Partial skeletal muscle-specific Drp1 knockout enhances insulin sensitivity in diet-induced obese mice, but not in lean mice

OBJECTIVE

Dynamin-related protein 1 (Drp1) is the key regulator of mitochondrial fission. We and others have reported a strong correlation between enhanced Drp1 activity and impaired skeletal muscle insulin sensitivity. This study aimed to determine whether Drp1 directly regulates skeletal muscle insulin sensitivity and whole-body glucose homeostasis.

METHODS

We employed tamoxifen-inducible skeletal muscle-specific heterozygous Drp1 knockout mice (mDrp1+/-). Male mDrp1+/- and wildtype (WT) mice were fed with either a high-fat diet (HFD) or low-fat diet (LFD) for four weeks, followed by tamoxifen injections for five consecutive days, and remained on their respective diet for another four weeks. In addition, we used primary human skeletal muscle cells (HSkMC) from lean, insulin-sensitive, and severely obese, insulin-resistant humans and transfected the cells with either a Drp1 shRNA (shDrp1) or scramble shRNA construct. Skeletal muscle and whole-body insulin sensitivity, skeletal muscle insulin signaling, mitochondrial network morphology, respiration, and H2O2 production were measured.

RESULTS

Partial deletion of the Drp1 gene in skeletal muscle led to improved whole-body glucose tolerance and insulin sensitivity (P < 0.05) in diet-induced obese, insulin-resistant mice but not in lean mice. Analyses of mitochondrial structure and function revealed that the partial deletion of the Drp1 gene restored mitochondrial dynamics, improved mitochondrial morphology, and reduced mitochondrial Complex I- and II-derived H2O2 (P < 0.05) under the condition of diet-induced obesity. In addition, partial deletion of Drp1 in skeletal muscle resulted in elevated circulating FGF21 (P < 0.05) and in a trend towards increase of FGF21 expression in skeletal muscle tissue (P = 0.095). In primary myotubes derived from severely obese, insulin-resistant humans, ShRNA-induced-knockdown of Drp1 resulted in enhanced insulin signaling, insulin-stimulated glucose uptake and reduced cellular reactive oxygen species (ROS) content compared to the shScramble-treated myotubes from the same donors (P < 0.05).

CONCLUSION

These data demonstrate that partial loss of skeletal muscle-specific Drp1 expression is sufficient to improve whole-body glucose homeostasis and insulin sensitivity under obese, insulin-resistant conditions, which may be, at least in part, due to reduced mitochondrial H2O2 production. In addition, our findings revealed divergent effects of Drp1 on whole-body metabolism under lean healthy or obese insulin-resistant conditions in mice.

[Early results of 131I therapy of differentiated thyroid carcinoma in children]

Between 1970-1995 because of thyroid carcinoma 105 children were diagnosed or treated by radioiodine at the Maria Skłodowska-Curie Institute of Oncology and Cancer Center in Gliwice, 101 of them with differentiated thyroid cancer (DTC). All children were operated, in 91% by total thyroidectomy. Papillary carcinoma was diagnosed in 73% of children, follicular cancer in 27%. Lymph node metastases were present in 58.5%. Complementary radioiodine was given to 62 children--to 44 children for thyroid ablation, to 18 for complementary treatment of lymph node metastases. Very good outcome of combined surgical and radioiodine treatment was observed in 95% of children. Post-therapy scans revealed pulmonary micrometastases in 3 patients. Twelve children with pulmonary metastases were treated by radioiodine. Complete remission was observed after radioiodine therapy in 9 of them, in others the treatment is continued.

Multivariate analysis of prognostic factors for differentiated thyroid carcinoma in children

At most centres, the standard treatment for differentiated thyroid cancer (DTC) comprises total thyroidectomy, radioiodine treatment and thyroid-stimulating hormone (TSH) suppressive therapy. There is, however, considerable disagreement over the appropriate treatment for DTC in children. Some dispute the use of total thyroidectomy and/or question the routine application of iodine-131 therapy in children. The aim of this study was to perform a retrospective analysis of treatment results and prognostic factors for DTC in children treated at our centre. The study included 109 children with DTC (aged 6-17 years). The primary treatment comprised total thyroidectomy in 81 cases, radioiodine therapy in 85 cases and TSH suppressive therapy with L-thyroxine in all patients. Uni- and multivariate analysis of prognostic factors for disease-free survival was performed using the Cox regression method. The actuarial survival rate was 100%, and the 5- and 10-year actuarial disease-free survival rates were 80% and 61% respectively. Univariate analysis revealed that older age, total thyroidectomy and radioiodine treatment had a positive impact on disease-free survival whereas there were no statistical differences with regard to the child's sex, histological type of cancer or lymph node status. On multivariate analysis, radical surgery was estimated to be the most significant factor (P=0.007) for disease-free survival, while less than total thyroidectomy increased the relative risk of relapse by a factor of 10. Radioiodine treatment decreased the relative risk of relapse by a factor of 5, but with borderline significance (P=0.07). Permanent postoperative complications were observed in 17% of children: in 11 laryngeal palsy occurred, in six there was hypoparathyroidism, and one suffered from both. It is concluded that total thyroidectomy and radioiodine treatment significantly improve recurrence-free survival in children and should be routinely applied even in young children as the primary treatment of DTC.