Selective regulation of cellular and secreted multimeric adiponectin by antidiabetic therapies in humans

Abstract

Adiponectin, an insulin-sensitizing factor secreted from adipose tissue, is decreased in individuals with type 2 diabetes (T2D) and increased in response to thiazolidinedione (TZD) therapy. Changes in its secretion and assembly into higher-order forms affect insulin sensitivity. To determine the relative potency of TZDs on intra-adipocyte multimerization and secretion of adiponectin, we assessed the impact of in vivo low- or high-dose rosiglitazone treatment alone or combined with metformin in subjects with T2D. T2D subjects received high-dose rosiglitazone (8 mg/day), high-dose metformin (2,000 mg/day), or low-dose combination rosiglitazone-metformin therapy (4 mg + 1,000 mg/day) for 4 mo. All subjects were then switched to high-dose rosiglitazone-metformin combination therapy (8 mg + 2,000 mg/day) for another 4 mo. Low-dose rosiglitazone increased serum adiponectin, whereas the high dose increased both adipocyte content and serum adiponectin levels. TZDs selectively increased the percentage of circulating adiponectin in the potent, high-molecular-weight (HMW) form. No TZD effects were evident on multimer distribution in the cell. Expression of the chaperone protein ERp44, which retains adiponectin within the cell, was decreased by TZD treatment. No changes occurred in Ero1-Lalpha expression. Metformin had no effect on any of these measures. Increases in adiponectin correlated with improvements in insulin sensitivity. In vivo, TZDs have apparent dose-dependent effects on cellular and secreted adiponectin. TZD-mediated improvements in whole body insulin sensitivity are associated with increases in circulating but not cellular levels of the HMW adiponectin multimer. Finally, TZDs promote the selective secretion of HMW adiponectin, potentially, in part, through decreasing the expression of the adiponectin-retaining protein ERp44.

Fig. 1.

Study design.

Fig. 2.

Changes in circulating adiponectin levels (A) and adipocyte adiponectin protein content (B) following treatment with high-dose rosiglitazone (R), high-dose metformin (M), low-dose combined treatment (r+m), and following conversion of all subjects to high-dose combination (R+M) treatment. Results are expressed as absolute difference from baseline value (see Table 1) average + SE. For serum measurements, n = 10, 9, 11, and 34 for R, M, r+m, and R+M, respectively. For adipocytes, n = 10, 10, 13, and 33, respectively. *P < 0.05 vs. paired baseline value.

Fig. 3.

Representative Western blots of adiponectin multimerization in adipocyte (left) and serum (right) treatment effects. Samples were collected at baseline (B) and after R and R+M treatments. Samples are of electrophoresis under nonreducing conditions, as detailed in materials and methods.

Fig. 4.

Changes in percent total adiponectin in the HMW form in serum (A) and adipocytes (B) following treatment as described in Fig. 1. Results are expressed as absolute difference from baseline value (see Table 1) and are average + SE; n = 8 each for R, M, and r+m; n = 24 for R+M. *P < 0.05 vs. paired baseline.

Fig. 5.

Changes in adipocyte protein expression of Ero1-Lα (A) and ERp44 (B) following treatment as described in Fig. 2. Results are expressed as absolute difference from baseline value (see Table 1). Representative autoradiograph is shown above data. Results are average + SE; n = 8 each for R, M, and r+m; n = 23 for R+M. *P < 0.05 vs. paired baseline.