Sulfotransferase 2B1b, Sterol Sulfonation, and Disease

Abstract

The primary function of human sulfotransferase 2B1b (SULT2B1b) is to sulfonate cholesterol and closely related sterols. SULT2B1b sterols perform a number of essential cellular functions. Many are signaling molecules whose activities are redefined by sulfonation-allosteric properties are switched "on" or "off," agonists are transformed into antagonists, and vice versa. Sterol sulfonation is tightly coupled to cholesterol homeostasis, and sulfonation imbalances are causally linked to cholesterol-related diseases including certain cancers, Alzheimer disease, and recessive X-linked ichthyosis-an orphan skin disease. Numerous studies link SULT2B1b activity to disease-relevant molecular processes. Here, these multifaceted processes are integrated into metabolic maps that highlight their interdependence and how their actions are regulated and coordinated by SULT2B1b oxysterol sulfonation. The maps help explain why SULT2B1b inhibition arrests the growth of certain cancers and make the novel prediction that SULT2B1b inhibition will suppress production of amyloid β (Aβ) plaques and tau fibrils while simultaneously stimulating Aβ plaque phagocytosis. SULT2B1b harbors a sterol-selective allosteric site whose structure is discussed as a template for creating inhibitors to regulate SULT2B1b and its associated biology. SIGNIFICANCE STATEMENT: Human sulfotransferase 2B1b (SULT2B1b) produces sterol-sulfate signaling molecules that maintain the homeostasis of otherwise pro-disease processes in cancer, Alzheimer disease, and X-linked ichthyosis-an orphan skin disease. The functions of sterol sulfates in each disease are considered and codified into metabolic maps that explain the interdependencies of the sterol-regulated networks and their coordinate regulation by SULT2B1b. The structure of the SULT2B1b sterol-sensing allosteric site is discussed as a means of controlling sterol sulfate biology.

Graphical abstract

Fig. 1

SULT2B1b functions in cancer biology. The levels of SULT2B1b and cholesterol sulfate (CS) increase in many cancers (see II.A. Cancer). SULT2B1b (2B1b, aqua) is shown converting cholesterol (C) to CS in the upper right quadrant of the cancer cell. CS is transported to the extracellular environment by a wide range of potential transporters including members of the organic anion transporter family (OAT3, OAT4, OAT5, OAT6, OAT7, and/or OAT9) (Nigam et al., 2015) and ATP-binding cassette transporters subfamily C (ABCC1, ABCC3, and/or ABCC4) (Konig et al., 1999). CS is transported into mature killer T cells via ABCG2 transporter (Thurm et al., 2021). Upon entering a mature killer T cell, which does not express SULT2B1b, CS binds DOCK-2, a guanine nucleotide exchange factor (brown), which prevents it from binding and activating Ras-related C3 botulinum toxin substrate GTPase (RAC; green)—a GTPase critical for T-cell migration and tumor penetration. CS also binds the monomer form of the TCR (orange) transmembrane domain, which inhibits its dimerization and prevents it from forming a functional complex with MHC (green; antigen, red). Formation of the MHC:TCR complex initiates the immune response, resulting in the export of cytokines, including TNFα (blue trimer). The tumor necrosis factor α receptor (TNFR) is the central receptor of a massive complex (TNFR Complex), whose signaling switches between cell survival and apoptosis in response to CS levels, which regulate the activities of certain components of the complex, notably Fas-associated death domain protein (FADD) and inhibitor κB kinase (IKK). The TNFR complex is shown in the lower right segment of the cancer cell membrane in three signaling states (I–III) that differ in the activation status of FADD and IKK—activation is indicated by a color change to red. Addition of TNFα to Complex I forms Complex II and activates the proapoptosis pathway by enabling the so-called death domain of FADD to bind procaspase 8 (P8, purple oval/red square), causing it to autocleave to caspase 8 (Casp 8, purple), which, in turn, cleaves more caspases and commits the cell to apoptosis. Through an as yet unknown mechanism, CS shifts Complex II toward Complex III and, thus, biases signaling toward survival and away from apoptosis. The prosurvival pathway is initiated by the activation of IKK, which phosphorylates inhibitor κB (IκB, orange), causing release of nuclear factor κB (NF-κB, gray) and exposing its nuclear-localization signaling peptide. As a result, NF-κB translocates to the nucleus and alters gene expression to inhibit apoptosis and promote survival. Oxysterols (small concentric red and yellow circles, upper right quadrant of cancer cell) produced either by the cancer cell or obtained from systemic circulation can bind the LXR receptor (LXR, teal), causing it to heterodimerize with the RXR receptor (R, purple) and enter the nucleus. In the nucleus, LXR upregulates expression of proteins related to C export, including ABCA1 (lime green) and APOE, a key component of chylomicrons (multicolored brown circle). In addition, the dimer can bind and inactivate sterol regulatory element binding protein-2 (SREBP, teal), a major transcriptional activator of C biosynthesis. The slowed synthesis and enhanced elimination of C inhibits cell growth. SULT2B1b sulfonates oxysterols that bind tightly and inhibit LXR (gray LXR dimer), which increases C and stimulates growth of the cancer cell. Finally, SULT2B1b expression upregulates two transcription factors, specific protein 1 and activating protein 2 (not shown), that enhance expression of vascular endothelial growth factor α (VEGF, orange), which stimulates vascularization of the tumor. The mechanism linking SULT2B1b and VEGF is not known—perhaps SULT2B1b sulfonates an as yet unidentified molecule (blue) that signals upregulation of SP1 and/or AP2. This figure was created using BioRender (BioRender.com).

