Identification of the Sigma-2 Receptor: Distinct from the Progesterone Receptor Membrane Component 1 (PGRMC1)

σRs are unique intracellular chaperone proteins [8] initially thought to be opioid receptor subtypes [9]. They have been classified into two subtypes based on specific radioligand binding assays using [3H](+)pentazocine for σ1Rs and [3H]1,3-di-o-tolylguanidine ([3H]DTG, in the presence of dextrallorphan to mask the σ1R) for σ2Rs in rat liver and kidney membranes [10]. Currently, the more selective σ1R ligand (+)-pentazocine has replaced dextrallorphan to mask the σ1R [7,11-14]. The σ1R has already been cloned as a 25-29 kDa chaperone protein composed of 223 amino acids [4,8,15]. It is widely distributed throughout the body [16-20]. Upon binding with agonists or under cellular stress, σ1Rs translocate from their primary endoplasmic reticulum (ER) location to different subcellular compartments where they can regulate ion channels and G-protein-coupled-receptor (GPCR) signaling [8,21-24]. In vivo functional studies on σ1Rs suggest that they play a substantial role in various cellular functions. Drugs acting at this receptor have been studied for their potential therapeutic effects in cancer, human immunodeficiency virus (HIV) infection, various psychiatric disorders, and substance abuse [1,25].

(CHO) cells have demonstrated that the selective σ 1 R ligands PRE-084 and (+)pentazocine can dose-dependently cause the dissociation of σ 1 R from a binding immunoglobulin protein/78 kDa glucose-regulated protein (BiP/GRP-78), another ER chaperone [8,26]. Thus, they serve as agonists. In contrast, the σ 1 R ligands haloperidol and 4-methoxy-3-(2phenylethoxy)-N,N-dipropylbenzeneethanamine (NE-100) alone do not affect the σ 1 R-BiP association but both completely inhibit the dissociation of σ 1 R from BiP caused by (+)pentazocine: they serve as antagonists [8,26]. In vivo, however, there is--as yet--no established functional assay for the σR subtypes. However, there is evidence showing a dose-dependent antagonism in vivo using the in vitro σ 1 R antagonists against the in vitro σ 1 R agonists using drug self-administration procedures [7,12,27,28]. Thus, it appears that the in vitro agonist-antagonist relationship will apply some in vivo responses.
Due to the lack of a known σ 2 R amino acid sequence, photoaffinity labeling remains the most viable approach for visualizing the receptor using sodium dodecyl sulfate (SDS) gels [29]. The basic principle is to covalently combine a photoactivatable σ 2 R-binding probe with the receptor such that the probe (radioactive-or fluorescent-labeled) remains with the protein even after denaturation with SDS [29]. Using a novel photoaffinity probe for σ 2 Rs, WC-21, a recent study identified the σ 2 R as the PGRMC1 in rat livers [3]. For example, the non-selective σ 1/2 R ligand DTG prevented the photolabeling of PGRMC1 (with WC-21) [3]. Further, an immunocytochemical study revealed that both PGRMC1 and (1R,3r,5S)-9-(10- , a fluorescent σ 2 R ligand, colocalize with molecular markers of the ER and mitochondria in HeLa cells [3]. As noted for the σ 1 R, studies utilizing various in vitro techniques indicated that σ 2 Rs are intracellular proteins.
However, the affinity of DTG for the PGRMC1 was not reported in the study [3].
Nonetheless, it appears that the identification of the σ 2 R as the PGRMC1 [3] has been accepted widely. However, two recent studies [1,2] demonstrated a more viable data set against this identification as follows:

4.
Progesterone has been reported to be a high-affinity (Kd=35 nM) ligand for PGRMC1 (Table 1). However, the Ki value of progesterone for the σ 2 R [4] is approximately 406-fold higher than the Kd value for PGRMC1 in rat liver membranes (Table 1). Further, the Ki value of DTG for the PGRMC1 is 472,000 ± 420,000 nM (Table 1) using cold (+)-pentazocine to block the σ 1 R [4], which is approximately 15,000-fold higher than that for the σ 2 R [4] (Table  1). However, the Ki value of DTG for the PGRMC1 [4] was shown to be >1,000-fold lower than that obtained in a previous study [37] (Table 1). This discrepancy likely results from the lack of use of a selective cold blocker at the σ 1 R in the previous study [37] since DTG can also bind the σ 1 R with high affinity ( Table 1). The binding profile of DTG for the PGRMC1 has been consistent with that for haloperidol, another non-selective σ1/2R ligand [4] ( Table 1). Thus, the PGRMC1 is not a high-affinity DTG binding site, which also means that the PGRMC1 is not the σ 2 R.
Together, these new data [4,5] clearly suggest that the σ 2 R and PGRMC1 are two different molecular entities. Furthermore, the photo affinity probe containing a σ 2 R-directing moiety that led to the identification of PGRMC1 [3] as the σ 2 R (with WC-21), likely binds both σ 2 R and PGRMC1. The identification of the σ 2 R as distinct from the PGRMC1 [4,5] should have considerable impact especially in the cancer study field since the σ 2 R has been developed as a biomarker for various tumor cells [38]. Other studies have attempted to examine the correlation between the binding affinity of various σR ligands and their ability to produce effects both in vitro and in vivo through the σ 2 R [35,39]. However, the evidence for σ 2 Rmediated actions from these studies is not compelling because of the mixed use of σR agonist-like and antagonist-like ligands. Thus, the pharmacology and physiological role of σ 2 Rs remain undetermined due to unsuccessful efforts to clone the receptor and a lack of activation of the σ 2 R resulted in the synthesis and release of dopamine in the rat brain [6,7]. Thus, future studies that further explore σ 2 R pharmacology may result in a better understanding of the dopamine-mediated reinforcing mechanism associated with stimulant abuse and other dopamine-related diseases (e.g. Parkinson's disease and schizophrenia).