Pluronic L-81 ameliorates diabetic symptoms in db/db mice through transcriptional regulation of microsomal triglyceride transfer protein

Abstract

Aim: To test whether oral L-81 treatment could improve the condition of mice with diabetes and to investigate how L-81 regulates microsomal triglyceride transfer protein (MTP) activity in the liver.

Methods: Genetically diabetic (db/db) mice were fed on chow supplemented with or without L-81 for 4 wk. The body weight, plasma glucose level, plasma lipid profile, and adipocyte volume of the db/db mice were assessed after treatment. Toxicity of L-81 was also evaluated. To understand the molecular mechanism, HepG2 cells were treated with L-81 and the effects on apolipoprotein B (apoB) secretion and mRNA level of the MTP gene were assessed.

Results: Treatment of db/db mice with L-81 significantly reduced and nearly normalized their body weight, hyperphagia and polydipsia. L-81 also markedly decreased the fasting plasma glucose level, improved glucose tolerance, and attenuated the elevated levels of plasma cholesterol and triglyceride. At the effective dosage, little toxicity was observed. Treatment of HepG2 cells with L-81 not only inhibited apoB secretion, but also significantly decreased the mRNA level of the MTP gene. Similar to the action of insulin, L-81 exerted its effect on the MTP promoter.

Conclusion: L-81 represents a promising candidate in the development of a selective insulin-mimetic molecule and an anti-diabetic agent.

Figure 1

L-81 reduced obesity. A: Body appearances of lean control (left), untreated db/db (middle), and db/db mice fed with chow sprayed with 1% L-81 (right); B: Weight of db/db mice or lean control mice receiving different drug treatments over an observation period of 30 d (n = 3 in each group); C: Plasma total cholesterol and (D) plasma triglyceride level of mice treated with the indicated drugs; E: The size of adipocytes in epididymal fat pads of mice treated with the indicated drugs as observed under 100 × magnification.

Figure 2

L-81 ameliorated diabetes. A: Food intake and (B) water intake of mice fed on a chow diet, with or without the indicated drugs (n = 3 in each group). C: Fasting plasma glucose levels of lean control mice or db/db mice treated with L-81 or rosig before and after a 21 d treatment of the indicated drugs; D: Glucose tolerance of mice treated with the indicated drugs. Mice injected with glucose had their blood glucose level monitored by Onetouch Ultra Blood Glucose Monitoring system. E: Plasma insulin level of db/db mice treated with the indicated drugs for 4 wk; F: Plasma adiponectin level of db/db mice treated with the indicated drugs for 4 wk. ^aP < 0.05 compared with untreated control.

Figure 3

L-81 regulated MTP transcription. A: The human MTP promoter was linked with luciferase reporter. Deletion or mutation constructs of MTP promoter-luciferase were transfected into HepG2 cells. The cells were treated with (black bars) or without (white bars) 1% L-81 in the medium. The luciferase activity in the cell lysate was measured. xxx = disabling mutations; B: L-81 inhibited apoB secretion in HepG2 cells. HepG2 cells were treated with the indicated concentration of L-81. The concentration of apoA-I and apoB in the culture medium were measured and normalized against the untreated control.

Figure 4

L-81 was not toxic to the liver. A: Plasma ALT level of mice treated with the indicated drugs; B: CTA test of mice treated with the indicated drugs; C: Histological examinations of db/db mice treated with indicated drugs. ^aP < 0.05 compared with untreated db/db.