X-irradiation of the contusion site improves locomotor and histological outcomes in spinal cord-injured rats

We have determined whether X-irradiation of the injury site can oppose tissue loss and improve recovery of locomotor function following contusion injury of the spinal cord. Contusion injury was produced in rats at the level of T10 with a weight drop device. Localized X-irradiation (20 Gy) of the injury site was performed at 20 min and 1, 2, 4, 7, and 17 days postinjury. Locomotor recovery was then determined with the 21-point Basso, Beattie, and Bresnahan (BBB) scale. X-irradiation enhanced recovery of locomotor function during a subsequent 6-week observation period when administered 20 min and 1 or 2 days following contusion injury (final BBB score approximately 7-8). X-irradiation at 4-17 days postinjury did not significantly affect final locomotor scores compared with unirradiated rats (final BBB score approximately 2), in marked contrast to previous studies where X-irradiation applied only at 17-18 days benefitted transection injury. The extent of recovery was directly related to measurements of sparing of spinal cord tissue at the contusion center. Because the treatment time window occurred earlier in contusion than reported for transection injury, the results suggest that contusion injury rapidly initiates underlying radiation-sensitive processes that occur only following a delay of several weeks after transection injury. Further optimization of X-ray treatment may lead to a useful therapeutic modality for use in spinal cord contusion injury.

Clenbuterol reduces degeneration of exercised or aged dystrophic (mdx) muscle

Evidence of dystrophic muscle degeneration in the hind limb muscles of young (20-week-old) treadmill-exercised or aged (87-week-old) sedentary mdx mice was greatly reduced by treatment with clenbuterol, a beta(2)-adrenoceptor agonist. Daily treadmill exercise for 10 weeks increased the size of regions within the mdx plantaris but not the soleus or gastrocnemius muscles, in which necrotic muscle fibers or the absence of fibers was observed. Clenbuterol reduced the size of these abnormal regions from 21% of total muscle cross-sectional area to levels (4%) found in sedentary mdx mice. In addition, the muscles obtained from aged clenbuterol-treated mdx or wild-type mice did not display the extensive fibrosis or fiber loss observed in untreated mdx mice. These observations are consistent with a mechanism of dystrophic muscle degeneration caused by work load-induced injury that is cumulative with aging and is opposed by beta(2)-adrenoceptor activation. Optimization of beta(2)-agonist treatment of muscular dystrophy in mdx mice may lead to a useful therapeutic modality for human forms of the disease.

Cellular physiology of STAT3: Where's the cytoplasmic monomer?

In the standard model of cytokine-induced signal transducer and activator of transcription (STAT) protein family signaling to the cell nucleus, it is assumed that STAT3 is recruited to the cytoplasmic side of the cell surface receptor complex from within a cytosolic monomer pool. By using Superose-6 gel-filtration chromatography, we have discovered that there is little monomeric STAT3 (91 kDa) in the cytosol of liver cells (human hepatoma Hep3B cell line and rat liver). The bulk of STAT3 (and STAT1, STAT5a, and -b) was present in the cytosol as high molecular mass complexes in two broad distributions in the size range 200-400 kDa ("statosome I") and 1-2 MDa ("statosome II"). Upon treatment of Hep3B cells with interleukin-6 (IL-6) for 30 min (i) cytosolic tyrosine-phosphorylated STAT3 was found to be in complexes of size ranging from 200-400 kDa to 1-2 MDa; (ii) a small pool of monomeric STAT3 and tyrosine-phosphorylated STAT3 eluting at 80-100 kDa was observed, and (iii) most of the cytoplasmic DNA-binding competent STAT3 (the so-called SIF-A "homodimer") co-eluted with catalase at 230 kDa. In order to identify the protein components of the 200-400-kDa statosome I cytosolic complexes, we used the novel technique of antibody-subtracted differential protein display using anti-STAT3 antibody. Eight polypeptides in the size range from 20 to 114 kDa co-shifted with STAT3; three of these (p60, p20a, and p20b) were co-shifted in an IL-6-dependent manner. In-gel tryptic fragmentation and mass spectroscopy identified the major IL-6-dependent STAT3-co-shifted p60 protein as the chaperone GRP58/ER-60/ERp57. Taken together, these data (i) emphasize the absence of a detectable STAT3 monomer pool in the cytosol of cytokine-free liver cells as posited by the standard model, and (ii) suggest an alternative model for STAT signaling in which STAT3 proteins function in the cytoplasm as heteromeric complexes with accessory scaffolding proteins, including the chaperone GRP58.

Clenbuterol, a beta(2)-adrenoceptor agonist, improves locomotor and histological outcomes after spinal cord contusion in rats

An important goal of rehabilitation following spinal cord injury is recovery of locomotor function and muscular strength. In the present studies, we determined whether the beta(2)-agonist, clenbuterol, can improve recovery of locomotor function following spinal cord injury. A model of spinal cord injury was examined in which four graded levels of contusion injury were produced in rats at the level of T10 with a weight-drop device. Locomotor recovery was determined with the Basso, Beattie, and Bresnahan (BBB) scale, which distinguishes between 22 progressive levels of recovery. As observed previously, recovery during the 6 weeks following injury was inversely related to the severity of injury. However, clenbuterol caused substantial enhancement of recovery of locomotor function at the two most severe levels of injury (BBB scores 10-12 vs 2-4). In addition, the extent of recovery was directly related to sparing of spinal cord tissue at the contusion center in both untreated and clenbuterol-treated spinal cords. Optimization of beta(2)-agonist treatment may lead to a useful therapeutic modality for treatment of spinal cord contusion injury.

