Comparison of the effects of thyroxine and triiodothyronine on protein turnover and apoptosis in primary chick muscle cell cultures

Quantification of Nτ -Methylhistidine and Nπ-Methylhistidine in Chicken Plasma by Liquid Chromatography-Tandem Mass Spectrometry

The concentration of N^τ-methylhistidine in plasma provides an index of skeletal muscle protein breakdown. This study aimed to establish a quantitative method for measuring the concentrations of N^τ-methylhistidine and its isomer N^π-methylhistidine in chicken plasma, using liquid chromatography-tandem mass spectrometry with stable isotope dilution analysis. The acceptable linear ranges of detection were 1.56-50.00 μmol/L for N^τ-methylhistidine and 0.78-25.00 μmol/L for N^π-methylhistidine. The proposed method detected changes in the plasma levels of N^τ-methylhistidine and N^π-methylhistidine in response to fasting and re-feeding. These results suggest that the method developed in this study can be used for the simultaneous measurement of N^τ-methylhistidine and N^π-methylhistidine in chicken plasma.

Triiodothyronine Aggravates Global Cerebral Ischemia-Reperfusion Injury in Mice

Thyroid hormones (THs) have been suggested to play an important role in both physiological and pathological events in the central nervous system. Hypothyroidism, which is characterized by low levels of serum THs, has been associated with aggravation of ischemic neuronal injuries in stroke patients. We hypothesized that administration of T3, the main active form of THs, may attenuate the ischemic neuronal injuries. In mice, global cerebral ischemia (GCI), which is induced by transient occlusion of the bilateral common carotid artery, causes neuronal injuries by inducing neuronal death and activating inflammatory responses after reperfusion in the hippocampus. In this study, we examined the effect of T3 administration on DNA fragmentation induced by neuronal death and the activation of inflammatory cells such as astrocytes and microglia in the hippocampus following GCI. The content of nucleosomes generated by DNA fragmentation in the hippocampus was increased by GCI and further increased by T3 administration. The protein expression levels of glial fibrillary acidic protein (GFAP), an astrocytic marker, and Ionized calcium binding adaptor protein 1 (Iba1), a microglial marker, in the hippocampus were also increased by GCI and further increased by T3 administration. The levels of T3 in both the serum and hippocampus were elevated by T3 administration. Our results indicate that T3 administration aggravates GCI-reperfusion injury in mice. There may be an increased risk of aggravation of ischemic stroke by the excessive elevation of T3 levels during the drug treatment of hypothyroidism.

The energy cost of feather replacement is not intrinsically inefficient

Feathers serve a diversity of functions in birds and their continuous use and exposure to the environment requires a scheduled moult to maintain their full functionality. As feathers represent about 25% of a bird’s protein content, moult is expected to impose substantial energy and nutrient demands, but perhaps not to the extent reported. Energy conversion efficiencies for feather formation are among the lowest for any biological structure examined, but this assumes that increases in maintenance energy requirements (minimum resting metabolic rate (RMRmin)) during moult are predominately due to feather synthetic costs. We tested this assumption by comparing the RMRminand protein turnover rates of House Sparrows (Passer domesticus (Linnaeus, 1758)) during peak moult and in a non-moulting cohort before and 12 days after having a similar amount of feathers plucked. Replacement of plucked feathers had no effect on metabolic rate, whereas RMRminwas 28% higher in moulting than in non-moulting House Sparrows. Protein turnover rates were lowest in non-moulting birds, but rate differences between non-moulting and moulting birds were threefold higher than those between non-moulting and plucked House Sparrows. Thus, the energy inefficiencies reported for feather replacement are mainly due to costs associated with coincident processes rather than being a direct cost of feather synthesis per se.

