Effects of monoglycerides on P-glycoprotein: modulation of the activity and expression in Caco-2 cell monolayers

The purpose of this study was to analyze the effects of two common monoglyceride components of lipid excipients, 1-monoolein and 1-monostearin, on the activity and expression of P-glycoprotein (P-gp) in Caco-2 cells. Non-cytotoxic concentrations of 1-monoolein and 1-monostearin were determined by assessing membrane permeability and mitochondrial activity in Caco-2 cells, a human colon adenocarcinoma cell line. Concentrations of 500 and 100 microM were used to evaluate P-gp activity through Rh123 accumulation and bifunctional transport studies. The P-gp protein expression levels were quantified through the use of immunoblots. The changes in cell membrane fluidity and nuclear membrane integrity upon the addition of monoglycerides were analyzed by fluorescence anisotropy using DPH and TMA-DPH as the fluorescent labels and by using increasing salt concentrations to release the nuclear contents, respectively. The absorptive flux (apical to basolateral) in the bifunctional transport studies was not found to be statistically significant for the non-cytotoxic concentrations of 1-monoolein and 1-monostearin. However, treatments of 500 and 100 microM of 1-monoolein or 1-monostearin displayed statistically lowered efflux (basolaterial to apical, P < 0.05) compared to the controls (7.9 +/- 0.8, 12.9 +/- 2.6 x 10 (6) cm/s for 1-monoolein or 11.1 +/- 2.0, 11.4 +/- 2.3 x 10 (6) cm/s for 1-monostearin, respectively, compared to the untreated control, 21.1 +/- 2.9 x 10 (6) cm/s, n = 5). Rh123 accumulation was also found to be enhanced upon 24 h incubation with both concentrations of the monoglycerides; however, only concentrations of 500 muM of the monoglycerides were shown to significantly reduce the P-gp protein expression. The results from this study suggest that these two monoglycerides, common components in various lipid excipients, are inhibitors of P-gp.