Regulation of IL-6 signaling by p53: STAT3- and STAT5-masking in p53-Val135-containing human hepatoma Hep3B cell lines

The influence of p53 on cytokine-triggered Janus kinase-STAT signaling was investigated in human hepatoma Hep3B cell lines engineered to constitutively express the temperature-sensitive Val135 mutant of p53. In comparison to the parental p53-free Hep3B cells, these p53-Val135-containing Hep3B cell lines displayed a reduced response to IL-6 at the wild-type-like p53 temperature (32.5 degrees C). In these cells, IL-6 induced a marked reduction in the immunologic accessibility of cytoplasmic and nuclear STAT3 and STAT5 within 20 to 30 min that lasted 2 to 4 h (STAT-masking) provided that the cells had been previously cultured at 32.5 degrees C for at least 18 to 20 h. The onset of IL-6-induced STAT-masking required protein tyrosine kinase, protein tyrosine phosphatase, proteasomal, phospholipase C, and mitogen-activated protein kinase kinase 1 activities. The maintenance of IL-6-induced STAT-masking was dependent on continued signaling through the phosphatidylinositol-dependent phospholipase C pathway. Despite a reduction in IL-6-induced STAT3 DNA binding activity in the nuclear compartment during STAT-masking, there was increased and prolonged accumulation of tyrosine-phosphorylated STAT3 in both the cytoplasmic and nuclear compartments, indicating that the capacity of tyrosine-phosphorylated STAT3 to bind DNA was reduced during STAT-masking. Thus, IL-6-induced STAT-masking, as dramatically evident on immunomicroscopy, is a visible consequence of a novel cellular process by which a p53-Val135-induced gene product(s) regulates the association of masking protein(s) with and the DNA-binding capacity of STAT3.

Clenbuterol, a beta2-adrenoceptor agonist, reduces scoliosis due to partial transection of rat spinal cord

Injury to the spinal cord often results in abnormal lateral curvature of the spine, or scoliosis, that is associated with neuromuscular weakness. The lateral curvature of the spine is thought to be a consequence of insufficient or asymmetrical loading of the vertebrae. To study neuromuscular scoliosis, an animal model of spinal cord injury was used in which the spinal cord was partially (3/4) transected, with the left lateral columns left intact. Partial transection of the spinal cord in the rat caused scoliosis that was maximal four to five vertebrae distal to the lesion site. As in previous experiments involving unilateral spinal cord lesions, the scoliotic curves were convex on the weakened side. Subtotal transection at T5 or T11 resulted in lateral displacement of vertebrae T9-T12 or L2-L5, respectively, of up to 11 mm. Interestingly, this vertebral displacement is greatly reduced by clenbuterol, a beta2-adrenoceptor agonist that has been found to retard loss of muscle contractility and bone mineralization due to denervation. Together these results suggest that stimulation of beta2-receptors opposes vertebral unloading due to neuromuscular weakness and thereby acts as a countermeasure to scoliosis.

Proteasome- and p53-dependent masking of signal transducer and activator of transcription (STAT) factors

Hepatoma Hep3B cell lines stably expressing a temperature-sensitive p53 species (p53-Val-135) displayed a reduced response to interleukin-6 (IL-6) when cultured at the wild-type (wt) p53 temperature (Wang, L., Rayanade, R., Garcia, D., Patel, K., Pan, H., and Sehgal, P. B. (1995) J. Biol. Chem. 270, 23159-23165). We now report that in such cultures IL-6 caused a rapid (20-30 min) and marked loss of cellular immunostaining for STAT3 and STAT5, but not for STAT1. The loss of STAT3 and STAT5 immunostaining was transient (lasted 120 min) and tyrosine kinase-dependent, and even though the loss was blocked by the proteasome inhibitors MG132 and lactacystin it was not accompanied by changes in cellular levels of STAT3 and STAT5 proteins suggesting that IL-6 triggered a rapid masking but not degradation of these transcription factors. STAT3 and STAT5 masking was accompanied by a reduction in IL-6-induced nuclear DNA-binding activity. The data suggest that p53 may influence Jak-STAT signaling through a novel indirect mechanism involving a wt p53-dependent gene product which upon cytokine addition is activated into a "STAT-masking factor" in a proteasome-dependent step.

Phosphorylation of the proteasome activator PA28 is required for proteasome activation

PA28, also referred to as 11S regulator, is a potent activator of the peptidase activities of the proteasome (multicatalytic proteinase complex). Although the role(s) of PA28-20S proteasome complexes in cellular proteolytic processes remain to be defined, these particles have been implicated in antigen processing of major histocompatibility complex (MHC) class I molecules. Our results demonstrate that PA28 is phosphorylated as evidenced by 32P incorporation into a single PA28 species in rabbit reticulocytes. In reticulocytes as well as human erythrocytes, PA28 is normally found in a phosphorylated state as detected by phosphoserine antibody. In human erythrocytes, this antibody recognizes three polypeptides which are also detected by antibody to PA28 on Western blot analysis. Dephosphorylation with alkaline phosphatase treatment completely abolishes the ability of PA28 to activate hydrolysis of Suc-Leu-Leu-Val-Tyr by proteasomes. After exposure to phosphatase, the three polypeptides are no longer recognized by phosphoserine antibody, although binding to PA28 antibody is unaffected. These results suggest that phosphorylation may function in transduction of cytokine and growth factor signals that, in turn, modulate antigen presentation and other processes which involve PA28-20S proteasome complexes.

