Prostate-specific antigen forms a complex with and cleaves alpha 1-protease inhibitor in vitro

BACKGROUND

Complexes between prostate-specific antigen (PSA) and alpha 1-protease inhibitor (API) occur in serum and they are of potential interest in the diagnosis of prostate cancer. Pure PSA-API complexes are needed for development of specific assays, but complex formation has not earlier been achieved in vitro.

METHODS

PSA was incubated with an excess of API at 37 degrees C. Complexes formed were quantitated by an immunofluorometric assay using antibodies to PSA and API. The products were further characterized by SDS-PAGE, immunoblotting and amino-acid sequencing. PSA-API was purified by gel filtration and immunoaffinity chromatography.

RESULTS

PSA formed an SDS-stable 80-kDa one-to-one complex with API. The rate of formation of PSA-API was slow compared to that of PSA-alpha 2-macroglobulin (A2M) or PSA-alpha 1-antichymotrypsin (ACT), and only about 15% of PSA complexed with a 5-fold molar excess of API at 37 degrees C in 7 days. A major part of API was cleaved between 358-Met and 359-Ser, causing loss of inhibitory activity. PSA-API formed in vitro was purified by gel filtration and immunoaffinity chromatography with anti-PSA antibody. After incubation for 7 days at 37 degrees C, 30-40% of the complex had dissociated causing release of active PSA and proteolytically cleaved inactive API. The dissociation was accelerated in the presence of serum, and released PSA complexed with A2M and ACT.

CONCLUSIONS

PSA forms a complex with API in vitro, but the reaction is slow and part of the API is cleaved. Complex formation is reversible and released PSA is enzymatically active, whereas API is inactivated. Purified PSA-API will facilitate development of quantitative immunoassays for this complex.

Association of the mitochondrial tRNA(A4336G) mutation with Alzheimer's and Parkinson's diseases

Alzheimer's and Parkinson's diseases (AD, PD) are among the most common neurodegenerative disorders in adults. Both AD and PD have a complex aetiology, and it is widely considered that genetic factors, acting independently or in concert with other genetic and/or environmental factors, modify the risk of developing them. While the apolipoprotein E (ApoE) epsilon 4 allele represents an established risk factor for familial and sporadic late-onset AD, it has been suggested that a common polymorphism in the alpha 1-antichymotrypsin gene modifies the ApoE epsilon 4 dosage effect in AD. Moreover, it has been proposed that a mitochondrial tRNA(Gln) sequence variant (A4336G transition) confers an increased risk for both AD and PD. This finding is of particular interest as the A4336G mutation seems to predispose to two clinically and neuropathologically distinct neurodegenerative disorders. We have examined the allelic frequencies of these putative susceptibility genes in 28 neuropathologically confirmed cases of AD, 23 cases with Lewy-body PD and 100 age-matched controls without clinical or histological evidence of neurodegenerative disease. The ApoE epsilon 4 allele frequency was significantly overrepresented in AD patients vs controls (0.35 vs 0.11) but we could not find evidence for an association between the alpha 1-antichymotrypsin AA genotype, the ApoE epsilon 4 allele and AD. In contrast, the mtDNA(A4336G) mutation was present in one of AD cases and in two of 23 PD patients, whereas no mutation was found in 100 age-matched controls (P < 0.05). Our data therefore support the hypothesis that the mitochondrial A4336G mutation represents a risk factor for AD and PD.

Prostate-specific antigen: characterization of epitopes by synthetic peptide mapping and inhibition studies

To improve our understanding of the prostate-specific antigen (PSA) antigenic regions, we studied the association targets of one anti-PSA polyclonal antibody and 10 anti-PSA monoclonal antibodies (mAbs). We also examined the ability of the mAbs to inhibit PSA enzymatic activity and block the association of PSA with alpha 1-antichymotrypsin (ACT). Linear epitope mapping with a polyclonal antibody indicated the presence of six major antigenic regions in PSA. Examination of the panel of mAbs established that three of them bind to linear epitopes. Five of the mAbs inhibited > 90% of PSA enzymatic activity. However, inhibition of PSA enzymatic activity and hindrance of PSA-ACT association by mAbs cannot be used to predict whether the mAbs bind to free PSA, the PSA-ACT complex, or both. Some of the mAbs may block PSA-ACT association through peripheral occlusion of the binding site, or through induction of conformational changes in PSA.

