Oral route of mini-proinsulin-expressing Ganoderma lucidum decreases blood glucose level in streptozocin-induced diabetic rats

Oral administration of insulin to control blood glucose has become a hot area of diabetes research. The major issue is to produce enough insulin and prevent insulin degradation in the acidic environment of the stomach. We hypothesize that the natural structure of the cell wall and the endoplasmic reticulum in Ganoderma lucidum should help resist the digestion of insulin produced in these cells, making this fungus a feasible production and a new delivery system for oral insulin. A new mini-proinsulin gene was constructed and was transformed into G. lucidum by the Agrobacterium-mediated method. Driven by a strong promoter of GPD isolated from Lentinus edodes, expression of mini-proinsulin reached ≤10.4% of total soluble protein, equal to 174 µg/g wet weight, which is sufficient for oral route. Oral route of transgenic G. lucidum significantly reduced blood glucose in streptozocin-induced diabetic rats, as compared to rats fed with saline (P<0.0002) or non-transgenic G. lucidum (P<0.0002). Blood glucose was reduced ≤64% in 80% of diabetic rats. Evaluation of pancreatic pathology showed that 54.5% rats in the transgenic group had no pathological changes, as compared to 25% in the saline group and 33.3% in the non-transgenic group. Rats in transgenic group had insulitis score <2, while 30% of rats in the saline group had insulitis score >2. These experimental results indicate that oral route of mini-proinsulin-expressing G. lucidum can be used to control blood glucose in diabetes and to improve pancreatic cell function.