Differential alternative splicing of human transglutaminase 4 in benign prostate hyperplasia and prostate cancer

Transglutaminase 4 is a member of enzyme family that catalyzes calcium-dependent posttranslational modification of proteins. Although transglutaminase 4 has been shown to have prostate-restricted expression pattern, little is known about the biological function of transglutaminase 4 in human. To gain insight into its role in prostate, we analyzed the expression status of human transglutaminase 4 in benign prostate hyperplasia (BPH) and prostate cancer (PCa). Unexpectedly, RT-PCR and nucleotide sequence analysis showed four alternative splicing variants of transglutaminase 4: transglutaminase 4-L, -M (-M1 and -M2) and -S. The difference between transglutaminase 4-M1 and -M2 is attributed to splicing sites, but not nucleotide size. The deduced amino acid sequences showed that transglutaminase 4-L, -M1 and -M2 have correct open reading frames, whereas transglutaminase 4-S has a truncated reading frame. RT-PCR analysis of clinical samples revealed that transglutaminase 4-M and -S were detected in all tested prostate tissue (80 BPH and 48 PCa). Interestingly, transglutaminase 4-L was found in 56% of BPH (45 out of 80) and only in 15% of PCa (7 out of 48). However, transglutaminase 4-L expression did not correlate with serum prostate-specific antigen (PSA) level, prostate volumes or PSA densities. These results will provide a clue to future investigation aiming at delineating physiological and pathological roles of human transglutaminase 4.

Transglutaminase 2 inhibits apoptosis induced by calciumoverload through down-regulation of Bax

An abrupt increase of intracellular Ca2+ is observed in cells under hypoxic or oxidatively stressed conditions. The dysregulated increase of cytosolic Ca2+ triggers apoptotic cell death through mitochondrial swelling and activation of Ca2+-dependent enzymes. Transglutaminase 2 (TG2) is a Ca2+-dependent enzyme that catalyzes transamidation reaction producing cross-linked and polyaminated proteins. TG2 activity is known to be involved in the apoptotic process. However, the pro-apoptotic role of TG2 is still controversial. In this study, we investigate the role of TG2 in apoptosis induced by Ca2+-overload. Overexpression of TG2 inhibited the A23187-induced apoptosis through suppression of caspase-3 and -9 activities, cytochrome c release into cytosol, and mitochondria membrane depolarization. Conversely, down-regulation of TG2 caused the increases of cell death, caspase-3 activity and cytochrome c in cytosol in response to Ca2+-overload. Western blot analysis of Bcl-2 family proteins showed that TG2 reduced the expression level of Bax protein. Moreover, overexpression of Bax abrogated the anti-apoptotic effect of TG2, indicating that TG2-mediated suppression of Bax is responsible for inhibiting cell death under Ca2+-overloaded conditions. Our findings revealed a novel anti-apoptotic pathway involving TG2, and suggested the induction of TG2 as a novel strategy for promoting cell survival in diseases such as ischemia and neurodegeneration.

Different inhibition characteristics of intracellular transglutaminase activity by cystamine and cysteamine

The treatment of cystamine, a transglutaminase (TGase) inhibitor, has beneficial effects in several diseases including CAG-expansion disorders and cataract. We compared the inhibition characteristics of cystamine with those of cysteamine, a reduced form of cystamine expected to be present inside cells. Cystamine is a more potent inhibitor for TGase than cysteamine with different kinetics pattern in a non- reducing condition. By contrast, under reducing conditions, the inhibitory effect of cystamine was comparable with that of cysteamine. However, cystamine inhibited intracellular TGase activity more strongly than cysteamine despite of cytoplasmic reducing environment, suggesting that cystamine itself inhibits in situ TGase activity by forming mixed disulfides.