The gene for human SULT2B1, as a result of an alternative exon 1 and differential splicing, encodes for two mRNAs, i.e. SULT2B1a and SULT2B1. The use of exon 1A produces SULT2B1a, whereas to produce SULT2B1b exon 1B plus a portion of exon 1A is required. While SULT2B1a avidly sulfonates the steroid pregnenolone, SULT2B1b is the physiologic cholesterol sulfotransferase. SULT2B1b is selectively expressed in a tissue-specific manner e.g. skin, whereas SULT2B1a is essentially globally silenced. DNA analysis revealed that the proximal promoter regions of both SULT2B1 isoforms contain multiple CpG dinucleotides where cytosines are subject to methylation. SULT2B1a and SULT2B1b promoters in human cells that do not express these isoforms, are hypermethylated. In contrast, the proximal promoter of SULT2B1b in keratinocytes that do highly express this isoform, is completely unmethylated. Removal of the methyl groups leads to a striking induction of expression, whereas in vitro methylation of SULT2B1a and SULT2B1b promoter/reporter constructs markedly reduces promoter activity after transfection. Thus, expression of the SULT2B1 isoforms is regulated, at least in part, by methylation of CpG dinucleotides in their proximal promoter regions and suggests an explanation for both the global silencing of SULT2B1a as well as the tissue-specific expression of SULT2B1b. Similar to human and mouse genes the rat SULT2B1 gene consists of an alternative exon I; however, as a result of exonic rearrangement, the genic locations of exons IA and IB are reversed in the rat gene. Where exon IA is located downstream of exon IB in human and mouse SULT2B1 genes, in the rat SULT2B1 gene, exon IA is located upstream of exon IB. Furthermore, unlike the case with human and mouse SULT2B1 genes where differential splicing is necessitated since a portion of exon IA is fused with exon IB to complete the SULT2B1b mRNA, this step is not required with the rat gene. Especially interesting concerning the rearrangement of the rat SULT2B1 gene is that there is not just a relocation of exon IA to be upstream of exon IB, which is the reverse of the situation in human and mouse genes, but that only that portion of exon IA encoding for the unique amino terminus of the SULT2B1a isoform is relocated. This is opportune for otherwise the SULT2B1b protein would sustain a substantial amino acid deletion rendering it inactive. The part of exon IA encoding for common amino acid sequence of the two isoforms remains in the same relative gene position as it is in the human and mouse genes and becomes exon II in the rat gene. Cholesterol sulfate, which binds with high affinity to the retinoid-related orphan nuclear receptor alpha (RORalpha), induces expression for the barrier protein, filaggrin (a relatively small histidine-rich basic protein that is derived from profilaggrin, a large precursor containing 10-12 identical copies of the mature filaggrin protein arranged in tandem; profilaggrin is extensively phosphorylated and packaged into granules, which protects profilaggrin from proteolysis until filaggrin is needed for the aggregation of keratin filaments; dephosphorylation exposes profilaggrin to proteolysis resulting in the release of filaggrin monomers) when added to primary cultures of human keratinocytes (NHEK). Furthermore, RORalpha, SULT2B1b (cholesterol sulfotransferase) and filaggrin co-localize to the outer granular layer of the human epidermis suggesting a functional relationship. NHEK undergo terminal differentiation when subjected to an increased calcium concentration in the medium and under these conditions SULT2B1b, filaggrin and RORalpha are induced in a similar manner and time frame. Association of RORalpha with filaggrin production was suggested when expression of the gene for RORalpha by NHEK was inhibited by 95% using siRNA, which resulted in a parallel reduction in the expression of profilaggrin mRNA by 80%; furthermore, adding cholesterol sulfate to the medium failed to produce a recovery in the expression of profilaggrin mRNA. Additionally, knocking down the gene for SULT2B1b also led to a reduction in profilaggrin mRNA expression; however, in this case, profilaggrin expression could be successfully restored following addition of cholesterol sulfate to the medium. These studies strongly suggest that cholesterol sulfate produced by the SULT2B1b activates the gene for profilaggrin and does so via an interaction with RORalpha. This is the first demonstration of a molecular action for cholesterol sulfate that is reminiscent of a typical hormone. Oxysterols constitute a class of cholesterol derivatives that exhibit broad biological effects ranging from cytotoxicity to regulation of nuclear receptors such as LXR, which is involved in the regulation of genes engaged in fatty acid and cholesterol metabolism. The role of oxysterols such as 7-ketocholesterol (7-KC) in the development of retinal macular degeneration and atheromatous lesions is of particular interest but little is known of their metabolic fate. A major oxysterol found in atheromas as well as other tissues is 7-KC, which is known from cell culture studies to induce cell injury at concentrations present in vivo, and for this reason, there exists a particular focus on metabolic pathways that can lead to a reduction in its toxicity. We established that the steroid/sterol sulfotransferase, SULT2B1b, known to efficiently sulfonate cholesterol, also effectively sulfonates a variety of oxysterols including 7- KC. The cytotoxic effect of 7-KC on 293T cells was attenuated when these cells, which do not express SULT2B1b, were transfected with SULT2B1b cDNA. Importantly, protection from 7-KC-induced loss of cell viability with transfection correlated with synthesis of SULT2B1b protein and production of the 7-KC sulfoconjugate (7-KCS). Moreover, when 7-KCS was added to the culture medium of 293T cells in amounts equimolar to 7-KC no loss of cell viability occurred. Additionally, MCF-7 cells, which highly express SULT2B1b, were significantly more resistant to the cytotoxic effect of 7-KC. We extended the range of oxysterol substrates for SULT2B1b to include 7alpha/7beta-hydroxycholesterol and 5alpha,6alpha/5beta,6beta-epoxycholesterol as well as the 7alpha-hydroperoxide derivative of cholesterol. Thus, SULT2B1b by acting on a variety of oxysterols offers a potential pathway for modulating in vivo the injurious effects of these compounds. Importantly, we have also demonstrated by physiochemical means for the first time that the sulfoconjugate of 7-KC does, indeed, occur in vivo as demonstrated for human atheromatous tissue. It now appears that oxysterols have a broad range of biological effects and that SULT2B1b plays a significant role throughout the spectrum. SULT2B1b not only inactivates classes of oxysterols that are cytotoxic, it also inactivates classes of oxysterols involved in cell signaling.
Showing the most recent 10 out of 18 publications
