OCT1 Expression in adipocytes could contribute to increased metformin action in obese subjects

Abstract

Objective: Metformin has been well characterized in vitro as a substrate of liver-expressed organic cation transporters (OCTs). We investigated the gene expression and protein levels of OCT-1 and OCT-2 in adipose tissue and during adipogenesis and evaluated their possible role in metformin action on adipocytes.

Research design and methods: OCT1 and OCT2 gene expressions were analyzed in 118 adipose tissue samples (57 visceral and 61 subcutaneous depots) and during human preadipocyte differentiation. To test the possible role of OCT1 mediating the response of adipocytes to metformin, cotreatments with cimetidine (OCT blocker, 0.5 and 5 mmol/l) and metformin were made on human preadipocytes and subcutaneous adipose tissue (SAT).

Results: OCT1 gene was expressed in both subcutaneous and visceral adipose tissue. In both fat depots, OCT1 gene expression and protein levels were significantly increased in obese subjects. OCT1 gene expression in isolated preadipocytes significantly increased during differentiation in parallel to adipogenic genes. Metformin (5 mmol/l) decreased the expression of lipogenic genes and lipid droplets accumulation while increasing AMP-activated protein kinase (AMPK) activation, preventing differentiation of human preadipocytes. Cotreatment with cimetidine restored adipogenesis. Furthermore, metformin decreased IL-6 and MCP-1 gene expression in comparison with differentiated adipocytes. Metformin (0.1 and 1 mmol/l) decreased adipogenic and inflammatory genes in SAT. OCT2 gene expression was not detected in adipose tissue and was very small in isolated preadipocytes, disappearing during adipogenesis.

Conclusions: OCT1 gene expression and protein levels are detectable in adipose tissue. Increased OCT1 gene expression in adipose tissue of obese subjects might contribute to increased metformin action in these subjects.

FIG. 1.

OCT1, FASN, ACC1, PPARγ and adiponectin gene expression during differentiation of human preadipocytes from obese (A) and lean subjects (B). *P < 0.05 vs. day 0.

FIG. 2.

OCT1 and FASN protein levels in differentiated and nondifferentiated preadipocytes at day 14. *P < 0.05 vs. nondifferentiated preadipocytes.

FIG. 3.

FASN, ACC1, PPARγ, adiponectin, OCT1, FABP4, IL-6, MCP-1 gene expressions after metformin (5 mmol/l) and cimetidine (0.5 and 5 mmol/l) cotreatments in differentiated human adipocytes. *P < 0.05 vs. differentiated adipocytes; +P < 0.05 vs. metformin (5 mmol/l) treatment.

FIG. 4.

Oil red O staining and LDH activity in differentiated human adipocytes, and after metformin (5 mmol/l) and cimetidine (0.5 and 5 mmol/l) cotreatment. *P < 0.05 vs. differentiated adipocytes; +P < 0.05 vs. metformin (5 mmol/l) treatment. (A high-quality color representation of this figure is available in the online issue.)

FIG. 5.

Effects of metformin (5 mmol/l) and cimetidine (0.5 and 5 mmol/l) cotreatment on ^p172ThrAMPK, ^p79SerACC1, and ACC1 concentrations. *P < 0.05 vs. differentiated adipocytes; +P < 0.05 vs. metformin (5 mmol/l) treatment.

FIG. 6.

FASN, ACC1, PPARγ, adiponectin, OCT1, IL-6, MCP-1 gene expressions after metformin (0.1 and 1 mmol/l) in subcutaneous adipose tissue explants. *P < 0.05 vs. control (vehicle) treatment.

FIG. 7.

OCT1 relative gene expression in both visceral and subcutaneous adipose tissue according to obesity status.

FIG. 8.

A: OCT1 protein levels in subcutaneous adipose tissue according to obesity status. Non-normalized OCT1 protein and normalized for β-Actin values (relative levels) are shown. B: OCT1 gene expression in stromovascular cells and mature adipocytes from subcutaneous adipose tissue.