Dose-dependent effects of ionizing radiation on in vitro myoblast fusion

Protective effects of insulin-like growth factor-I on the decrease in myogenic differentiation by ionizing radiation

PURPOSE

The aim of the work is to evaluate the effects of insulin-like growth factor-1 (IGF-1) on the decrease in myotube formation induced by ionizing radiation.

MATERIALS AND METHODS

We induced C2C12 cells to a myogenic linage following X-ray irradiation at 2 and 4 Gy. Myogenic differentiation was estimated using immnocytochemical staining with anti-myosin antibody, and the anti-myosin antibody positive areas, the total number of nuclei, the number of nuclei included in multinucleated myotubes per field, and the myotube formation ratio were analyzed.

RESULTS

In the myogenic differentiation in the presence of IGF-1, the decrease in anti-myosin antibody positive areas, the nuclei included in myotubes, and the myotube formation ratio induced by X-ray irradiation at 2 Gy was restored to control levels.

CONCLUSIONS

The addition of IGF-1 protected against the decrease myotube formation induced by X-ray irradiation at 2 Gy. Since X-ray irradiation at 2 Gy is usually used for multi-fractionated irradiation in radiotherapy, our findings suggest that IGF-1 could be useful to protect against impairment of muscle repair induced by therapeutic dose radiation.

The use of adeno-associated virus to circumvent the maturation-dependent viral transduction of muscle fibers

Muscle-based gene therapy using adenovirus, retrovirus, and herpes simplex virus has been hindered by viral cytotoxicity, host immune response, and the maturation-dependent viral transduction of muscle fibers. The development of new mutant vectors has greatly reduced the toxicity and the immune rejection problems, but the inability of viral vectors to penetrate and transduce mature myofibers remains an important issue. Research has been focused on the characterization of barriers to viral transduction in mature myofibers to develop strategies to circumvent the maturation-dependent viral transduction of myofibers. Here, we report that adeno-associated virus (AAV) can be used to overcome the maturation-dependent viral transduction of myofibers. We have investigated by which mechanism AAV can penetrate and efficiently transduce mature muscle fibers, and have shown that this viral vector is not blocked by the basal lamina and that AAV transduction of myofibers is independent of myoblast mediation. Although AAV can efficiently transduce mature myofibers, a differential transduction is still observed among the different types of myofibers that correlates with the expression of the heparan sulfate proteoglycan receptors, the muscle maturity, the number of viral particles used, and the time postinjection. The identification of the mechanisms by which AAV transduces mature myofibers will help in the development of strategies to achieve an efficient muscle-based gene therapy for inherited and acquired diseases.

Extended tropism of an adenoviral vector does not circumvent the maturation-dependent transducibility of mouse skeletal muscle

BACKGROUND

Efficient adenoviral gene delivery to mature skeletal muscle has been hindered by different factors. The low levels of adenoviral attachment receptor (CAR) that have been reported in this tissue may be a limiting factor. Therefore, adenoviral transduction of mature muscle may be improved by extending the tropism of the adenoviral vectors to attachment receptors that are highly expressed in mature myofibers. In this study, we have investigated whether an extended tropism adenoviral vector which additionally attaches to the broadly expressed heparan-containing receptors (AdPK) can bypass the maturation-dependent adenoviral transducibility of mouse skeletal muscle.

METHODS

The adenoviral vector AdPK carrying the LacZ gene was evaluated as a gene delivery vehicle in mouse skeletal muscle at different maturities in vitro and in vivo. The viral transduction efficiencies were determined by histochemical and ONPG analysis of the beta-galactosidase activity level.

RESULTS

Higher transduction efficiencies were detected in immature muscle from normal mice, and in mature muscle from merosin-deficient dy/dy mice (carrying myofibers with an impaired extracellular matrix) and dystrophin-deficient mdx mice (showing a high level of myoblast activity) when compared to mature muscle from normal mice.

CONCLUSION

Despite the enhanced attachment characteristics, the extended tropism adenoviral vector is, similarly to the wild-type adenoviral vector in previous studies, still hindered by both a protective extracellular matrix and the diminished myoblast-mediation in mature muscle.

Effects of hyaluronan viscous materials on cell membrane electrical properties

Hyaluronan [hyaluronic acid (HA)] has been implicated in various cellular processes such as proliferation, adhesion, migration, and differentiation. The secondary and tertiary structures of HA give it very important and unique viscoelastic properties. HA-composed materials are currently used intraocularly during ophthalmological surgery to facilitate surgical procedures and prevent tissue damage. To examine the effects of three viscous biomaterials composed of hyaluronan (Healon, IAL, and Biolon) used in ophthalmological surgery, the membrane electrical properties of the erythroleukemic K562 cell line exposed to these materials were investigated. Membrane conductivity, membrane permittivity, and the conductivity of the cytosol were evaluated using dielectric relaxation measurements in the radiofrequency range and fitting the experimental results to the general equations of the Maxwell-Wagner effect. The results demonstrate that while membrane permittivity and the conductivity of the cytosol are not significantly altered, the membrane conductivity of K562 cells exposed to all three biomaterials increases substantially and in a time-dependent manner with respect to untreated cells. These observations seem to indicate that hyaluronan perturbs ionic transport while it does not vary the type, quantity, or distribution of membrane components. In addition, the variations induced by these substances on the cell membrane are not dependent upon the molecular weight or on the biological origin of hyaluronan. These results may aid in elucidating the mechanisms involved in hyaluronan/cell membrane interaction and thus may provide a deeper understanding of the complications related to their use in ophthalmological surgery.

