Obesity-induced down-regulation of the mitochondrial translocator protein (TSPO) impairs placental steroid production

Abstract

Context: Low concentrations of estradiol and progesterone are hallmarks of adverse pregnancy outcomes as is maternal obesity. During pregnancy, placental cholesterol is the sole source of sex steroids. Cholesterol trafficking is the limiting step in sex steroid biosynthesis and is mainly mediated by the translocator protein (TSPO), present in the mitochondrial outer membrane.

Objective: The objective of the study was to investigate the effects of maternal obesity in placental sex steroid biosynthesis and TSPO regulation.

Design/participants: One hundred forty-four obese (body mass index 30-35 kg/m(2)) and 90 lean (body mass index 19-25 kg/m(2)) pregnant women (OP and LP, respectively) recruited at scheduled term cesarean delivery. Placenta and maternal blood were collected.

Setting: This study was conducted at MetroHealth Medical Center (Cleveland, Ohio).

Main outcome measures: Maternal metabolic components (fasting glucose, insulin, leptin, estradiol, progesterone, and total cholesterol) and placental weight were measured. Placenta (mitochondria and membranes separated) and cord blood cholesterol values were verified. The expression and regulation of TSPO and mitochondrial function were analyzed.

Results: Plasma estradiol and progesterone concentrations were significantly lower (P < .04) in OP as compared with LP women. Maternal and cord plasma cholesterol were not different between groups. Placental citrate synthase activity and mitochondrial DNA, markers of mitochondrial density, were unchanged, but the mitochondrial cholesterol concentrations were 40% lower in the placenta of OP. TSPO gene and protein expressions were decreased 2-fold in the placenta of OP. In vitro trophoblast activation of the innate immune pathways with lipopolysaccharide and long-chain saturated fatty acids reduced TSPO expression by 2- to 3-fold (P < .05).

Conclusion: These data indicate that obesity in pregnancy impairs mitochondrial steroidogenic function through the negative regulation of mitochondrial TSPO.

Figures

Figure 1.

Plasma estradiol and progesterone concentrations in lean and obese pregnant women and expression of rate limiting enzymes for their synthesis. Estradiol (A) and progesterone (B) plasma concentrations were measured in the study cohort (n = 234) as described in Table 1. C and D, 17β-HSD1 and 3β-HSD1 expression was assayed in RNA isolated from total placental tissue and normalized against β-actin. Results are expressed as fold changes against the lean group, considered as 1. Data are expressed as means ± SEM (n = 8 in each group) with *, P < .001.

Figure 2.

Cholesterol concentration in maternal and cord plasma and in placental cellular fractions. Maternal plasma (A; n = 50) and umbilical cord plasma (B; n = 50) total cholesterol concentrations are shown. Placental tissue was submitted to cell fractionation as described in Materials and Methods, and total cholesterol was measured in the cellular membranes (C; n = 12) and mitochondrial fractions (D; n = 12). Cholesterol content of each fraction of equal volume was measured as described in Materials and Methods and normalized against protein concentration in each sample. Data are expressed as means ± SEM with *, P < .05.

Figure 3.

Transporters involved in mitochondrial cholesterol trafficking. Protein expression of TSPO and the MLN64/StAR-related lipid transfer domain containing MLN64 was analyzed in the isolated mitochondria and total placental tissue, respectively. The STAR protein expression was verified in total placental tissue. MLN64 and TSPO but not STAR were detected in term human placenta. Protein lysates of mice adrenal glands and ovaries were used as a control for STAR expression. VDAC (loading control) expression was used for the validation mitochondria isolation. Ag, mice adrenal glands; Mito, mitochondria; Ov, mice ovaries.

Figure 4.

Effect of maternal obesity on the expression of placental cholesterol transporters and mitochondria biogenesis. Protein expression of TSPO (A) was characterized in isolated mitochondria and normalized against VDAC (loading control). The MLN64 protein expression (B) was characterized in total placental tissue lysates and normalized against β-actin. Upper panels represent the Western blots and lower graphs the densitometry analysis. Activity of the citrate synthase enzyme was analyzed in isolated mitochondria (C) to confirm mitochondria integrity and as an indirect method for quantification of mitochondria density. Mitochondrial DNA copy number, as a ratio of cytochrome B to β-actin was measured by real-time PCR (D). Results are expressed in obese vs lean group and bars are representative of 12 replicates in each group. Data are expressed as means ± SEM with *, P < .05. L, lean; Ob, obese.

Figure 5.

In vitro regulation of TSPO expression in primary trophoblasts. The TSPO mRNA expression was measured in cultured trophoblast cells treated with BSA (control) or LPS, or palmitic, myristic acid, and oleic acid for 24 hours. The TSPO expression was normalized using β-actin. The data are expressed as means ± SEM, and the statistically significant differences were determined by a Student's t test (*, P < .05.)