Two-dimensional gel electrophoresis detects prostate-specific antigen-alpha1-antichymotrypsin complex in serum but not in prostatic fluid

We investigated the interaction between prostate-specific antigen (PSA) and 1-antichymotrypsin (ACT) in prostatic secretions, identifying PSA and ACT in human serum, prostatic fluid, and seminal plasma by two-dimensional gel electrophoresis (2-D PAGE). Both PSA and ACT were detected in all three body fluids, but PSA-ACT complex was detected only in serum. Moreover, the 2-D PAGE Western blot staining profile for ACT from serum differed from that for prostatic fluid or seminal plasma. Incubation of prostatic fluid with purified ACT led to formation of PSA-ACT complex. Incubation of prostatic fluid with purified PSA, however, failed to form the complex, suggesting that the ACT in prostatic fluid was inactive or inhibited. Given that physiological concentrations of zinc inhibited the formation of PSA-ACT complex, we consider zinc a possible physiological inhibitor of the formation of the PSA-ACT complex. These results indicate that the failure to detect the PSA-ACT complex in prostatic fluid could be related to the inactivation of ACT, the presence of inhibitors (e.g., zinc), or simply the PSA:ACT ratio in the fluid.

Cytokine concentration in serum of lupus erythematosus patients: the effect on acute phase response

In order to get a better insight into cytokine network regulation in systemic lupus erythematosus (SLE), we analyzed levels of interleukin-10 (IL-10), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma) in the sera from 36 SLE patients. Moreover, C-reactive protein (CRP), alpha-1-acid-glycoprotein (AGP), and alpha-1-antichymotripsin (ACT) serum levels were evaluated. Serum levels of IL-10 and IL-6 were significantly increased when compared with healthy controls. TNF-alpha and IFN-gamma did not differ from normal values. We established the relationship between IL-10 and IL-6 as well as between IL-10 and TNF-alpha. None of the analyzed cytokines correlated with the acute phase protein levels. Based on the obtained data, we conclude that IL-10 may play the superior regulating role in SLE. A lack of correlation between the cytokines and acute phase proteins suggests their independence from cytokine regulation.

Free/total prostate-specific antigen ratio--hope and controversies

Prostate-specific antigen (PSA) forms in serum two stable complexes with alpha 1-antichymotrypsin and alpha 2-macroglobulin. PSA complexed to alpha 1-antichymotrypsin is the predominant fraction of PSA. A minor fraction of serum PSA is not associated with proteinase inhibitors. These molecular differences explain the possibility to distinguish free from total PSA (F/T ratio). Free and complexed PSA have different clearances and significant differences between clearance of free PSA after radical prostatectomy (RP) and after open surgery for benign prostatic hyperplasia (BPH) are observed. These differences are explained by the entire removal of prostatic cells responsible for PSA synthesis and storage during RP, i.e. the source of free PSA present in the intravascular pool. The proportion of free PSA is significantly lower in patients with prostate cancer than in patients with BPH. Thus, the mean F/T ratio in prostate cancer is lower than that in BPH and may be helpful to distinguish cancer from BPH especially in the gray zone of total PSA (4-10 ng/ml). The reason why complexed PSA increases in patients with prostate cancer remains unknown but could be explained by the requirement of an enzymatically active PSA released by the malignant prostate tissue to bind to alpha 1-antichymotrypsin. However, a consensual threshold value for L/T ratio is yet to be found to be of widespread clinical use in the differential diagnosis between cancer and BPH.