Fructated protein is more resistant to ATP-dependent proteolysis than glucated protein possibly as a result of higher content of Maillard fluorophores

Glycation by fructose (fructation) renders bovine serum albumin more refractory to degradation by an ATP-dependent proteolytic system from reticulocytes than glycation by glucose (glucation). It appears that the decrease in the protein's susceptibility to degradation is a complex effect of the various protein-bound moities that are generated at the different stages of the Maillard reaction and not only the result of primary amino group blockage. Advanced Maillard reaction fluorescent components may induce a decrease in proteolysis, whereas the intermediate Amadori groups possibly may enhance degradation. However, the inhibitory effect on degradation of the fluorophores would predominate at higher levels of glycation. Resistance of intracellular fructated proteins to ATP-dependent degradation may lead to alterations in the function of cells with an active sorbitol pathway and, in this way, underlie the complications of diabetes.

Clenbuterol, a beta 2-agonist, retards wasting and loss of contractility in irradiated dystrophic mdx muscle

Treatment with the adrenergic beta 2-receptor agonist clenbuterol prevented, in dystrophic muscle from mdx mice, a pronounced loss of contractile strength that is observed after blockade of muscle regeneration with gamma irradiation. In addition, muscle mass and myosin content were greater (62-109%) in irradiated hindlimbs from clenbuterol-treated mdx mice, whereas the effects of the beta 2-agonist were relatively smaller (12-21%) in the nonirradiated hindlimbs. Together, these results suggest that beta 2-agonists can antagonize degenerative processes occurring in muscle fibers lacking dystrophin.

240-kDa proteasome inhibitor (CF-2) is identical to delta-aminolevulinic acid dehydratase

The 240-kDa proteasome inhibitor has been reported to be an ATP-stabilized component (CF-2) of the 26 S proteasome complex. We now report that this inhibitory factor is indistinguishable from delta-aminolevulinic acid dehydratase (ALAD), the second enzyme in the pathway of heme synthesis, based upon the following observations: 1) common sequence of the first 14 N-terminal amino acids; 2) identical migration on native and SDS-polyacrylamide gel electrophoresis; 3) identical isoelectric points of pH 7.1; 4) cross-reactivity of specific polyclonal antibodies; 5) similar dehydratase and proteasome inhibitor specific activities in both proteins; and 6) the presence of both activities in recombinant ALAD. The dual role of this protein as CF-2 in the ATP/ubiquitin-dependent pathway and in heme synthesis may be an example of "gene sharing" and explains the unexpected abundance of ALAD noted in earlier studies.

Copurification of casein kinase II with 20 S proteasomes and phosphorylation of a 30-kDa proteasome subunit

The 20 S proteasome is a multicatalytic protease that has been implicated in several processes including ATP/ubiquitin-dependent proteolysis. However, the ATP requirement(s) related to proteasome function is undefined. We demonstrate that a protein kinase activity copurifies through multiple steps utilized to isolate latent 20 S proteasomes from human erythrocytes. The kinase phosphorylates serine residues within a single 30-kDa proteasome subunit. The activity is not sensitive to cyclic AMP or protein kinase inhibitor, indicating that it is not a cyclic AMP-dependent kinase. It is sensitive to nanomolar levels of heparin and is able to utilize both ATP and GTP as phosphodonors, similar to casein kinase II activity. Moreover, a polyclonal antibody specific for casein kinase II recognizes the alpha' subunit of casein kinase II in the 20 S preparation and specifically immunoprecipitates the proteasome-phosphorylating activity. These characteristics suggest that the proteasome kinase is similar or identical to casein kinase II. It is suggested that phosphorylation of the 30-kDa proteasome subunit by casein kinase II may be involved in regulating the activity and/or assembly of proteasome complexes.

Phosphorylation and ubiquitination of the 26S proteasome complex

This article reviews recent studies from our laboratory on protein regulators of the proteasome (multicatalytic proteasome complex) in red blood cells. A 240-kD inhibitory component (CF-2) exists in 26S proteasome complexes in a form which is conjugated to ubiquitin. Interestingly, this factor was shown to be identical to delta-aminolevulinic acid dehydratase (ALAD), involved in heme synthesis. A distinct 200-kD inhibitor of the proteasome is not present in the 26S complex. A 32-kD subunit of the 20S proteasome appears to be important for the latency of this core protease. Multiple isoelectric variants of the 32-kD subunit are consistent with phosphorylation. Another 20S proteasome subunit of 30 kD molecular weight is phosphorylated at specific serine residues by copurifying casein kinase II. It is suggested that ubiquitination and phosphorylation may account for at least part of the ATP dependency associated with the 26S proteasome complex. These modifications may play a role in the activity, assembly, translocation and/or turnover of this particle.

