Effects of all trans retinoic acid on treatment of type 2 diabetic neuropathy

Abstract

Peripheral neuropathy is one of the most common complications in diabetes mellitus. Nerve growth factor (NGF) is believed to regulate peripheral and central nervous system, neuronal differentiation, and regeneration of damaged nerves, and their role in diabetic neuropathy has been emphasing these days. Retinoic acid increases the endogenous expression of NGF. The aim of the present study was to investigate the effects of all trans retinoic acid (ATRA) on diabetic neuropathy in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, an animal model of type 2 diabetes and the effects of ATRA on the expression of the NGF and NGF Receptors p75NGFR and trkA in dorsal root ganglion (DRG) cells. In order to evaluate the effect of ATRA, twenty OLETF rats were received with an oral dose of 10 mg/kg/day ATRA for 16 weeks, and twenty OLETF rats and ten Long-Evans Tokushima Otsuka (LETO) rats were received with the cellulose as a vehicle once a day for 16 weeks. At the end of the treatment period, blood glucose levels and contents of NGF in serum and sciatic nerve were measured. Neurometer current stimulus test was conducted to examine improvements of the current stimulus thresholds in rats. Nerve capillary density was measured in search of morphological changes secondary to neuropathy and regeneration. The effects of ATRA on the neurite growth from DRG cells exposed to NGF were assessed and mRNA of NGF and NGF receptors p75NGFR and trkA in response to ATRA concentration in DRG cells were measured by RT-PCR and real-time PCR. Fasting glucose levels were significantly higher in the ATRA-non-treated OLETF rats (178.5  38.3 mg/dL) than in the ATRA-treated OLETF rats (151.0  25.1 mg/dL) (P<0.01) or in the LETO rats (105.4  14.4 mg/dL) (P<0.01). A decrease in serum glucose was observed in the ATRA-treated OLETF rats when compared with the ATRA-non-treated OLETF rats. Contents of NGF in LETO rats were 1423.50 + 503.21 pg/mL in serum and 243.2  41.9 pg/g in nerve; in ATRA-non-treated OLETF rats the values were 683.71  169.89 pg/mL in serum and 150.0  28.8 pg/g in nerve; and in ATRA-treated OLETF rats the values were 1855.17  667.34 pg/mL in serum and 377.9  61.5 pg/g in nerve (P<0.05). A significant threshold increase was observed in both groups of OELTF rats when compared with the LETO rats, a significant decrease of threshold at 2000, 250 Hz was observed in ATRA-treated OELTF rats when compared with the ATRA-non-treated OLETF rats. Nerve capillary density was decreased in ATRA-non-treated OLETF rats (173.20 + 42.4 /mm2) when compared with LETO rats (262.78 + 34.1 /mm2); in contrast, it was increased in ATRA-treated OLETF rats (203.28 + 29.2 /mm2) when compared with ATRA-non-treated OLETF rats, but there was no statistical difference. Neurite outgrowth and length in DRG cells increased in NGF and ATRA dose dependent. The high glucose (>30 mM) inhibited the NGF dependent neurite formation. These results suggest that high glucose concentration inhibits neurite formation. In the DRG cell, ATRA concentration regulated gene expression of NGF within 24 hr in a dose dependent way. But ATRA did not increase the expression of the NGFR, p75NGFR and trkA mRNA levels.Our results have shown that ATRA treatment increases serum and nerve contents of NGF in OLETF rats and influence nerve cell regeneration by promoting the synthesis of the NGF and suggest that ATRA has a potential therapeutic role for diabetic neuropathy.