Catalytic properties of 26 S and 20 S proteasomes and radiolabeling of MB1, LMP7, and C7 subunits associated with trypsin-like and chymotrypsin-like activities

20 and 26 S proteasomes were isolated from rat liver. The procedure developed for the 26 S proteasome resulted in greatly improved yields compared with previously published methods. A comparison of the kinetic properties of 20 and 26 S proteasomes showed significant differences in the kinetic characteristics with certain substrates and differences in the effects of a protein substrate on peptidase activity. Observed differences in the kinetics of peptidylglutamyl peptide hydrolase activity suggest that the 26 S complex cannot undergo the conformational changes of 20 S proteasomes at high concentrations of the substrate benzyloxycarbonyl (Z) -Leu-Leu-Glu-beta-naphthylamide. Various inhibitors that differentially affect the trypsin-like and chymotrypsin-like activities have been identified. Ala-Ala-Phe-chloromethyl (CH2Cl) inhibits chymotrypsin-like activity assayed with succinyl (Suc) -Leu-Leu-Val-Tyr-AMC, but surprisingly not hydrolysis of Ala-Ala-Phe-7-amido4-methylcoumarin (AMC). Tyr-Gly-Arg-CH2Cl inhibits Suc-Leu-Leu-Val-Tyr-AMC hydrolysis as well as trypsin-like activity measured with t-butoxycarbonyl (Boc) -Leu-Ser-Thr-Arg-AMC, while Z-Phe-Gly-Tyr-diazomethyl (CHN2) was found to inhibit only the two chymotrypsin-like activities. Radiolabeled forms of peptidyl chloromethane and peptidyl diazomethane inhibitors, [3H]acetyl-Ala-Ala-Phe-CH2Cl, [3H]acetyl- and radioiodinated Tyr-Gly-Arg-CH2Cl, and Z-Phe-Gly-Tyr-(125I-CHN2), have been used to identify catalytic components associated with each of the three peptidase activities. In each case, incorporation of the label could be blocked by prior treatment of the proteasomes with known active site-directed inhibitors, calpain inhibitor 1 or 3, 4-dichloroisocoumarin. Subunits of labeled proteasomes were separated either by reverse phase-HPLC and SDS-polyacrylamide gel electrophoresis or by two-dimensional polyacrylamide gel electrophoresis followed by autoradiography/fluorography and immunoblotting with subunit-specific antibodies. In each case, label was found to be incorporated into subunits C7, MB1, and LMP7 but in different relative amounts depending on the inhibitor used, consistent with the observed effects on the different peptidase activities. The results strongly suggest a relationship between trypsin-like activity and chymotrypsin-like activity. They also help to relate the different subunits of the complex to the assayed multicatalytic endopeptidase activities.

Catalytic components of proteasomes and the regulation of proteinase activity

The proteasome (multicatalytic proteinase complex) is a large multimeric complex which is found in the nucleus and cytoplasm of eukaryotic cells. It plays a major role in both ubiquitin-dependent and ubiquitin-independent nonlysosomal pathways of protein degradation. Proteasome subunits are encoded by members of the same gene family and can be divided into two groups based on their similarity to the alpha and beta subunits of the simpler proteasome isolated from Thermoplasma acidophilum. Proteasomes have a cylindrical structure composed of four rings of seven subunits. The 26S form of the proteasome, which is responsible for ubiquitin-dependent proteolysis, contains additional regulatory complexes. Eukaryotic proteasomes have multiple catalytic activities which are catalysed at distinct sites. Since proteasomes are unrelated to other known proteases, there are no clues as to which are the catalytic components from sequence alignments. It has been assumed from studies with yeast mutants that beta-type subunits play a catalytic role. Using a radiolabelled peptidyl chloromethane inhibitor of rat liver proteasomes we have directly identified RC7 as a catalytic component. Interestingly, mutants in Pre1, the yeast homologue of RC7, have already been reported to have defective chymotrypsin-like activity. These results taken together confirm a direct catalytic role for these beta-type subunits. Proteasome activities are sensitive to conformational changes and there are several ways in which proteasome function may be modulated in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell markers in gastrointestinal stromal tumors