Ubiquitinated proteasome inhibitor is a component of the 26 S proteasome complex

Western blot analysis, using a polyclonal antibody to the 240-kDa endogenous inhibitor of the 20 S proteasome, revealed that the inhibitor is a component of the 26 S complex. Although isolated inhibitor displayed a single 40-kDa band on SDS-PAGE, the antibody detected a 55-kDa component in the 26 S proteasome complex. Ubiquitin polyclonal antibody recognized the same 55-kDa component but did not react with free 40-kDa inhibitor subunit. Addition of purified 40-kDa inhibitor to a ubiquitin ligating system also generated the 55-kDa species. In crude erythrocyte extracts, most of the inhibitor migrated at 55 kDa in the presence of ATP but shifted to 40 kDa in the absence of ATP, consistent with removal of ubiquitin. It is suggested that ubiquitination of the inhibitor may be involved in regulating assembly and/or activity of the 26 S proteasome complex.

A monoclonal antibody that distinguishes latent and active forms of the proteasome (multicatalytic proteinase complex)

Monoclonal antibodies (mAbs) were generated to proteasome purified from human erythrocytes. Five of six proteasome-specific mAbs reacted with three subunits in the molecular mass range of 25-28 kDa, indicating a common epitope. The other mAb (AP5C10) exhibited a more restricted reactivity, recognizing a 32-kDa subunit of the proteasome purified in its latent state. However, when the proteasome is isolated in its active state, AP5C10 reacts with a 28-kDa subunit, evidence for processing of the proteasome subunits during purification. Purified proteasome preparations which exhibited partial latency have both AP5C10 reactive subunits. Although the 32-kDa subunit appears required for latency, loss of this component and generation of the 28-kDa component are not obligatory for activation. The 32- and 28-kDa subunits can each be further resolved into three components by isoelectric focusing. The apparent loss of 4 kDa during the conversion of the 32- to 28-kDa subunit is accompanied by a shift to a more basic pI for each polypeptide. Western blots of the early steps of proteasome purification reveal an AP5C10-reactive protein at 41 kDa. This protein was separated from proteasomes by sizing chromatography and may represent a pool of precursor subunits. Since the 32-kDa subunit appears necessary for latency, it is speculated to play a regulatory role in ATP-dependent proteolytic activity.

Isolation and characterization of a novel endogenous inhibitor of the proteasome

A novel endogenous inhibitor of the proteasome (high molecular weight multicatalytic protease) has been isolated and characterized from human erythrocytes. After purification by ion-exchange and sizing chromatography, the inhibitor displayed a native molecular mass of approximately 200 kDa and contained a single subunit of 50 kDa with an isoelectric point of 6.9. Although the inhibitor noncompetitively blocks proteolysis of [methyl-14C]-alpha-casein (Ki = 7.1 x 10(-8) M) and inhibits hydrolysis of Suc-Leu-Leu-Val-Tyr-AMC, it did not affect hydrolysis of other peptide substrates, such as MeOSuc-Phe-Leu-Phe-MNA and Z-Ala-Arg-Arg-MNA. To further characterize the 50-kDa inhibitor, a monoclonal antibody (MI-8) was generated that showed specific binding upon Western blot analysis of both native PAGE and SDS-PAGE. Immunoprecipitation with MI-8 specifically removed inhibitor activity against the proteasome. The 50-kDa inhibitor is distinct from a previously described 40-kDa inhibitor of the proteasome (Murakami, K., & Etlinger, J.D. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 7588-7592) on the basis of lack of cross-reactivity with MI-8 and dissimilar peptide digest patterns. It is suggested that these endogenous inhibitors may have a role in ATP/ubiquitin-dependent proteolysis and/or other cellular functions involving this protease.

Clenbuterol, a beta 2-receptor agonist, reduces net bone loss in denervated hindlimbs

Clenbuterol treatment for several weeks prevented up to one-third of the reduction in mineralization of femurs and tibias caused by sectioning of the sciatic nerve in young rats. The normalizing effect of clenbuterol on bone mineral content was directly proportional to similar alterations in muscle mass, which in turn could be abolished by ablation of the triceps surae or hindlimb unweighting and reduced by hypophysectomy. In contrast to the effects of inactivity, ovariectomy caused small reductions (2-4%) in bone density that were not affected by clenbuterol and were not accompanied by changes in ash weight. Together, our results suggest that the ability of beta 2-agonists to retard the loss in net muscle mass and enhance contractile tension can oppose net bone loss caused by denervation. Increases in contractile tension caused by beta 2-agonists may enhance the utility of exercise or electrical stimulation as countermeasures for the effects of scoliosis, prolonged bed rest, spinal cord injury, or weightlessness in space on bone mass.

Slow to fast alterations in skeletal muscle fibers caused by clenbuterol, a beta 2-receptor agonist

Chronic treatment of rats with clenbuterol, a beta 2-receptor agonist (8-12 wk), caused hypertrophy of histochemically identified fast- but not slow-twitch fibers within the soleus, while the mean areas of both fiber types were increased in the extensor digitorum longus (EDL). In contrast, treatment with the beta 2-receptor antagonist, butoxamine, reduced fast-twitch fiber size in both muscles. In the solei and to a lesser extent in the EDLs, the ratio of the number of fast- to slow-twitch fibers was increased by clenbuterol, while the opposite was observed with butoxamine. The muscle fiber hypertrophy observed in the EDL was accompanied by parallel increases in maximal tetanic tension and muscle cross-sectional area, while in the solei, progressive increases in rates of force development and relaxation toward values typical of fast-twitch muscles were also observed. Our results suggest a role of beta 2-receptors in regulating muscle fiber type composition as well as growth.

