HSD17B1 Human

What is HSD17B1 and what is its primary function in human steroid metabolism?

HSD17B1 (hydroxysteroid 17β-dehydrogenase type 1) is an enzyme that catalyzes the conversion of estrone (E1) to the more potent estradiol (E2). While the reaction is theoretically reversible in vitro, physiological conditions favor the production of estradiol, making this conversion the predominant reaction . The enzyme is encoded by the HSD17B1 gene located on chromosome 17q11-q21 . This conversion represents the last step in estrogen activation, making HSD17B1 a critical component in estrogen biosynthesis pathways . The enzyme functions by utilizing NADPH as a cofactor to reduce the 17-keto group of estrone to produce the 17β-hydroxyl of estradiol, significantly increasing estrogen potency at the receptor level.

How does HSD17B1 structure relate to its function?

The structure of HSD17B1 has been elucidated through crystallography studies, revealing important insights into its catalytic mechanism. Crystal structures of human HSD17B1 complexed with estrone (E1) and NADP+ have demonstrated critical enzyme-substrate-cofactor interactions . The enzyme contains a binding pocket that accommodates the steroid substrate and the NADP(H) cofactor. The active site includes key residues such as His221, which plays a significant role in substrate binding and catalysis . Interestingly, structural analysis has revealed that estrone can bind in a reverse orientation in the active site, which explains the substrate inhibition phenomenon observed with this enzyme. This reverse binding leads to the formation of a dead-end complex, which serves as a regulatory mechanism for enzymatic activity .

What is the tissue distribution of HSD17B1 in humans?

HSD17B1 is predominantly expressed in steroidogenic tissues, with highest expression in the placenta, ovaries, and breast tissue. In breast tissue, HSD17B1 expression contributes to local estrogen production, which is particularly significant in postmenopausal women when ovarian estrogen production has ceased. The expression pattern suggests its importance in reproductive tissues and hormone-responsive organs. In pathological conditions such as breast cancer, HSD17B1 expression can be elevated, contributing to increased local estrogen production and potentially driving tumor growth . Studies using transgenic models have shown that universal overexpression of human HSD17B1 leads to increased estrone to estradiol conversion in various tissues, including mammary gland .

How do polymorphisms in HSD17B1 affect Alzheimer's disease risk?

Genetic variants in HSD17B1 have been associated with altered risk and earlier onset of Alzheimer's disease (AD), particularly in women with Down syndrome. A study of 238 women with Down syndrome followed over 4.5 years identified specific single-nucleotide polymorphisms (SNPs) in the HSD17B1 gene region that were associated with elevated AD risk . Specifically, women homozygous for the minor allele at three SNPs - in intron 4 (rs676387), exon 6 (rs605059), and exon 4 in COASY (rs598126) - showed a two- to threefold increased risk of developing AD and earlier age of onset .

The TCC haplotype, based on the risk alleles for these three SNPs, conferred an almost twofold increased risk of developing AD (hazard ratio = 1.8, 95% CI, 1.1–3.1) . These findings support the neuroprotective role of estrogen in the brain and suggest that genetic variations affecting estrogen metabolism and activity can influence neurodegenerative disease susceptibility. The mechanism likely involves altered enzyme activity affecting local estrogen levels in brain tissue, which could impact estrogen's known neurotrophic and neuroprotective effects, including increasing cholinergic activity, antioxidant activity, and protection against beta-amyloid neurotoxicity .

What is the role of HSD17B1 in breast cancer development and progression?

HSD17B1 plays a significant role in breast cancer by enhancing local estrogen production within tumor tissue. Estradiol is a potent growth stimulator for estrogen receptor-positive breast cancers, and increased HSD17B1 activity can lead to elevated local estradiol concentrations that promote tumor growth . Research has demonstrated that HSD17B1 expression enhances estrogen sensitivity of breast cancer cells. In experiments with MCF-7 breast cancer cells stably transfected with human HSD17B1, the enzyme efficiently converted estrone to estradiol and enhanced estrogen-dependent growth both in vitro and in vivo .

Transgenic mice universally overexpressing human HSD17B1 (HSD17B1TG mice) showed increased estrone to estradiol conversion in mammary gland tissue and enhanced estrogen receptor signaling . These mice developed extensive lobuloalveolar development that increased with age, along with elevated serum prolactin concentrations. Notably, at old age, these transgenic females developed mammary cancers . When HSD17B1 expression was restricted to mammary tissue, it induced lesions at the sites of ducts and alveoli, accompanied by inflammation and disruption of the myoepithelial cell layer . These lesions were shown to be estrogen-dependent, as treatment with the antiestrogen ICI 182,780 reversed the phenotype even after lesions were established .

What is the significance of the HSD17B1/HSD17B2 ratio in cancer prognosis?

The ratio between HSD17B1 and HSD17B2 has emerged as an important prognostic indicator in breast cancer. While HSD17B1 converts estrone to the more active estradiol, HSD17B2 catalyzes the opposite reaction, converting estradiol back to estrone. Research has shown that patients with low HSD17B1 to HSD17B2 ratio have a better prognosis than those with a higher ratio .

Studies conducted in laboratories have demonstrated that a low HSD17B1 to HSD17B2 ratio is a positive predictive marker for tamoxifen treatment response . This ratio appears to reflect a more normal-like tissue phenotype, with reduced local estrogen activity. Treatment of breast cancer cell lines with dihydrotestosterone (DHT) was found to promote this more favorable phenotype by reducing HSD17B1 and increasing HSD17B2 expression . This effect was observed to be largely androgen receptor (AR)-dependent, as the changes in expression were reduced following treatment with the AR antagonist hydroxyflutamide . These findings suggest that the balance between these two enzymes is critical in determining local estrogen concentrations and subsequently influencing tumor growth and response to therapy.