Fig. 2

SULT2B1b functions in Alzheimer disease. Cholesterol, which cannot cross the blood-brain barrier, is oxidized prior to transport into systemic circulation. 24HC (labeled red sphere, top of neuron) is produced by the CYP46A-catalyzed oxidation of cholesterol. CYP46A (lavender symbol) is found exclusively in the neuron, where it associates with the smooth endoplasmic reticulum. Upon entering the astrocyte, 24HC can be uploaded onto chylomicrons and transported into an adjoining vessel or bind the LXRβ receptor (LXR, teal), causing it to heterodimerize with the RXR receptor (R, purple). The heterodimer enters the nucleus, where it regulates scores of activities including stimulating 24HC export by upregulating the oxysterol transporter (ABCA1, lime green) and APOE, a key structural component of chylomicrons. The LXR dimer can also bind and inactivate the sterol regulatory element binding protein-1 (SREBP). SREBP (teal symbol, nucleus) is shown upregulating cholesterol biosynthesis and forming an inactive (gray) complex with the LXR dimer. SULT2B1b (aqua, nucleus) converts 24HC to 24HCS, which slows cholesterol biosynthesis by binding to and inactivating LXR (gray complex); thus, the 24HC/24HCS balance regulates both synthesis and export of cholesterol from the astrocyte. In the astrocyte cytosol, cholesterol is seen binding to the APP, a cholesterol sensor located in the cell membrane (purple and orange structures). Cholesterol binding shifts the APP cleavage-product peptide from a 40-mer to the prodisease 42-mer, fostering formation of Aβ plaques (circular brown aggregate). Microglial phagocytosis of Aβ plaques is upregulated by LXR and inhibited by 24HCS binding and inactivation of LXR. SULT2B1b in the astrocyte cytosol converts cholesterol to CS, both of which are provided by the astrocyte to the mature neuron via the ATP-binding cassette transporter-2 (ABCA2) (Ahmad et al., 2019), which does not synthesize cholesterol. Once in the neuron, CS binds protein kinase C (PKC, brown), causing it to hyperphosphorylate (red dots) tau protein fibrils, which aggregate into toxic neurofibrillary tangles. Elevated cholesterol levels in the neuron upregulate CYP46A, resulting in an increase in 24HC, which, once transported into astrocytes, increases 24HC export into systemic circulation and downregulates cholesterol biosynthesis. This intracellular circuitry allows a neuron to monitor its cholesterol levels and signal to the astrocyte, via 24HC, the need to adjust the levels. The AD metabolic map predicts that SULT2B1b inhibition will significantly decrease cholesterol levels, Aβ plaques, and tau protein aggregates. This figure was created using BioRender (BioRender.com).

Fig. 3

Strata of normal human epithelium. Strata are labeled and color coded: Corneum (blue/black), Granulosum (green), Spinosum (purple), and Basale (red). This figure was adapted from Sokol et al. (2015).

Fig. 4

Immunofluorescent localization of SULT2B1b in normal human epithelium. (A) Localization of filaggrin (green), a marker for the corneum/granulosum interface. Granulosum keratinocytes are shown in blue. The scale bar (white) corresponds to 50 µM. (B) Localization of SULT2B1b (red). (C) Superposition of (A) and (B). This figure was adapted from Heinz et al. (2017).

Fig. 5

SULT2B1b and SULT1A1 allosteric pockets. (A) The SULT2B1b oxysterol-binding site. The binding site consists primarily of three structural elements, the cap (red), base (teal), and N-terminus (blue), and is bound to Qu. PAPS and cholesterol (CH) are bound at the active site. (B) Closeup of the oxysterol-binding site. Quercetin directly contacts one residue from each structural element—these appear to be the only direct contacts between the allostere and enzyme. (C and D) Comparison of the SULT2B1b and SULT1A1 allosteric sites. Unlike SULT2B1b, the N-terminus of SULT1A1 [small blue sphere (C)] is not long enough to contact quercetin, and SULT1A1 contacts quercetin at four (rather than three) points—two in the cap, one in the base, and one in a small loop (yellow) that is not present in any member of the SULT2 family. As is evident, quercetin orients quite differently in the two enzymes—inward toward PAPS in the SULT2B1b structure (D) and along the perimeter of SULT1A1.