Degradation of proteins with blocked amino groups by cytoplasmic proteases

The effect of blocking amino groups on the susceptibility of BSA and calmodulin to high molecular weight protease (HMP) and calpain, the two major cytosolic proteases, was studied. Both proteases hydrolyzed methylated vs. unmodified BSA more slowly. Methylation of BSA resulted in the accumulation of proteolytic intermediates, especially of larger sizes. However, similar fragments were generated from unmodified BSA indicating that rates of hydrolysis rather that sites of proteolytic cleavage were altered. Calmodulin from Dictyostelium discoideum was hydrolyzed rapidly by HMP whereas brain and muscle calmodulins which have a epsilon-N-trimethyl residue on the single surface lysine were relatively stable.

Clenbuterol, a beta 2-agonist, retards atrophy in denervated muscles

Denervated soleus, anterior tibialis, and gastrocnemius muscles, but not the extensor digitorum longus, contained 95-110% more protein after 2-3 wk of treatment with the adrenergic beta 2-receptor agonist, clenbuterol, than denervated controls. In addition, the twofold difference in the protein content of denervated solei was paralleled by similar changes in contractile strength and muscle fiber cross-sectional area. In contrast, when the innervated contralateral muscles were examined, the extensor digitorum longus and anterior tibialis showed relatively small increases in protein of 32 and 19%, respectively, whereas the soleus and gastrocnemius were unaffected. The magnitude of the effects of clenbuterol in sparing the mass and functional capacity of denervated muscle suggests that this agent may be important in studies of neuromuscular diseases and disuse atrophy.

Regulation of Ca2+-dependent protein turnover in skeletal muscle by thyroxine

Dantrolene, an agent that inhibits Ca2+ mobilization, improved protein balance in skeletal muscle, as thyroid status was increased, by altering rates of protein synthesis and degradation. Thyroxine (T4) caused increases in protein degradation that were blocked by leupeptin, a proteinase inhibitor previously shown to inhibit Ca2+-dependent non-lysosomal proteolysis in these muscles. In addition, T4 abolished sensitivity to the lysosomotropic agent methylamine and the autophagy inhibitor 3-methyladenine, suggesting that T4 inhibits autophagic/lysosomal proteolysis.

Endogenous inhibitor of nonlysosomal high molecular weight protease and calcium-dependent protease

An endogenous inhibitor of high molecular weight protease was purified from human erythrocytes and partially characterized. The inhibitor was isolated by DEAE-Sephacel ion-exchange chromatography followed by separation on a Bio-Gel A-0.5m column. The inhibitor displayed a native Mr of 240,000 and contained a single subunit of Mr 40,000 after NaDodSO4/polyacrylamide gel electrophoresis. The Mr 240,000 hexamer inhibited high molecular weight protease noncompetitively (Ki = 8.3 X 10(-8) M) and showed marked susceptibility to proteolytic digestion and heat treatment. The purified factor was also a potent inhibitor of calcium-dependent protease (Ki = 2.8 X 10(-8) M), whereas it had no effect on trypsin, chymotrypsin, or papain. Heat treatment (50-70 degrees C X 10 min) caused loss of inhibition against high molecular weight protease; however, inhibition of calcium-dependent protease was stable under the same conditions. This result is consistent with different domains on the inhibitor that interact with high molecular weight protease and calcium-dependent protease. Together with earlier studies in which repression of inhibitor by an ATP-ubiquitin-dependent process was proposed, the present results suggest a general mechanism for regulation of multiple nonlysosomal proteases that are complexed with endogenous inhibitors.

A soluble adenosine triphosphate-dependent proteolytic system in human peripheral red blood cells

A soluble adenosine triphosphate (ATP)-dependent proteolytic system has been detected in human peripheral blood erythroid cells. Hemolysates prepared from reticulocyte-rich blood of subjects with autoimmune hemolytic anemia, treated pernicious anemia, and iron deficiency anemia or from pools of red blood cells enriched for reticulocytes by density gradient centrifugation were tested against a radioactive casein standard. Up to 57% of the casein was rendered trichloroacetic acid (TCA) soluble after incubation with such hemolysates for 60 minutes in the presence of 1.0 mmol/L ATP. In the absence of ATP or in hemolysates prepared from reticulocyte-poor blood as little as 6% to 10% of the casein was hydrolyzed. The proteolytic activity was found in the 100,000-g supernatant of active hemolysates and was blocked by hemin, N-ethylmaleimide, and sodium vanadate and thus resembles a previously described activity in rabbit reticulocytes. In the presence of ATP, similar lysates prepared from rabbit reticulocytes preferentially hydrolyzed the abnormal human hemoglobins Leiden and Gun Hill compared with hemoglobin A. These results suggest that there is an active ATP-dependent proteolytic system in young human erythroid cells that can degrade certain abnormal globin chains; the enzymatic activity is lost in the transition from reticulocyte to erythrocyte.