Prostate-specific antigen concentrations in serum in acute illnesses

In light of recent studies showing that prostate-specific antigen (PSA) complexes with certain blood proteins, we studied the effects of acute-phase reactants and alpha 2-macroglobulin (A2MG) on serum concentrations of PSA. Serum samples were obtained from 419 men admitted to an acute-care facility. Various acute-phase reactants-including C-reactive protein, alpha 1-acid glycoprotein, alpha 1-antitrypsin, and alpha 1-antichymotrypsin-and A2MG were measured with a Beckman Array analyzer in parallel with determinations of PSA concentrations by two methods, the Hybritech Tandem RIA and the Abbott PSA IMx. Evaluation by Spearman rank correlation revealed a significant negative correlation of A2MG with PSA values (P < 0.01) and (as expected) a positive correlation of age with PSA values (P < 0.001). The former correlation suggests the possibility that patients with high serum concentrations of A2MG may give falsely decreased results for PSA concentrations in serum.

Addition of purified prostate specific antigen to serum from female subjects: studies on the relative inhibition by alpha 2-macroglobulin and alpha 1-antichymotrypsin

PURPOSE

Two forms of prostate specific antigen (PSA), 1 complexed to alpha 1-antichymotrypsin and the other a free PSA, are recognized by current commercially available immunoassays. A third form of PSA complexed to alpha 2-macroglobulin also is present in the serum. To study these 3 different molecular forms of PSA in vivo, we simulated leakage of PSA from the prostate into the serum in vitro.

MATERIALS AND METHODS

Purified seminal fluid PSA was incubated with fresh sera from female subjects at different concentrations. Following gel filtration chromatography, the 3 forms of PSA were studied by immunoassays and Western blot analysis.

RESULTS

Using a commercial immunoassay, 60% of immunoreactivity of seminal fluid PSA was lost after incubation with sera from female subjects. Western blot analysis showed that most of this loss in PSA signal was caused by complexation to alpha 2-macroglobulin. Minimal, if any, complexation to alpha 1-antichymotrypsin occurred even when excess alpha 1-antichymotrypsin was added to the serum.

CONCLUSIONS

Our studies demonstrated that alpha 2-macroglobulin is a much stronger inhibitor to PSA than alpha 1-antichymotrypsin. Further studies of these complexes may be important. They clearly explain why spiking PSA into sera from female subjects to be used as quality controls for PSA assays leads only to the free form of enzymatically inactive PSA in the serum, and not to the dominant form of complexed PSA and alpha 1-antichymotrypsin present in human serum.

Relative efficacies of amyloid beta peptide (A beta) binding proteins in A beta aggregation

The aggregation of amyloid beta peptide (A beta) into its fibrillar, cross beta-pleated configuration is generally viewed as a critical event in the pathophysiology of Alzheimer's disease (AD). A diverse group of molecules, the A beta binding proteins, has been evaluated for their effects on this process. However, most of these studies have used micromolar or greater reagent concentrations, and their different methods have not permitted quantitative comparisons of the efficacy of different A beta binding proteins in augmenting or inhibiting aggregation. In the present work we have undertaken a coherent analysis using fluorimetry of thioflavin T-stained experimental solutions. The complement protein C1q, serum amyloid P, and transthyretin significantly enhanced the formation of precipitable, cross beta-pleated aggregates in solutions of 800 nM A beta 1-42. Under these same experimental conditions, alpha 1-antichymotrypsin had no significant effect on the aggregation process, and both the E3 and E4 isoforms of apolipoprotein E were significant inhibitors. There was a non-significant trend toward the E3 isoform exhibiting greater inhibition than the E4 isoform. Of the aggregation-facilitating molecules, C1q was substantially and significantly the most potent.