Regulation of Autophagy in Chick Skeletal Muscle: Effect of mTOR Inhibition

Autophagy in the skeletal muscle increases under catabolic conditions resulting in muscle atrophy. This study investigated the effect of inhibition of mechanistic target of rapamycin (mTOR) on autophagy in chick skeletal muscle. We examined the effects of Torin1, an mTOR inhibitor, on autophagy. Chick myotubes were incubated with Torin1 (100 nM) for 3 h. It was observed that Torin1 inhibited the phosphorylation of AKT (Ser473), p70 ribosomal S6 kinase 1 (S6K1, Thr389), S6 ribosomal protein (Ser235/236), and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1, Thr37/46), which are used for measurement of mTOR activity. Torin1 significantly (P< 0.01) increased the LC3-II/LC3-I ratio, an index for autophagosome formation, while it did not influence the expression of autophagy-related genes (LC3B, GABARAPL1, and ATG12). In addition, Torin1 increased atrogin-1/MAFbx (a muscle-specific ubiquitin ligase) mRNA expression. Fasting for 24 h inhibited the phosphorylation of AKT (Ser473), S6K1 (Ther389), S6 ribosomal protein (Ser235/236), and 4E-BP1 (Thr37/46) in chick skeletal muscle and significantly (P<0.01) increased the LC3-II/LC3-I ratio. Fasting also increased GABARAPL1 and atrogin-1/MAFbx mRNA expression but not LC3B or ATG12 mRNA expression. These results indicate that mTOR signaling regulates autophagy and the ubiquitin-proteasome proteolytic pathway in chick skeletal muscle.

Effects of hyperthyroidism on the rectus muscles in mice

Background: Structural details of vertebrate extraocular muscles (EOMs) have shown an anatomically and functionally distinct laminar organization into an outer orbital (OL) and an inner global layer (GL). Since hyperthyroidism alters tissue oxidative metabolism through mitochondrial enzymes, it is expected that structural/mitochondrial changes may be seen in hyperthyroid EOMs. We investigated the alterations in the laminar organization and mitochondrial changes in hyperthyroid mouse EOMs. Methods: Hyperthyroidism was induced in C57BL/6 mice and fresh rectus muscles were obtained to identify functional mitochondria using MitoTracker® Green and confocal microscopy; frozen sections from rectus muscles were stained with anti-rabbit Troponin T (selectively present in the OL) to demonstrate changes in the OL and GL of the EOMs. Ultrastructural features of EOMs were studied using transmission electron microscopy (TEM). Results: Of all four rectus EOMs studied, the maximum change was seen in the inferior rectus muscle (IR) followed by medial rectus (MR). Myofiber cross-sectional area measurements and Troponin T staining in the control IR EOMs demonstrated a smaller OL (113.2 ± 3.66 μm²) and higher density staining with Troponin T (90%) and a larger GL (411 ± 13.84 μm²) with low intensity staining (10%), while hyperthyroidism resulted in an increased OL (205.9 ± 5.3 μm²) and decreased GL (271.7 ± 7.5 μm²) p = 0.001. Confocal microscopy demonstrated an intense staining especially in the outer rims in the hyperthyroid IR which was confirmed by TEM showing structural alterations in the mitochondria and a subsarcolemmal migration. Conclusions: The outer, thinner, OL of the mouse EOM contains smaller diameter myofibers and fewer mitochondria while the inner, larger GL contains larger diameter myofibers and larger density of mitochondria. Hyperthyroidism results in a significant alteration in the laminar organization and mitochondrial alterations of mouse EOMs.

Experimental hyperthyroidism in rats increases the expression of the ubiquitin ligases atrogin-1 and MuRF1 and stimulates multiple proteolytic pathways in skeletal muscle