Regulation of myofibrillar accumulation in chick muscle cultures: evidence for the involvement of calcium and lysosomes in non-uniform turnover of contractile proteins

The effect of calcium on myofibrillar turnover in primary chick leg skeletal muscle cultures was examined. Addition of the calcium ionophore A23187 at subcontraction threshold levels (0.38 microM) increased significantly rates of efflux of preloaded 45Ca+2 but had no effect on total protein accumulation. However, A23187 as well as ionomycin caused decreased accumulation of the myofibrillar proteins, myosin heavy chain (MHC), myosin light chain 1f (LC1f), 2f (LC2f), alpha-actin (Ac), and tropomyosin (TM). A23187 increased the degradation rate of LC1f, LC2f, and TM after 24 h. In contrast, the calcium ionophore caused decreased degradation of Ac and troponin-C and had no effect on the degradation of MHC, troponin-T, troponin-I, or alpha, beta-desmin (Dm). In addition, A23187 did not alter degradation of total myotube protein. The ionophore had little or no effect on the synthesis of total myotube proteins, but caused a marked decrease in the synthesis of MHC, LC1f, LC2f, Ac, TM, and Dm after 48 h. The mechanisms involved in calcium-stimulated degradation of the myofibrillar proteins were also investigated. Increased proteolysis appeared to involve a lysosomal pathway, since the effect of the Ca++ ionophore could be blocked by the protease inhibitor leupeptin and the lysosomotropic agents methylamine and chloroquine. The effects of A23187 occur in the presence of serum, a condition in which no lysosomal component of overall protein degradation is detected. The differential effect of A23187 on the degradative rates of the myofibrillar proteins suggests a dynamic structure for the contractile apparatus.

Regulation of protein degradation in muscle by calcium. Evidence for enhanced nonlysosomal proteolysis associated with elevated cytosolic calcium

Calcium-dependent regulation of intracellular protein degradation was studied in isolated rat skeletal muscles incubated in vitro in the presence of a large variety of agents known to affect calcium movement and distribution. A23187, KC1, sucrose, and 8-(diethylamino)octyl-3,4, 5-trimethoxybenzoate hydrochloride increase proteolysis while tetracaine, verapamil, and low extracellular calcium caused significant decreases. Additionally, dantrolene decreases proteolysis in the presence of depolarizing levels of potassium, while it has no effect on degradation in normal media. The dose dependence of calcium ionophore A23187 on proteolysis and contracture tension are parallel. Furthermore, excess KC1 and hypertonic solutions increased protein degradation at doses reported to cause tension. Thus, the parallel increase in proteolysis and tension in response to various agents supports the hypothesis that protein degradation in muscle is regulated by calcium. To determine the responsible proteolytic systems involved in calcium-dependent degradation, the effect of different classes of protease inhibitors was tested. Addition of the inhibitors leupeptin and E-64-c blocked the A23187-induced increase in degradation. Since proteases sensitive to these agents are present in both the sarcoplasm and lysosomes, known lysosomotropic agents, methylamine and chloroquine, as well as 3-methyladenine, a specific autophagy inhibitor, were used in combination with A23187. These agents did not inhibit calcium ionophore-induced proteolysis, although these three agents selectively inhibited enhanced degradation seen in the absence of insulin, demonstrating an autophagic/lysosomal pathway in these muscles. Thus, our results suggest that nonlysosomal leupeptin- and E-64-c-sensitive proteases are responsible for calcium-dependent proteolysis in muscle.

Endocytosis, proteolysis, and exocytosis of exogenous proteins by cultured myotubes

The endocytic activity of chick myotubes in cultures was investigated. Differentiated myotubes internalized the fluid-phase marker horseradish peroxidase in membrane-bound particles which typically displayed reaction product at the inner surface of the vesicle. Pulse-chase experiments demonstrated a rapid decrease in the number of horseradish peroxidase-containing vesicles and a redistribution from a uniform to a perinuclear pattern. Horseradish peroxidase uptake was extensively inhibited by incubation at 0-4 degrees C consistent with an endocytic mechanism. To further characterize the process, the fate of labeled protein was investigated. Following uptake [3H] hemoglobin A was extensively degraded (40-50%) to acid-soluble products within 10 h. Degradation displayed a biphasic pattern with a rapid early phase followed by a much slower second phase. The decreasing rate of proteolysis can be accounted for, in part, by a simultaneous exocytosis of a substantial fraction (25-30%) of acid-insoluble label from myotubes. The lysosomotropic agents methylamine, monensin, and chloroquine significantly inhibited (23-75%) proteolysis, indicating a lysosomal site of degradation. Part of the inhibitory effect results from an increase in exocytosis in the presence of these agents. Degradation of endocytosed [3H]hemoglobin A was not inhibited by insulin. In contrast degradation of endogenous myotube proteins was inhibited (40%) by insulin and blocked by methylamine. These results suggest that cultured myotubes possess a coupled endocytic/exocytic pathway for macromolecules and that a fraction of the internalized substrate is degraded by an insulin-insensitive lysosomal pathway.

A particle-associated ATP-dependent proteolytic activity in erythroleukemia cells

An ATP-dependent proteolytic activity has been detected in both mouse erythroleukemia (Friend) cells and human (K562) erythroleukemia cells. Exposure of the Friend cells to dimethyl sulfoxide, which stimulates differentiation, increased ATP-dependent proteolysis approximately 2-fold although inducing differentiation in the K562 line had no significant effect on proteolysis. In contrast to the previously described soluble ATP-dependent proteolytic system of reticulocytes, the activity in the more primative erythroid cells is associated with a particulate fraction and is readily sedimentable by centrifugation at 100,000 X g for 1 h. Like the soluble reticulocyte system, the particulate activity requires divalent cation and is inhibited by hemin as well as vanadate. The activity was isolated on a sucrose cushion (30%) and did not appear to be associated with membranes, cytoskeleton, or polysomes. This enzymatic activity which degrades abnormal globin chains may initially reside in a particulate fraction and then become solubilized during erythroid maturation to the reticulocyte stage. Alternatively, the particulate activity may disappear with cell maturation being replaced by a distinct soluble activity. ATP-dependent proteolytic activity is eventually lost with reticulocyte maturation and further aging of erythrocytes.