Role of the P2 residue in determining the specificity of serpins

The importance of the P2 residue in determining serpin specificity was examined by making a series of substitutions in the P2 position of recombinant alpha 1-antichymotrypsin that contained an arginine P1 residue. The importance of the P2 residue in governing the association rate constant (Kon) of the serpin varied with the protease examined. For trypsin, the P2 residue played a relatively minor role, whereas the nature of this residue markedly influenced the rates of inhibition of thrombin, factor Xa, and APC. A 1000-fold difference in Kon values was observed between the fastest (P2 proline) and the slowest (P2 threonine) inhibitors of thrombin. Similar differences were observed with factor Xa; the best inhibitor (P2 glycine) displayed a 200-fold higher Kon value than the poorest (P2 threonine). The nature of the P2 residue also affected whether the interaction of the serpin with the protease resulted in inhibition of the protease or cleavage of the serpin; a P2 proline residue increased the rate of cleavage of alpha 1-antichymotrypsin by trypsin. By using mutants of thrombin, it was possible to show that the B-insertion loop, which partially occludes the active site, is important in determining the P2 specificity of this enzyme. Deletion of three amino acids from this loop yielded a protease (des-PPW) that became more like trypsin in its specificity. In addition, it was shown that Glu192 dramatically restricts thrombin's ability to accommodate a threonine in the P2 position. Taken together, the results demonstrated the importance of complementary interactions between the P2 residue of the serpin and the S2 binding site of the protease in regulating the specific interaction between serpin and protease.

Synthesis and secretion of active alpha 1-antichymotrypsin by murine primary astrocytes

Activated astrocytes have been identified as the main source of the serine protease inhibitor alpha 1-antichymotrypsin (ACT), an acute phase protein that is tightly associated with amyloid plaques in Alzheimer's disease (AD) and in normal aged human and monkey brain. We analyzed the synthesis of ACT by cultured murine astrocytes in vitro. The murine astrocytes expressed an ACT-like antigen that crossreacted with antibodies to human ACT. The murine ACT-like protein is secreted by the astrocytes and is able to form an SDS-resistant complex with the serine protease cathepsin G, indicating that the secreted ACT is biologically active. We conclude that cultured primary astrocytes synthesize and secrete murine ACT in an active form. We, therefore, suggest that the ACT present within AD plaques is locally derived from plaque-associated activated astrocytes as a part of a glia-mediated local inflammatory response that is associated with the neurodegeneration seen in AD.

The prognostic significance of histomorphometry and immunohistochemistry in giant cell tumors of bone

Eighty-two cases of giant cell tumor (GCT) were reviewed. Hematoxylin-eosin-and hematoxylin, phloxine, saffron, and alcian green-stained sections (82 cases) were examined for mitotic rate, the number of giant cells, and the pleomorphism of the stromal cells. In 29 cases, the tumor was stained for CD68, alpha 1-antichymotrypsin (AIACT), S100 protein, Muramidase, and von Willebrand factor (factor VIII). The staining properties of mononuclear and multinucleated giant cells were compared. Morphometric analysis was performed on 14 cases with a LECO 2001 computer-assisted image analyzer (LECO Instruments Ltd, Mississauga, Ontario, Canada) and included absolute cell count, nuclear area, perimeter, roughness, roundness, and aspect and nuclear versus cytoplasmic ratios, measured both in the stromal cells and giant cells. The cases were divided into four groups: (1) cases with metastasis, (2) cases with recurrence, (3) cases with both metastasis and recurrence, and (4) cases with neither metastasis nor recurrence. Immunohistochemistry revealed a stronger AIACT than muramidase positivity in general. The staining was stronger in stromal cells than in giant cells. Giant cells in all tumors were positive for CD68. Stromal cells showed weaker positivity for the same stain. The number of asymmetrical mitotic figures was significantly greater in group 3 than in group 4 (P < .05). Morphometric assessment has identified a statistically significant difference in the aspect ratio and the roundness of the nuclei between these two groups. The other parameters did not differ significantly. In this article, the significance of these findings in prognostication and the histogenesis of the giant cell tumor are discussed. Their clinical applicability is yet to be determined.