Muscle wasting is commonly seen in patients with hyperthyroidism and is mainly caused by stimulated muscle proteolysis. Loss of muscle mass in several catabolic conditions is associated with increased expression of the muscle-specific ubiquitin ligases atrogin-1 and MuRF1 but it is not known if atrogin-1 and MuRF1 are upregulated in hyperthyroidism. In addition, it is not known if thyroid hormone increases the activity of proteolytic mechanisms other than the ubiquitin-proteasome pathway. We tested the hypotheses that experimental hyperthyroidism in rats, induced by daily intraperitoneal injections of 100 microg/100 g body weight of triiodothyronine (T3), upregulates the expression of atrogin-1 and MuRF1 in skeletal muscle and stimulates lysosomal, including cathepsin L, calpain-, and caspase-3-dependent protein breakdown in addition to proteasome-dependent protein breakdown. Treatment of rats with T3 for 3 days resulted in an approximately twofold increase in atrogin-1 and MuRF1 mRNA levels. The same treatment increased proteasome-, cathepsin L-, and calpain-dependent proteolytic rates by approximately 40% but did not influence caspase-3-dependent proteolysis. The expression of atrogin-1 and MuRF1 remained elevated during a more prolonged period (7 days) of T3 treatment. The results provide support for a role of the ubiquitin-proteasome pathway in muscle wasting during hyperthyroidism and suggest that other proteolytic pathways as well may be activated in the hyperthyroid state.

Effects of elevated thyroid hormone on adult rabbit extraocular muscles

PURPOSE

Human extraocular muscles (EOM) are preferentially susceptible to thyroid eye disease. Although the specific cause of this autoimmune disorder is unknown, it is often associated with elevated thyroid hormone levels. Thus, the effect of elevated thyroid hormone levels on cross-sectional area, myofiber size, satellite cells, and myosin heavy chain (MyHC) isoform expression was examined in adult rabbit EOMs, to determine how elevated thyroid hormone alters EOM biology.

METHODS

After 1 month of elevated thyroid hormone levels, the EOMs were removed and prepared for histologic examination. Total muscle mass, myofiber size, patterns of MyHC isoform expression, and the number of satellite cells were determined.

RESULTS

Elevated thyroid hormone levels significantly decreased muscle mass, total number of myofibers, and mean cross-sectional area of the myofibers. Alterations in MyHC isoform expression were extremely complex, but several basic patterns emerged. The percentages of neonatal- and developmental-positive myofibers decreased in almost all EOM regions examined, and the percentages of slow-positive myofibers significantly increased. In contrast to normal EOMs, which retain a population of activated satellite cells throughout life, elevated thyroid hormone levels resulted in the virtual disappearance of MyoD-positive cells and a decrease in Pax7-positive cells.

CONCLUSIONS

The reductions in EOM size, number of fibers expressing developmental and neonatal MyHC, and number of MyoD- and Pax7-positive satellite cells suggest that elevated thyroid hormone levels decrease the ongoing myofiber remodeling normally seen in the EOM. These catabolic changes have important implications for maintenance of function in the EOMs.

Triiodothyronine but not thyroxine accelerates myofibrillar proteolysis via ATP production in cultured muscle cells

These experiments were done to clarify that the differential effects of thyroxine (T(4)) and triiodothyronine (T(3)) on skeletal muscle protein turnover are caused by their roles on ATP production. Primary cultured chick muscle cells were treated with a physiological level of T(4) (60 ng/ml), T(3) (12 ng/ml), or ATP (0.5 mM) for 6 days and the protein content, ATP production, proteasome activity, and myofibrillar protein breakdown were measured. The protein content measured as an index of cell growth was not affected by T(4), T(3), or ATP. The cellular ATP level was increased by T(3) and ATP, but not by T(4). Proteasome activity and N(tau)-methylhistidine (MeHis) release measured as an index of myofiblillar protein breakdown was also increased by T(3) and ATP, but not by T(4). These results indicate that T(3) but not T(4) increases ATP production followed by an increase in proteasome activity, and thus stimulates myofibrillar proteolysis.