Inhibition of intracellular proteolysis in muscle cultures by multiplication-stimulating activity. Comparison of effects of multiplication-stimulating activity and insulin on proteolysis, protein synthesis, amino acid uptake, and sugar transport

The effects of the insulin-like growth factor, multiplication-stimulating activity (MSA), on chick myotube cultures were investigated. In serum-free media, MSA at levels reported to be present in fetal serum (5 ng/ml) significantly inhibited overall rates of protein degradation and stimulated protein synthesis and amino acid uptake. Half-maximal effects on protein degradation (-30%), synthesis (+25%), and amino acid uptake (+50%) occurred at approximately 0.05 micrograms/ml. In contrast, 10(2)-10(3)-fold higher concentrations (5 micrograms/ml) were required to stimulate transport of the glucose analog 2-deoxyglucose. The results indicate that MSA is an effective anabolic agent regulating protein metabolism and amino acid uptake, but not sugar transport in these cells. Parallel studies conducted with insulin demonstrated similar size effects on protein metabolism and amino acid uptake in serum-free media. However, unlike MSA, insulin levels (10(-2) units/ml) well in excess of its normal physiological range were required to produce significant effects. In addition, the relative sensitivity of sugar transport with respect to protein metabolic effects differed for insulin and MSA. Thus, 2-deoxyglucose transport was approximately 10 times more sensitive to insulin than protein synthesis, proteolysis, or amino acid uptake in contrast to MSA where the reverse was true. However, despite the relatively higher sensitivity of sugar transport to insulin, supraphysiological levels (10(-3) units/ml) of this hormone were still required for significant stimulation. These results suggest a generally low insulin sensitivity in cultured chick myotubes relative to adult tissues. In contrast, the effects of MSA are consistent with a possible role of this or similar factors in regulating growth and development of embryonic muscle.

ATP stimulates proteolysis in reticulocyte extracts by repressing an endogenous protease inhibitor

An endogenous inhibitor of the reticulocyte ATP-dependent proteolytic system has been purified partially by ammonium sulfate precipitation from rabbit reticulocyte and erythrocyte extracts. Inhibitor-free protease rapidly degrades 21-40% of the substrate [14C]methyl-alpha-casein per hour, resembling ATP-dependent activity in reticulocyte extracts. This proteolytic activity is not stimulated by ATP and does not respond to ubiquitin. Adding back the inhibitory fraction to reticulocyte inhibitor-free protease results in a significant decrease (65-75%) in proteolysis, both in the presence and absence of ATP. In contrast, inhibition is repressed when both ATP and the ubiquitin-containing fraction are present, resulting in an 80-350% stimulation of proteolysis by these components. These results suggest that ATP, in the presence of ubiquitin, may act in releasing the protease(s) from its endogenous inhibitor. Erythrocyte extracts, unlike reticulocyte extracts, exhibit low levels of ATP-dependent proteolytic activity. However, ion-exchange chromatography reveals that erythrocytes contain levels of proteolytic activity that are comparable to the reticulocyte's inhibitor-free protease. Addition of ubiquitin and inhibitor to erythrocyte protease results in a highly ATP-dependent activity that resembles levels of ATP-dependence (3- to 4-fold) seen in reticulocyte extracts. Thus, the proteolytic and inhibitory components of the ATP-dependent proteolytic system appear to be retained with reticulocyte maturation. However, some other component(s) of the system are lost or modified with maturation so that the protease remains inactive.

Easily releasable myofilaments from skeletal and cardiac muscles maintained in vitro. Role in myofibrillar assembly and turnover

Gentle treatment with an ATP-containing relaxing solution of isolated myofibrils from rat diaphragm, soleus, extensor digitorum longus, and left atria maintained in vitro releases a small amount of myofilaments constituting less than 5% of total myofibrillar protein. Successive extraction of myofibrils produced little further filament release. Releasable myofilaments lack alpha-actinin (Mr = 95,000), certain very high molecular weight proteins (greater than 200,000), and possibly M-line protein but contain other myofibrillar proteins. After pulse-labeling with [3H]leucine for 8 min, specific activity of the myosin heavy chain in the easily releasable myofilaments is 3-6 times higher than the specific activity of myosin heavy chain in the residual myofibrils, although 85-90% of total label is in the myofibrillar myosin. In the absence of protein synthesis, releasable filament specific activity decreases, with a half-time of 60-90 min, to that of the myofibrillar myosin. This labeling pattern appears inconsistent with a simple precursor-product relationship between releasable filaments and myofibrils suggesting that the filaments originate largely from myofibrils. Preincubation of muscles with several factors known to decrease proteolysis, i.e. passive stretch, leupeptin, colchicine, and cycloheximide, reduced the size of the releasable filament fraction. Treatment of muscles with the calcium ionophore A23187, which accelerates proteolysis, and pretreatment of myofibrils with either trypsin or calcium-dependent protease increased filament release. Therefore, the releasable filament fraction may contain intermediates in the breakdown of myofibrils. The labeling kinetics may indicate a mixing of myofilaments within myofibrils which functions in the movement of contractile protein to its possible site of degradation, i.e. the myofibrillar surface.