A pilot study of prednisone in Alzheimer's disease

Preliminary to a multicenter trial, an open-label study was conducted of prednisone treatment in Alzheimer's disease. Prednisone was given at an initial dose of 10 mg (part 1) or 20 mg (part 2) and tapered over 7 weeks. There were no serious adverse events attributed to the medication, and there were no significant changes in either mean cognitive or behavioral assessment scores with treatment during either part. Serum levels of the acute phase proteins alpha-1-antichymotrypsin and C-reactive protein did not change significantly during part 1, but were suppressed by the higher dose given in part 2. Thus, a prednisone regimen with an initial dose of 20 mg is tolerable and results in suppression of the acute phase response in Alzheimer's disease.

Matrix metalloproteinases in the neocortex and spinal cord of amyotrophic lateral sclerosis patients

Matrix metalloproteinases (MMPs) were analyzed by immunohistochemistry and zymography in amyotrophic lateral sclerosis (ALS) and control brain and spinal cord specimens. Three major bands of enzyme activity (70, 100, and 130 kDa) were consistently observed and were subsequently identified as MMP-2 (70 kDa; also known as EC 3.4.24.24 or gelatinase A) and MMP-9 (100 and 130 kDa; also known as EC 3.4.24.35 or gelatinase B). Immunohistochemical studies established the presence of MMP-2 in astrocytes and MMP-9 in pyramidal neurons in the motor cortex and motor neurons in the spinal cord of ALS patients. Although a significant decrease in MMP-2 activity was noticed in the ALS motor cortex, statistically significant increases in MMP-9 (100-kDa) activity were observed in ALS frontal and occipital cortices (BA10 and 17) and all three spinal cord regions when compared with control specimens. The highest MMP-9 (100-kDa) activities in ALS were found in the motor cortex and thoracic and lumbar cord specimens. The abnormally high amount of MMP-9 and its possible release at the synapse may destroy the structural integrity of the surrounding matrix, thereby contributing to the pathogenesis of ALS.

Distortion of the active site of chymotrypsin complexed with a serpin

There is no complete understanding of how serine protease inhibitors of the serpin family inhibit their target enzymes. Structural and biochemical studies have suggested that serpins utilize a mechanism that is distinct from the standard mechanism of inhibition proposed for most small protein protease inhibitors. Proton nuclear magnetic resonance spectroscopy was used in the present study to demonstrate a fundamental difference in the atomic environment of the catalytic triad of enzyme in complex with serpins when compared to uncomplexed enzyme and enzyme in complex with standard mechanism inhibitors. This work demonstrates that the active site of chymotrypsin is distorted when complexed to a serpin and makes tenable a mechanism of inhibition in which the serpin induces a conformational change in the enzyme that dramatically reduces or completely abrogates the catalytic activity of the protease.

Diagnostic value of molecular forms of prostate-specific antigen for female breast cancer

OBJECTIVES

To examine the diagnostic value of prostate-specific antigen (PSA) subfractions in the serum of female breast cancer patients.

DESIGN AND METHODS

PSA subfractions (free PSA, F-PSA; PSA bound to alpha 1-antichymotrypsin, PSA-ACT) were determined in the serum of patients with breast cancer and in the serum of healthy women. Serum was injected into a high-performance liquid chromatography column and all fractions were analyzed for PSA using a highly sensitive PSA immunofluorometric assay. We studied 3 normal male sera, 3 sera from prostate cancer patients who underwent radical prostatectomy (all for comparative purposes), 3 sera from healthy women, 3 sera from women with breast cancer obtained presurgically, and 7 sera from women with breast cancer, postsurgically.

RESULTS

All male sera contained mostly PSA-ACT complexes and very little free PSA. Sera from all healthy women also contained mostly PSA-ACT complexes and nondetectable or traces of free PSA. All 3 presurgical sera from patients with breast cancer contained predominantly free PSA. Patients who had surgical resection of the breast tumor and were in remission had postsurgical serum PSA subfractions similar to those of healthy women (i.e., mostly PSA-ACT complexes).

