abolishes their binding to classical nuclear receptors, and considerably increases their polarity.
Sulfonated steroids are thus hardly able to penetrate biological membranes by passive diffusion.
As a result their distribution volume in the body is substantially reduced (Strott 1996, 2002).
Sulfonation of steroids increases their binding to plasma proteins, predominantly to albumin.
Nevertheless, sulfonated steroids are efficiently eliminated in the kidney, thus they are found at high concentrations in the urine (Dawson 2012).
Sulfonation abolishes the biological activity of steroids by preventing their binding to classical nuclear steroid receptors (Strott 1996). However, there is now clear evidence that sulfonated steroids may be more than just inactivated metabolites destined for excretion. They have longer half-time in plasma in comparison to their free counterparts and they may circulate in significantly higher concentrations. During the past three decades, especially from studies on steroid metabolism in human breast cancer tissue increasing evidence came up that free active steroids may be produced locally from sulfonated precursors in cells expressing the enzyme steroid sulfatase (“sulfatase pathway”; see chapter 2.8.3).
Whereas there is now a considerable number of studies on the role of the sulfatase pathway in the progression of estrogen-dependent breast cancer tissue including clinical trials using a sulfatase inhibitor (Nakata et al. 2003, Pasqualini 2004, Secky et al. 2013), virtually no information is available on the occurrence and role of sulfatase pathways in physiological settings with the exception of the situation in feto-placental unit in human pregnancy, where the production of estrogens in the syncytiotrophoblast largely depends on the provision of sulfonated C19-precursors by the fetal and maternal adrenals (Fritz & Speroff 2010).
The role of sulfonated precursors for human placental estrogen productions becomes obvious in cases of an inherited steroid sulfatase deficiency, which are characterized by very low levels of pregnancy-associated estrogens (Lykkesfeldt et al. 1984). Under physiological conditions the role of a sulfatase pathway may consist in the limitation of steroid effects to a subset of potentially responsive cells, which in addition to the specific receptor are characterized by the expression of steroid sulfatase and an uptake transporter, providing additional levels for local regulatory mechanisms. Moreover sulfonation of steroids enables the transport of high amounts
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of steroidal substrates from a producing tissue into a target cell without interfering with the function of other steroidogenic tissues. Different transporter proteins have been shown or suggested to mediate the transport of sulfonated steroids through the plasma membrane of cells, including the sodium-dependent organic anion transporter (SOAT, SLC10A6, Geyer et al.
2006), the sodium-independent organic anion transporting polypeptides (OATPs) and the ASBT (apical sodium-dependent bile acid transporter (Moitra et al. 2011).
As mentioned above, sulfonation of steroids abolishes their capacity of interacting with classical nuclear steroid receptors. However, they may have direct effects at other receptor types.
Sulfonation in the brain modulates the nongenomic actions of neurosteroids on GABA A, N-methyl-D-aspartate, glutaminergic and σ-opioid receptors, usually in opposing ways (Kríz et al. 2008). For example, pregnenolone sulfate is a picrotoxin-like antagonist, whereas unconjugated pregnenolone is a barbiturate-like agonist. In addition, DHEA sulfate stimulates acetylcholine release from the hippocampus but unconjugated DHEA does not. These findings may be relevant to the association of pregnenolone sulfate and DHEA sulfate with enhanced cognitive function in animals (Kríz et al. 2008). These findings, together with the detection of SULT1A1, SULT1E1, SULT2A1, SULT2B1 and steroid sulfatase in the fetal and adult brain, suggests that sulfonation and deconjugation of neurosteroids contributes to neurodevelopment and maintenance of brain function. Another cytosolic sulfotransferase, SULT4A1, is most highly expressed in selective regions of the brain. However, its substrate and physiological role is yet unknown (Liyou et al. 2003, Minchin et al. 2008).
Sulfonated steroids are possibly moved through the plasma membrane of cells by several different transporter proteins. The ATP binding cassette (ABC) proteins transporters are ubiquitously expressed and are mostly considered responsible for the efflux of steroid substrates, whereas SOAT and OATPs mediate tissue-specific bi-directional transport of steroid sulfates across the plasma membrane of cells (Geyer et al. 2006, Roth et al. 2012, Moitra et al.
2011). SOAT transports steroid sulfates, including estrone-3-sulfate, pregnenolone sulfate and DHEA sulfate. Four families of OATP (OATP1, OATP2, OATP3 and OATP4) have been shown to transport DHEA sulfate and estrone-3-sulfate. The OATP1 genes are expressed throughout the body, with highest expression levels for sub-family member OATP1A2 in the brain, liver, lung, kidney and testis. OATP1B1 and OATP1B3 are specifically expressed in the liver and OATP1C1 in the brain and testis. The OATP2 sub-family member OATP2B1 is expressed in numerous tissues, including liver, syncytiotrophoblasts of the placenta, mammary gland, heart, skeletal muscle and endothelial cells of the blood-brain barrier. The OATP3A1_v1
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transporter is expressed in the germ cells of the testis, as well as in the choroid plexus and frontal cortex. Two OATP4 sub-family members have been identified in the following tissues:
OATP4A1 in heart, lung, liver, skeletal muscle, kidney, pancreas and syncytiotrophoblasts in the placenta; whereas OATP4C1 is localized to the basolateral membranes of renal proximal tubules. Whilst certain sulfonated steroids (i.e. DHEA sulfate and estrone-3-sulfate) have been used to test the substrate specificity of the above ABC, SOAT and OATPs, further studies are required to investigate all known naturally occurring (as well as synthetic) steroid sulfate substrates (Dawson 2012).
2.7 Cytosolic sulfotransferases (SULTs)
SULTs sulfonate small endogenous and exogenous compounds, such as hormones, bioamines, drugs, and various xenobiotic agents. To date, 13 human cytosolic SULTs have been identified (Allali-Hassani et al. 2007). The significant substrate overlap between SULTs caused a lot of confusion in early naming schemes, and the nomenclature of these enzymes has a long and confusing history. Accordingly, in many older studies based on the measurement of enzyme activities only, it does not come clear which of the SULTs definitely contributed to the substrate conversions measured. Similarly, the significance of studies based on immunological methods (immunohistochemistry, Western blot) may have been impaired by the potential reactivity of the antibodies used with more than one of the structurally closely SULTs. Only after the availability of more precise biochemical, biophysical, and genetic methods this issue has been resolved (Chapman et al. 2004). However, many different names for the same enzyme may still be encountered in the literature.
The SULTs characterized so far have been recognized as structurally closely related members of a gene family. Their current nomenclature is based on 45% of the amino acids that members of this family have in common in their structure. Isoforms within a subfamily are labeled using Arabic numbers following the subfamily designation. In human, three SULT families have been characterized: (1) the SULT1 family (“phenol family”) including eight subfamilies: A1, A2, A3, A4, B1, C2, C4, and E1; (2) the SULT2 family (“hydroxysteroid family”) consisting of the SULTs 2A1 and 2B1; and (3) the “brain-specific” SULT4 family (Pasqualini 2009). An overview on human SULT superfamilies and some structural features and substrate preferences of individual SULTs are presented in Table 1.
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Table 1: The human cytosolic SULT superfamily (according to Pasqualini 2009).
SULT common
SULT1E1 hEST/-1 4q13.2 294 estrogens (high
affinity) 50.1
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Table 1 (continued): The human cytosolic SULT superfamily (according to Pasqualini 2009).
SULT common