Dimethylarginine dimethylaminohydrolase overexpression enhances insulin sensitivity

Abstract

Objective: Previous studies suggest that nitric oxide (NO) may modulate insulin-induced uptake of glucose in insulin-sensitive tissues. Asymmetrical dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase (NOS). We hypothesized that a reduction in endogenous ADMA would increase NO synthesis and thereby enhance insulin sensitivity.

Methods and results: To test this hypothesis we used a transgenic mouse in which we overexpressed human dimethylarginine dimethylaminohydrolase (DDAH-I). The DDAH-I mice had lower plasma ADMA at all ages (22 to 70 wk) by comparison to wild-type (WT) littermates. With a glucose challenge, WT mice showed a prompt increase in ADMA, whereas DDAH-I mice had a blunted response. Furthermore, DDAH-I mice had a blunted increase in plasma insulin and glucose levels after glucose challenge, with a 50% reduction in the insulin resistance index, consistent with enhanced sensitivity to insulin. In liver, we observed an increased Akt phosphorylation in the DDAH-I mice after i.p. glucose challenge. Incubation of skeletal muscle from WT mice ex vivo with ADMA (2 mumol/L) markedly suppressed insulin-induced glycogen synthesis in fast-twitch but not slow-twitch muscle.

Conclusions: These findings suggest that the endogenous NOS inhibitor ADMA reduces insulin sensitivity, consistent with previous observations that NO plays a role in insulin sensitivity.

Figure 1. Alterations in plasma ADMA, glucose, insulin, and IR index

Plasma ADMA levels (A) were reduced in DDAH transgenic mice. Furthermore, glucose (B) and insulin (C) levels trended lower in DDAH animals and the IR index (D) was significantly reduced by 63%. Values are mean ± SEM for 9 animals in each group. *p<0.05 DDAH vs. WT, †p<0.025 vs. WT.

Figure 2. Plasma ADMA increases after a glucose challenge

Basal ADMA plasma concentrations were significantly lower in DDAH animals. Plasma ADMA levels increased in WT animals following an intraperitoneal glucose challenge. The increase in plasma ADMA levels was blunted in the DDAH transgenic mice. Values are mean ± SEM for 9 animals in each group. WT = wild-type littermates, DDAH = DDAH-1 transgenic mice. Curve for DDAH animals is expressed as dashed line. *p<0.05 DDAH vs. WT (AUC analysis). †p<0.01 vs. WT, for each timepoint.

Figure 3. Effect of ADMA on glycogen synthesis in incubated EDL and soleus muscle

a. Exogenous ADMA (2μM) significantly reduced insulin-induced glucose conversion to glycogen in isolated EDL muscles ex vivo. Glucose incorporation into glycogen was significantly increased by insulin at all concentrations in vehicle-treated EDL muscle. By contrast, insulin-induced glucose incorporation was abrogated by ADMA except at the highest concentration of insulin. b. Soleus muscle, in contrast to EDL muscle, exhibited a higher basal incorporation of glucose into glycogen and overall a greater response to insulin. The effect of insulin to increase glycogen synthesis was unaffected by ADMA. Values are mean ± SEM. Control = C57BL/6J mice, ADMA = C57BL/6J mice with 2μM ADMA concentration in the incubation media. *p<0.05 ADMA vs. control,

Figure 3. Effect of ADMA on glycogen synthesis in incubated EDL and soleus muscle

a. Exogenous ADMA (2μM) significantly reduced insulin-induced glucose conversion to glycogen in isolated EDL muscles ex vivo. Glucose incorporation into glycogen was significantly increased by insulin at all concentrations in vehicle-treated EDL muscle. By contrast, insulin-induced glucose incorporation was abrogated by ADMA except at the highest concentration of insulin. b. Soleus muscle, in contrast to EDL muscle, exhibited a higher basal incorporation of glucose into glycogen and overall a greater response to insulin. The effect of insulin to increase glycogen synthesis was unaffected by ADMA. Values are mean ± SEM. Control = C57BL/6J mice, ADMA = C57BL/6J mice with 2μM ADMA concentration in the incubation media. *p<0.05 ADMA vs. control,

Figure 4. Glucose-stimulated insulin signaling in whole liver lysates

Western analysis reveals that hepatic Akt phosphorylation was increased to a greater degree in the DDAH transgenic mice 30 min post glucose challenge. This enhancement of insulin signaling was not due to changes in Jun N-terminal kinase (JNK) activation or inhibitory serine-phosphorylation of insulin receptor substrate-1 (IRS-1) (dns).