Long term reduction in hyperglycemia in STZ-induced diabetic mice by virtue of HK M. Aurum preventive treatment : possible effects on glucose utilization, mitochondrial uncoupling and oxidative stress in liver and skeletal muscle

Abstract

Insulin presents the only efficient symptom-treating strategy to treat diabetic patients. However, it doesn’t come without its negative long term side effects, notably hypoglycemia, which can be life threatening specially in patients receiving intense treatment. In this regard, scientists aim to develop new strategies to prevent or treat diabetes by harvesting the potential beneficial immunomodulatory properties of mycobacterial species. In fact, vaccination with the Bacillus Calmette-Guerin (BCG), an attenuated live strain of Mycobacterium bovis (M. bovis) resulted in a reduction of hyperglycemia in both diabetic patients and diabetic murine models. A novel heat-killed (HK) whole cell preparation of M. aurum is currently being developed and investigated for its potential immunomodulatory properties. HK M. aurum was found to be phylogenetically related to M. bovis and thus we speculate that they might share similar immune properties. The aim of this study is to investigate whether HK M. aurum pre-treatment can reduce hyperglycemia in STZ-induced (Streptozotocin) diabetic mice. We also assess glucose transport and utilization in addition to mitochondrial uncoupling and oxidative stress in the liver and skeletal muscle, essential organs for glycemic control, in diabetic mice pre-treated with multiple doses of HK M. aurum. HK M. aurum does not induce hypoglycemia in normo-glycemic and healthy conditions. Pre-treatment with HK M. aurum was found to be able to reduce hyperglycemia and urine glucose levels in STZ-induced diabetic mice. However, it had no effect on preventing weight loss. These results reveal an important property highlighting HK M. aurum’s safety and its conditional glucose-lowering ability that is only active in hyperglycemic conditions. Pre-treatment with HK M. aurum was found to prevent the STZ-induced reduction in hepatic α-lactate dehydrogenase (α-LDH) levels but was unable to rescue the STZ-induced reduction in GLUT2 expression. In other words, the STZ-induced increase in hepatic glucose production, which accentuates hyperglycemia in diabetic mice, was found to be possibly prevented by HK M. aurum pre-treatment. In skeletal muscle, the STZ-induced up-regulation of α-LDH was not prevented by HK M. aurum pre-treatment, an indication that HK M. aurum may not be able to prevent the exacerbation in glycogen breakdown and lactate build-up observed in diabetic conditions. The expression of GLUT4 in skeletal muscle, which should reflect its ability to take-up excess blood glucose, was inconclusive since type II muscle fibers which are insensitive to insulin might blur the possible changes in GLUT4 expression in response to insulin insufficiency as well as HK M. aurum vaccination. Furthermore, HK M. aurum pre-treatment prevented the STZ-induced up-regulation of UCP3 levels in skeletal muscle, which indicates that HK M. aurum might have prevented lipid peroxidation and overproduction of ROS in skeletal muscle of diabetic mice. Despite the fact that HK M. aurum did not avert the reduction in ROS levels in the liver of STZ-induced diabetic mice, it succeeded in rescuing the STZ-induced decrease in hepatic UCP2 levels. We speculate that HK M. aurum pre-treatment seem to prevent the possible cutback in ATP production in the liver of diabetic mice, allowing for uncoupling via UCP2 to occur, while an STZ-induced ATP deprived state might not allow for such uncoupling. In addition, hepatic catalase expression was found to be similar in non-diabetic, as well as in diabetic mice pre-treated with saline or HK M. aurum. These findings set the stage for further exploration of the potential of HK M. aurum and other HK mycobacterial species in treating or preventing such metabolic or even auto-immune diseases.