increase in pH. Deamidation of these asparagines is a signal for the degradation of Bcl-x_L, which is a component of the apoptotic response to DNA damage. It has previously been shown that the increase in pH is mediated by the ion transporter Na⁺/H⁺ exchanger 1 in some cells. Here we demonstrate that one or more additional ion transporters also have a role in the regulation of Bcl-x_L deamidation in at least some tumor cell lines and fibroblasts. As a second, independent finding, we report that there are histidines in close proximity to the Bcl-x_L deamidation sites that are highly conserved in land-dwelling species and we present evidence that deamidation of human Bcl-x_L is intramolecularly catalyzed in a manner that is dependent upon these histidines. Further, we present evidence that these histidines act as a pH-sensitive switch that enhances the effect of the increase in pH on the rate of Bcl-x_L deamidation. The conservation of such histidines implies that human Bcl-x_L is in essence "designed" to be deamidated, which provides further evidence that deamidation serves as a bona fide regulatory post-translational modification of Bcl-x_L.

Control of cellular Bcl-xL levels by deamidation-regulated degradation

Deamidation of two asparagines activates a conditional PEST sequence to target Bcl-xL for degradation.

The cellular concentration of Bcl-x_L is among the most important determinants of treatment response and overall prognosis in a broad range of tumors as well as an important determinant of the cellular response to several forms of tissue injury. We and others have previously shown that human Bcl-x_L undergoes deamidation at two asparaginyl residues and that DNA-damaging antineoplastic agents as well as other stimuli can increase the rate of deamidation. Deamidation results in the replacement of asparginyl residues with aspartyl or isoaspartyl residues. Thus deamidation, like phosphorylation, introduces a negative charge into proteins. Here we show that the level of human Bcl-x_L is constantly modulated by deamidation because deamidation, like phosphorylation in other proteins, activates a conditional PEST sequence to target Bcl-x_L for degradation. Additionally, we show that degradation of deamidated Bcl-x_L is mediated at least in part by calpain. Notably, we present sequence and biochemical data that suggest that deamidation has been conserved from the simplest extant metazoans through the human form of Bcl-x_L, underscoring its importance in Bcl-x_L regulation. Our findings strongly suggest that deamidation-regulated Bcl-x_L degradation is an important component of the cellular rheostat that determines susceptibility to DNA-damaging agents and other death stimuli.

Cellular levels of the pro-survival protein Bcl-x_L are an important determinant of cellular susceptibility to many death stimuli, including most cancer therapies. We previously showed that human Bcl-x_L undergoes deamidation – the conversion of two neutral asparaginyl side-chains into negatively charged aspartyl side-chains – a process that occurs spontaneously but is accelerated by the treatment of tumor cells with DNA-damaging agents. Here, we show that deamidation activates a hitherto undetected signal sequence within Bcl-x_L that targets it for degradation by a pathway involving the proteolytic enzyme calpain. This increased degradation of Bcl-x_L, and the consequent enhanced cellular susceptibility to programmed cell death, may contribute to the ability of DNA-damaging agents to kill tumors. We also demonstrate that deamidation of Bcl-x_L has likely been conserved from the simplest metazoans to humans, underscoring the importance of deamidation in the regulation of Bcl-x_L.

The transcription factor PAX4 acts as a survival gene in INS-1E insulinoma cells

The paired/homeodomain transcription factor Pax4 is essential for islet beta-cell generation during pancreas development and their survival in adulthood. High Pax4 expression was reported in human insulinomas indicating that deregulation of the gene may be associated with tumorigenesis. We report that rat insulinoma INS-1E cells express 25-fold higher Pax4 mRNA levels than rat islets. In contrast to primary beta-cells, activin A but not betacellulin or glucose induced Pax4 mRNA levels indicating dissociation of Pax4 expression from insulinoma cell proliferation. Short hairpin RNA adenoviral constructs targeted to the paired domain or homeodomain (viPax4PD and viPax4HD) were generated. Pax4 mRNA levels were lowered by 73 and 50% in cells expressing either viPax4PD or viPax4HD. Transcript levels of the Pax4 target gene bcl-xl were reduced by 53 and 47%, whereas Pax6 and Pdx1 mRNA levels were unchanged. viPax4PD-infected cells displayed a twofold increase in spontaneous apoptosis and were more susceptible to cytokine-induced cell death. In contrast, proliferation was unaltered. RNA interference-mediated repression of insulin had no adverse effects on either Pax4 or Pdx1 expression as well as on cell replication or apoptosis. These results indicate that Pax4 is redundant for proliferation of insulinoma cells, whereas it is essential for survival through upregulation of the antiapoptotic gene bcl-xl.