Stromal tumors of the gastrointestinal (GI) tract have generated considerable controversy about their direction and level of differentiation, particularly about whether the tumor cells are smooth muscle or Schwann cells. In an attempt to characterize these tumors, the immunohistochemical staining patterns of desmin, vimentin, actin, and S-100 protein were studied in 41 GI stromal tumors, using the avidin-biotin method, and compared with normal host smooth muscle and nerve and with esophageal and uterine leiomyomas. Twenty gastric and one rectal tumor stained diffusely with vimentin and actin, but not with desmin, and had scattered strongly S-100-positive cells that might either be trapped Schwann cells or tumor cells. Twenty small bowel tumors stained similarly to the gastric tumors with regard to vimentin, actin, and desmin, but most (17/20) had a unique, strongly positive geographic staining pattern with S-100. No differences in staining were noted between benign and malignant tumors in either gastric or small bowel sites, and most histologic patterns in tumors from similar locations stained similarly. These results suggest that GI stromal tumors are not truly "leiomyomas and leiomyosarcomas," but relatively undifferentiated tumors, with the expression of various antigens depending on their location in the gut.

The effect of lactation on peak adult shaft and ultra-distal forearm bone mass in women

The effect of long-term vs short-term lactation (three to four children with an average of 10.7 mo vs 2.8 mo duration for each child) on maternal forearm bone-mass sites that are either primarily trabecular or cortical in composition was determined. The habitual calcium intake of each subject exceeded 90% of her recommended dietary allowance (RDA) for nonpregnant, pregnant, and lactating states. Trabecular and cortical bone mass were estimated using single-photon densitometry at ultra-distal and midshift sites on the radius, respectively. Bone mass at both sites was lower in the long-term than in the short-term lactation group and, at the ultra-distal site (trabecular bone), the effect was statistically significant (p less than 0.05). These data indicate that even when women consume the RDA for calcium, long-term lactation can deplete ultra-distal bone mass in the forearm.

Juvenile (cellular) adenofibromas. A clinicopathologic study

"Juvenile" adenofibromas that presented in 25 patients were reviewed. All of the patients were in the second decade of life. The tumors were solitary in 19 patients and multiple and bilateral in six patients. All were distinguished microscopically by prominent cellularity of both epithelium and stroma. Patients who presented with solitary tumors, regardless of size, microscopic pattern, or manner of excision, had no recurrence. In contrast, all patients who presented with multiple tumors developed additional benign masses, often requiring re-excision. We believe that solitary "juvenile adenofibromas," regardless of size, should be excised so as to preserve as much breast tissue as possible. Those patients with multiple, bilateral tumors may anticipate recurrences, but malignant change is not seen. Tumors with this microscopic pattern also may occur, albeit uncommonly, in adults.

Pituitary adenomas. An immunohistochemical study of hormone production and chromogranin localization

Tumors from 42 surgically resected pituitaries and from 13 autopsy cases were studied immunohistochemically with polyclonal antisera to 7 anterior pituitary hormones and with a newly developed monoclonal antibody directed against human chromogranin for evaluation of the distribution of chromogranin in normal and neoplastic pituitaries. In addition, a prospective study was done for assessment of the prevalence, morphology, and endocrine cell types of pituitary tumors in 100 autopsy subjects. When these 55 pituitary adenomas were examined with monoclonal antibody (LK2H10) directed against human chromogranin, selective staining of normal adenohypophyseal cell types and pituitary tumors was observed. Most null-cell adenomas (12/14) were positive for chromogranin, whereas all prolactin (PRL)-producing adenomas (19/19) were negative. Growth hormone (GH) adenomas were focally positive (9/9). All oncocytomas (2/2), 1 thyrotropin (TSH) adenoma, and a follicle-stimulating hormone/luteinizing hormone adenoma were positive for chromogranin. One or more adenomas were present in 14% of the autopsy cases. The tumors occurred most frequently in patients in the fifth through the seventh decades of life. Immunohistochemical staining of 13 adenomas revealed 1 TSH, 1 ACTH, and 4 PRL-producing tumors, whereas 7 other tumors, which were null-cell or undifferentiated adenomas, failed to stain for any of the seven principle pituitary hormones. These results indicate that antibody LK2H10 to human chromogranin is useful in the immunohistochemical characterization of pituitary adenomas. Incidental pituitary microadenomas from autopsy-derived pituitaries most commonly produce PRL, or they belong to the null-cell or undifferentiated tumor group.