A soluble ATP-dependent proteolytic system responsible for the degradation of abnormal proteins in reticulocytes

Reticulocytes, like other cells, selectively degrade certain abnormal proteins by an energy-dependent process. When isolated rabbit reticulocytes incorporate the valine analog 2-amino-3chlorobutyric acid (ClAbu) in place of valine, they produce an abnormal globin that is degraded with a half-life of 15 min. Normal hemoglobin, in contrast, undergoes little or no breakdown within these cells. Cell-free extracts from reticulocytes have been shown to rapidly hydrolyze these abnormal proteins. The degradative system is located in the 100,000 X g supernatant, has a pH optimum of 7.8, and does not appear to be of lysosomal origin. This breakdown of analog-containing protein was stimulated severalfold by ATP, and slightly by ADP. AMP and adenosine-3':5'-cyclic monophosphate had no significant effect on proteolysis. Experiments with ATP analogs suggest that the terminal high energy phosphate is important in the degradative process. Proteolysis in the cell-free system and in intact reticulocytes was inhibited by the same agents (L-l-tosylamido-2-phenyl-ethylchloromethyl ketone, N-alpha-p-tosyl-L-lysine chloromethyl ketone, N-ethylmaleimide, iodoacetamide, and o-phenanthroline). In addition, the relative rates of degradation of several polypeptides in the cell-free extracts paralleled degradatives rates within cells. Thus, a soluble nonlysosomal proteolytic system appears responsible for the energy-dependent degradation of abnormal proteins in reticulocytes.

Compositional studies of myofibrils from rabbit striated muscle

The localization of high-molecular-weight (80,000-200,000-daltons) proteins in the sarcomere of striated muscle has been studied by coordinated electron-microscopic and sodium dodecyl sulfate (SDS) gel electrophoretic analysis of native myofilaments and extracted and digested myofibrils. Methods were developed for the isolation of thick and thin filaments and of uncontracted myofibrils which are devoid of endoproteases and membrane fragments. Treatment of crude myofibrils with 0.5% Triton X-100 results in the release of a 110,000-dalton component without affecting the myofibrillar structure. Extraction of uncontracted myofibrils with a relaxing solution of high ionic strength results in the complete disappearance of the A band and M line. In this extract, five other protein bands in addition to myosin are resolved on SDS gels: bands M 1 (190,000 daltons) and M 2 (170,000 daltons), which are suggested to be components of the M line; M 3 (150,000 daltons), a degradation product; and a doublet M 4, M 5 (140,000 daltons), thick-filament protein having the same mobility as C protein. Extraction of myofibrils with 0.15% deoxycholate, previously shown to remove Z-line density, releases a doublet Z 1, Z 2 (90,000 daltons) with the same mobility as alpha-actinin, as well as proteins of 60,000 daltons and less, and small amounts of M 1, M 2, M 4, and M 5; these proteins were not extracted with 0.5% Triton X-100. The C, M-line, and Z-line proteins and/or their binding to myofibrils are very sensitive to tryptic digestion, whereas the M 3 (150,000 daltons) component and an additional band at 110,000 daltons are products of proteolysis. Gentle treatment of myofibrils with an ATP relaxing solution results in the release of thick and thin myofilaments which can be pelleted by 100,000-g centrifugation. These myofilaments lack M-and Z-line structure when examined with the electron microscope, and their electrophoretograms are devoid of the M 1, M 2, Z 1, and Z 2 bands. The M 4, M 5 (C-protein doublet), and M 3 bands, however, remain associated with the filaments.

Removal of Z-lines and alpha-actinin from isolated myofibrils by a calcium-activated neutral protease

A calcium-activated factor (CaAF) has been isolated and partially purified from the post-myofibrillar supernatant fraction of rabbit skeletal muscle. The 200-fold purified CaAF hydrolyzed denatured casein, [3-H]acetyl hemoglobin, and N-ethyl[3-H]maleimide-labeled alpha-actinin. The proteolytic activity has a pH optimum at 6.9 and is dependent on the presence of Ca2+ (optimum concentration, 10 mM). Digestion of isolated myofibrils with CaAF results in removal of Z-lines and in a parallel loss of a 90, 000-dalton protein that has a mobility identical with that of alpha-actinin as determined by polyacrylamide gel electrophoresis. A protein with the properties of alpha-actinin (identical electrophoretic mobility, and ability to accelerate the Mg2+-activated ATPase of reconstituted actomyosin) was isolated from the supernatant of CaAF-treated myofibrils. The release of alpha-actinin from myofibrils by the calcium-activated neutral protease occurs in the absence of detectable change in the electrophoretic profiles of the other myofibrillar proteins, or in the ethylene glycol bis(beta-aminoethyl ether)-N, N' tetraacetic acid (EGTA) sensitivity of Mg2+-activated ATPase. In contrast to the specific removal of Z-lines and of alpha-actinin by CaAF, trypsin treatment of myofibrils results in extensive degradation of myosin heavy chains and of the inhibitory component of troponin (TN-I), and in loss of EGTA sensitivity of myofibrillar ATPase. The degradation of TN-I and loss of EGTA sensitivity occur before the Z-line disappearance.