**1. Introduction**

Our previous human studies indicated a dysregulation of the tumor suppressor genes retinoic acid receptor responsive proteins (retinoic acid receptor responders, RARRES) 1 and 2 in the third trimester placentas complicated by preeclampsia (PE) and PE conjoined with intrauterine growth restriction (IUGR) [1,2]. We observed an induction of RARRES1 expression in primary villous cytotrophoblasts isolated from PE and PE/IUGR placentas with a concomitant increase in RARRES1 syncytial staining. RARRES2 mRNA expression, on the contrary, seemed reduced, ye<sup>t</sup> unaltered at the protein level in third trimester villous placental samples [1]. These results are controversial, as others have found increased RARRES2 protein expression in samples from total placentas in pregnancies complicated by PE [3]. Furthermore, we had previously determined that RARRES1 and 2 were located in distinct functional placental compartments [1]. RARRES1 (also known as Tazarotene-induced gene 1 (TIG1), Latexin-like (LXNL), or Phorbol Ester-induced gene 1 (PERG-1) [4]) was located in human villous and extravillous trophoblast cells (EVT) [1], while RARRES2 (also known as chemerin, HP10433, and TIG2 [5]) was

specifically expressed in human placental EVTs [1]. In contrast, Garces et al. described an additional placental RARRES2 expression in cytotrophoblasts and Hofbauer cells [6].

RARRES1 stimulates the expression of antioxidant enzymes, inhibits angiogenesis, and stimulates autophagy via mTOR [7]. In line with its proposed tumor suppressor function [8–10], RARRES1, along with RARRES2, was reduced in choriocarcinoma [2] and its expression was also significantly reduced in certain choriocarcinoma cell lines (i.e., Jeg-3 and BeWo) [1].

While RARRES1 is located intracellularly [10,11], RARRES2 is a secreted adipocytokine that requires activation of its pro-form by proteolytic cleavage to exert its functions via chemokine-like receptor 1 (CMKLR1, ChemR23) [12,13]. Wang et al. [3] were able to show that RARRES2 exerts anti-inflammatory functions by inducing endothelial nitric oxide synthase (eNOS) expression in human umbilical vein endothelial cells (HUVECs) and by significantly decreasing TNFα-induced nuclear factor (NF)-kappa B, and vascular cell adhesion molecule (VCAM)-1 production [3]. RARRES2 further modulates chemotaxis and activation of dendritic cells and macrophages via CMKLR1 [14,15], which is expressed in various leukocyte populations [16].

Pregnancy represents a state of constant metabolic adaptation and increased inflammation. In this respect, IUGR and PE represent two extreme gestational disturbances [17–19]. In PE the production of placental inflammatory cytokines [18,19] is increased. It is known that adipocytokine and interleukin signaling interact [20–22]. Recently IL-11, a member of the IL-6 family also known as adipogenesis inhibitory factor (AGIF) [23], has been found by others to be upregulated in PE and leads to inflammation and preeclampsia-like features in mice [24]. Treatment of mice with IL-11 negatively affects placentation, including trophoblast invasion and spiral artery remodeling, a key process in the pathogenesis of human PE [24,25]. IL-11 further increases systolic maternal blood pressure and leads to PE-like proteinuria in dams [24,25]. Mice with an eNOS-deficiency (eNOS−/− [26–29]) display PE-like features (e.g., vascular placental impairment [19,27,30] and an increased inflammatory state [31–34]).

To confirm this, we tested placental IL-11 expression in these mice. Moreover, we analyzed Rarres expression in second and third trimester placenta of eNOS−/− mice, because Garces et al. detected a maximum of placental Rarres2 expression at this gestational age in rodents [6].

To expand our findings from third trimester human placenta [1], we investigated Rarres1, Rarres2, and Cmklr1 expression in third trimester rodent placenta. Moreover, we analyzed the influence of maternal protein restriction in rats (IUGR-like features [35,36]) on placental Rarres1 and 2 expression. We additionally compared placentas in the context of fetal sex, given the di fferences in Rarres2 expression that were already described by Watts et al. [37] for male and female fetuses.