CONCLUSION

The serum PSA subfractions of breast cancer patients are substantially different from serum PSA subfractions of male patients, healthy females, and females who have apparently been treated successfully for breast cancer. These findings may form the basis for a serological diagnostic test for breast cancer.

Characterisation of the tumour-associated carbohydrate epitope recognised by monoclonal antibody 4D3

The tumour-associated epitope recognised by monoclonal antibody (MAb) 4D3 is expressed on a high m.w. mucin glycoprotein preparation known as small intestinal mucin antigen (SIMA). This epitope is detected in tissue from a high proportion of patients with colorectal cancer, and elevated levels occur in serum from a significant number of such patients, highlighting the potential clinical utility of MAb 4D3. In the present study, insight into the composition and structure of the carbohydrate epitope recognised by MAb 4D3 was gained following characterisation of 2 glycopeptides that co-purified with SIMA. Sequence analysis of 1 of these glycopeptides revealed that it was identical to the glycoprotein alpha-1-anti-chymotrypsin. This glycoprotein was subsequently deglycosylated to yield 5 forms corresponding to alpha-1-anti-chymotrypsin substituted with 4, 3, 2, 1 or no branched glycans. MAb 4D3 was reactive with each of the glycosylated forms, including the form carrying only 1 branched glycan, but did not react with fully deglycosylated alpha-1-anti-chymotrypsin. MAb 4D3 also reacted to different extents with ovine, bovine or porcine submaxillary mucins, each of which has a different amount of the O-linked sialylated disaccharide known as sialosyl Tn. Of these mucins, MAb 4D3 was most reactive with ovine submaxillary mucin, in which almost all of the carbohydrate chains are sialosyl Tn. Reactivity of MAb 4D3 towards isolated glycans, sialosyl Tn and related structures led to the conclusion that the preferred MAb 4D3 epitope involves the sialylated N-acetyl galactosamine disaccharide as well as an additional monosaccharide present on a neighbouring carbohydrate chain. Although the preferred epitope recognised by MAb 4D3 involves this sialylated disaccharide, the specificity of MAb 4D3 was different from that of other MAbs with a reported specificity for sialosyl Tn.

Structural investigation of the alpha-1-antichymotrypsin: prostate-specific antigen complex by comparative model building

Prostate-specific antigen (PSA), produced by prostate cells, provides an excellent serum marker for prostate cancer. It belongs to the human kallikrein family of enzymes, a second prostate-derived member of which is human glandular kallikrein-1 (hK2). Active PSA and hK2 are both 237-residue kallikrein-like proteases, based on sequence homology. An hK2 model structure based on the serine protease fold is presented and compared to PSA and six other serine proteases in order to analyze in depth the role of the surface-accessible loops surrounding the active site. The results show that PSA and hK2 share extensive structural similarity and that most amino acid replacements are centered on the loops surrounding the active site. Furthermore, the electrostatic potential surfaces are very similar for PSA and hK2. PSA interacts with at least two serine protease inhibitors (serpins): alpha-1-antichymotrypsin (ACT) and protein C inhibitor (PCI). Three-dimensional model structures of the uncleaved ACT molecule were developed based upon the recent X-ray structure of uncleaved antithrombin. The serpin was docked both to PSA and hK2. Amino acid replacements and electrostatic complementarities indicate that the overall orientation of the proteins in these complexes is reasonable. In order to investigate PSA's heparin interaction sites, electrostatic computations were carried out on PSA, hK2, protein C, ACT, and PCI. Two heparin binding sites are suggested on the PSA surface and could explain the enhanced complex formation between PSA and PCI, while inhibiting the formation of the ACT-PSA complex, PSA, hK2, and their preliminary complexes with ACT should facilitate the understanding and prediction of structural and functional properties for these important proteins also with respect to prostate diseases.