Bacterial ceramides and sphingophospholipids induce apoptosis of human leukaemic cells

The genus Sphingobacterium, whose members are Gram-negative non-fermentative rods, possesses ceramides and related sphingophospholipids (SPLs) with isoheptadecasphinganine and 2-hydroxy or non-hydroxy isopentadecanoic acid. This paper reports evidence that ceramides isolated from Sphingobacterium spiritivorum ATCC 33861 induce endonucleolytic DNA cleavage in human myeloid leukaemia HL-60 cells in vitro, which is the primary characteristic biochemical marker for apoptosis or programmed cell death. Ceramides and SPLs also induced DNA fragmentation and caspase-3 activation, followed by changes in morphology, such as alterations in the size of nuclei and cells, and cell cycle shortening. Apoptotic activity correlated with the ceramide structure. Ceramide with a 2-hydroxy fatty acid showed stronger apoptotic activity than ceramide with a non-hydroxy fatty acid. Furthermore, the major five SPLs (ceramide phosphorylethanolamine-1 and -2, ceramide phosphorylinositol-1 and -2, and ceramide phosphorylmannose-1) showed apoptosis-inducing activity in HL-60 cells, indicating that the ceramide moiety of the SPLs plays a crucial role as the intracellular second messenger but that their hydrophilicity is less important in this regard. The hydrophilic part of SPLs may play a role in other cellular response systems. The involvement of Fas antigen was implicated in the apoptotic event since Fas antigen expression was observed after 3 or 4 h stimulation of HL-60 cells with bacterial ceramides. However, a time-course study for caspase-3 activation indicated maximal activity at 1 h after stimulation with bacterial ceramides, suggesting that two (or possibly more) mechanisms of signal transduction, Fas-dependent and Fas-independent, may be involved. Fas antigen expression and caspase-3 activation by five kinds of SPLs were observed after 3 or 4 h. These results indicate that there is a difference in the response of HL-60 cells to bacterial ceramides and SPLs.

Effect of hyperthyroidism on the orbicularis oculi muscle in rabbits

PURPOSE

To determine the effect of hyperthyroidism on both myofiber number and myosin heavy-chain isoform composition within the palpebral orbicularis oculi muscle in rabbits.

METHODS

Four New Zealand White rabbits were made hyperthyroid by injection of 3,3,3'-triiodothyroinine intraperitoneally every other day for 1 month. Four rabbits were used as control animals. After 1 month the rabbits were euthanized, and the eyelids were excised and sectioned in a cryostat. The sections were immunostained to determine the presence of fast, slow, and neonatal myosin heavy-chain isoforms. To determine alterations in myofiber number, differential counts of myofiber number and the cross-sectional areas of the muscle fibers were performed with the use of computerized morphometry.

RESULTS

The orbicularis oculi muscle in the palpebral portion of the eyelids from hyperthyroid rabbits had significantly fewer myofibers compared with control eyelids, predominantly as the result of a loss of myofibers in the preseptal region. The remaining fibers showed continued expression of fast myosin but upregulated coexpression of slow myosin isoform.

CONCLUSIONS

Hyperthyroidism led to reduced orbicularis oculi muscle in the rabbit model and an alteration in the myosin heavy-chain isoform composition. This finding may help explain the clinical finding of eyelid retraction in patients with Graves orbitopathy.

Induction of apoptosis by all-trans-retinoic acid and C2-ceramide treatment in rat stromal-vascular cultures

Apoptosis of preadipocytes and adipocytes contributes to the balance of adipose tissue mass by reducing adipocyte number. To address this phenomenon, we treated cultured rat S-V cells with all-trans-retinoic acid (RA) (10 microM) or C2-ceramide (50 microM) during adipogenesis. Gel electrophoresis of DNA from treated cells cultured in serum-free medium showed that 10 microM RA or 50 microM ceramide induced a distinct laddering pattern of DNA fragments. Cellular caspase 3 activity, another marker of apoptosis, was increased by RA (10 microM) (P < 0.05), but not by 50 microm C2-ceramide. RT-PCR results showed that RA (10 microM) decreased the expression of Bcl-2 mRNA. These results suggest that fat cell loss by apoptosis can be regulated, in part, by RA (10 microM) which increases caspase 3 activity and decreases Bcl-2 expression in rat S-V cells. C2-ceramide apparently works through a different cellular mechanism to induce apoptosis.