Polylysine induces changes in membrane electrical properties of K562 cells

The ionic environment of the cell membrane is of extreme importance in maintaining cell integrity and the numerous functions necessary for cell growth, differentiation, etc., as well as in cell-biomaterial interactions. In this study, the effects of polylysine (a basic poly-amino acid with a net positive charge which is often used to coat biomaterials surfaces) on the erythroleukemic K562 cell membrane were investigated. In particular, the effects of this polycation were evaluated using dielectric relaxation studies in the radiofrequency range with which it is possible to measure both active ionic transport across the cell membrane (membrane conductivity) and the static charge distribution present on the cell surface due to the structural components of the cell membrane (membrane permittivity). The conductivity of the cytosol can also be determined. The results demonstrate that while the conductivity of the cytosol is not significantly altered, both the conductivity and permittivity of the K562 cell membrane are varied by exposure of these cells to polylysine. These observations indicate that both active ionic transport and the type, quantity, or distribution of membrane components such as lipids, proteins, and polysaccharides may also be altered. Although the precise mechanisms by which these variations in K562 cells occur are unknown, it can be hypothesized that changes in the growth characteristics of these cells may be in part responsible. In particular, as demonstrated by light microscopic examination of K562 cells directly in the culture flasks, the cells in polylysine-coated flasks do not grow in suspension as do the controls, but rather show anchorage-dependent-like behavior. It is this important change from suspension to monolayer growth induced by polylysine that may be responsible for the changes in membrane electrical parameters.

Irradiation enhances cellular uptake of carboplatin

PURPOSE

Because radiation is known to damage cellular membranes, the purpose of this study was to determine whether irradiation of cultured cells might modify the cellular uptake of the chemotherapy agent carboplatin.

METHODS AND MATERIALS

Total intracellular platinum was measured using atomic absorption spectrometry in cultured V79 cells and in four Chinese hamster ovary (CHO) cell lines.

RESULTS

Intracellular carboplatin concentrations increased linearly with radiation dose (10-50 Gy) under both hypoxic and oxic irradiation conditions. Similar doses of radiation did not significantly increase the uptake of a nontoxic platinum compound [Pt(NH3)4Cl2.H2O] (p > 0.5). Compared to unirradiated controls, there was no increase in intracellular carboplatin concentrations when carboplatin was irradiated prior to administration to the cell cultures (p > 0.5). Within the 32.5 min or less required to deliver the radiation, a dose of 50 Gy produced approximately a 50% increase in intracellular platinum in V79 cells and approximately an increase of a factor of 1.3-1.6 in the CHO cell lines. Although the increase in drug uptake would be expected to be less than 10% for most cell lines at the doses of radiation used to investigate radiosensitization by carboplatin, this level of increase may play a significant role in the radioenhancement observed in UV41 cells because these excision-repair--deficient cells are much more sensitive to carboplatin as measured by cytotoxicity.

CONCLUSION

These results suggest that some of the enhanced cell killing that results when cells are exposed to carboplatin in combination with radiation may be attributed to an increased cellular uptake. One mechanism of radiopotentiation may be an enhanced chemotoxicity resulting from a radiation-induced increase in carboplatin uptake.

Recent developments in the radiobiology of cellular membranes

We have reviewed the literature on cellular membrane radiobiology over the last ten years and, in particular, report on the development of rapid techniques used to identify damage soon after irradiation. It is clear that damage can now be quantified after low doses, and further refinements can be expected. From the work summarised, it would appear that changes to membranes at low doses may occur soon after damage to other important macromolecules by intercommunicating processes. We believe that there now exists a variety of rapid methods of measuring deposition of damage in vital macromolecules, such as cellular membranes and DNA, which can give a fuller picture of the overall effects of radiation and lead to predictions of eventual cellular mortality.

The fusion radiosensitivity of differentiating chick embryo myoblasts in vitro is not determined by the plasma membrane

We have recently demonstrated by dielectric relaxation studies in the radiofrequency range that the sharp drop in the conductivity and permittivity of the membranes of chick embryo myoblasts in vitro, representative of fusion, is either delayed or completely blocked by sublethal doses of ionizing radiation (Santini et al. 1990a). The lowest of the doses investigated (3.25 Gy) caused a 10 h delay in myoblast membrane fusion when the cells were exposed at 24 h of culture, indicating that radiation-induced membrane injury had occurred. The purpose of this study was to determine if the myoblast system under investigation shows the same radiosensitive characteristics if irradiated with 3.25 Gy at various stages of differentiation. Consequently, the myoblasts were exposed to this dose at two different stages of differentiation (12 h or 48 h of culture). We show here that the time at which the myogenic cells are irradiated (state of differentiation) does not seem to affect the magnitude of the fusion delay (which was used as a measure of radiosensitivity of the myoblasts). In fact, the sharp drop in both membrane conductivity and membrane permittivity occurs with the same 10 h delay independent of the time of exposure. The role played by the plasma membrane in determining myoblast response to radiation damage is discussed.