[Glycoforms of alpha-1 antichymotrypsin in infectious process. Diagnostic value and follow-up]

OBJECTIVES

Three alpha 1-antichymotrypsin (alpha 1-ACT) glycans have been identified. We followed their levels during the septic processes in 24 patients in order to determine their diagnostic value in correlation with C-reactive peptide in patients with infection.

METHODS

Sera were collected for assay on days 0, 3 and 6 after starting antibiotics. Erythrocyte sedimentation rate, alpha 1-ACT and C-reactive protein were determined.

RESULTS

Erythrocyte sedimentation rate changed little while C-reactive protein fell sharply as in other inflammatory processes. Crossed immunoaffinoelectrophoresis showed a decreased formation of the Con-A non reactive fraction which disappeared rapidly after initiating antibiotics. Glycan microheterogeneity returned to normal at 6 days while C-reactive protein and alpha 1-ACT were still elevated.

CONCLUSION

Synthesis and glycosylation of alpha 1-ACT are independent. Study of alpha 1-ACT glycan microheterogeneity may provide a useful test in the diagnosis of inflammatory processes of unknown origin and may be helpful in following-up patients with sepsis.

The defective secretion of a naturally occurring alpha-1-antichymotrypsin variant with a frameshift mutation

A newly found variant alpha-1-antichymotrypsin (ACT), ACT Isehara-2, has a deletion of two bases (AA) at codon 391 near the carboxyl terminus. This frameshift mutation caused a change in the amino acid sequence and generated 10 extra amino acids (408 amino acids total) [Tsuda, M., Sei, Y., Matsumoto, M., Kamiguchi, H., Yamamoto, Y., Shinohara, Y., Igarashi, T. & Yamamura, M. (1992) Hum. Genet. 91. 467-468]. The serum ACT levels in three unrelated heterozygotes with this mutant ACT gene were 37% 49% and 54% that of the normal individuals. To examine the reduced serum levels, the normal ACT and the mutant ACT created by site-directed mutagenesis were transfected into COS-7 cells for comparison. The value for the retention rate (intracellular ACT/total ACT) was apparently higher in the cells expressing mutant ACT Isehara-2 than those bearing the normal gene. In the pulse-chase experiments, the secretion of the synthesized mutant ACT into the medium was not observed, whereas the normal ACT was mostly secreted as a 64-kDa form. The endoglycosidase H digestion and an electron microscopic analysis indicated that the retained mutant ACT was present in the endoplasmic reticulum. These results provide the biochemical basis for the decreased serum ACT level of individuals with ACT Isehara-2, and suggest the importance of the carboxyl-terminal region for its secretion.

Degradation of nuclear matrix and DNA cleavage in apoptotic thymocytes

In dexamethasone-treated thymocyte cultures an increase in nuclear proteolytic activity paralleled chromatin fragmentation and the appearance of small apoptotic cells. The elevation of nuclear proteolytic activity was accompanied by site-specific degradation of nuclear mitotic apparatus protein and lamin B, two essential components of the nuclear matrix. Nuclear mitotic apparatus protein phosphorylation and cleavage into 200 and 48 kDa fragments occurred within 30 minutes of dexamethasone treatment. Cleavage of lamin B, which generated a fragment of 46 kDa consistent with the central rod domain of the protein, was also detected after 30 minutes of exposure to the steroid hormone. The level of lamin B phosphorylation did not change as a result of the dexamethasone treatment and the lamina did not solubilize until the later stages of apoptosis. Initial DNA breaks, detected by the terminal transferase-mediated dUTP-biotin nick end labeling assay, occurred throughout the nuclei and solubilization of lamina was not required for this process to commence. The data presented in this paper support a model of apoptotic nuclear destruction brought about by the site-specific proteolysis of key structural proteins. Both the nuclear mitotic apparatus protein and lamin B were specifically targeted by protease(s) at early stages of the cell death pathway, which possibly initiate the cascade of degradative events in apoptosis.