How can researchers create experimental models to study HSD17B1 function in vivo?

Several experimental approaches have been developed to study HSD17B1 function in vivo. One effective approach is the creation of transgenic mouse models overexpressing human HSD17B1. Such models have been instrumental in understanding the enzyme's role in various tissues and disease contexts . For example, transgenic mice universally overexpressing human HSD17B1 (HSD17B1TG mice) have been used to investigate the significance of this enzyme in mammary gland development and tumorigenesis .

Another approach involves transplantation techniques, where HSD17B1-expressing mammary epithelium is transplanted into cleared mammary fat pads of wild-type females. This method allows for investigation of the effects of local HSD17B1 expression on mammary gland estradiol production, epithelial cells, and the myoepithelium . This technique is particularly valuable for studying tissue-specific effects while avoiding systemic alterations that might occur in whole-body transgenic models.

Cell line models also provide important tools for HSD17B1 research. Stably transfected breast cancer cell lines such as MCF-7, T-47D, and ZR-75-1 expressing human HSD17B1 have been used to study the enzyme's effects on estrogen-dependent growth both in vitro and in xenograft models . These cell-based models can be implanted into immunodeficient mice to create xenograft tumors that allow for investigation of HSD17B1's role in tumor growth and response to treatments, including specific HSD17B1 inhibitors .

What methods are available for measuring HSD17B1 enzyme activity?

Several methodologies have been developed to assess HSD17B1 enzyme activity in research settings. In vitro enzymatic assays typically measure the conversion of radiolabeled or isotope-labeled estrone to estradiol. These assays can be performed with purified enzyme, cellular extracts, or intact cells expressing HSD17B1. The products are usually separated by chromatography methods and quantified using scintillation counting or mass spectrometry.

In cell-based models, researchers often measure HSD17B1 activity by incubating cells with estrone and then quantifying the estradiol production using techniques such as enzyme immunoassays, radioimmunoassays, or liquid chromatography-mass spectrometry (LC-MS). This approach has been used with stably transfected breast cancer cell lines to demonstrate enhanced conversion of estrone to estradiol compared to non-transfected controls .

For in vivo studies, transgenic mouse models expressing human HSD17B1 have been used to demonstrate increased estrone to estradiol conversion in target tissues such as the mammary gland . Tissue samples can be collected and analyzed for local estradiol concentration using sensitive assay methods. Additionally, the biological effects of enhanced estrogen action can be assessed through evaluation of estrogen-responsive gene expression using techniques such as quantitative PCR or by measuring estrogen response element (ERE)-driven reporter activity .

What approaches exist for inhibiting HSD17B1 activity in research settings?

Several approaches have been developed for inhibiting HSD17B1 activity in research contexts. Specific small molecule HSD17B1 inhibitors have been designed and tested both in vitro and in vivo. These compounds typically compete with the natural substrate for binding to the enzyme active site or interfere with cofactor binding. Research has demonstrated that specific HSD17B1 inhibitors can markedly inhibit the estrogen-dependent growth of HSD17B1-expressing xenografts in the presence of estrone .

In one study, administration of a specific HSD17B1 inhibitor at 5 μmol/kg per day significantly reduced the tumor size of HSD17B1-expressing MCF-7 xenografts by 59.8% compared to untreated tumors . Importantly, this inhibition was specific to the tumor tissue and did not affect uterine growth in the mice, suggesting tissue-specific efficacy . This was consistent with the induction of apoptosis observed in the tumors.

Other approaches include genetic knockdown strategies using RNA interference techniques such as siRNA or shRNA targeting HSD17B1 mRNA. These approaches have been used in cell culture models to reduce HSD17B1 expression and assess the subsequent effects on estrogen metabolism and cellular responses.

A novel approach to HSD17B1 inhibition emerges from structural studies revealing the mechanism of substrate inhibition. Research has demonstrated that estrone can bind in a reverse orientation in the enzyme active site, forming a dead-end complex . This finding suggests a new inhibitor design strategy based on compounds that promote dead-end complex formation, which could efficiently target estrogen-dependent diseases .

How do steroid hormones regulate HSD17B1 expression?

Steroid hormones play significant roles in regulating HSD17B1 expression, with different hormones exerting distinct effects. Research has investigated the effects of both estrogens and androgens on HSD17B1 expression in various cell models.

Dihydrotestosterone (DHT), a potent androgen, has been shown to regulate HSD17B1 expression in breast cancer cell lines. Studies in ERα- and AR-positive cell lines (ZR-75-1, MCF7, and T-47D) demonstrated that DHT treatment resulted in a small but significant reduction of HSD17B1 expression . The effect varied between cell lines and treatment durations, with ZR-75-1 showing reduction at 24 and 48 hours, MCF7 showing reduction only at early time points, and T-47D showing reduction at 48 hours and 7 days .

Interestingly, while DHT reduced HSD17B1 expression, it markedly increased the expression of HSD17B2 (which catalyzes the opposite reaction) in all tested cell lines at 48 hours and 7 days . This dual regulation promotes a phenotype resembling healthy tissue by increasing the HSD17B2 to HSD17B1 ratio, which would reduce local estrogen activity. This effect appears to be largely androgen receptor-dependent, as AR-negative cell lines showed different responses, and treatment with the AR antagonist hydroxyflutamide reduced these effects .

What microRNAs regulate HSD17B1 expression?

Studies have investigated multiple miRNAs predicted to influence HSD17B1 expression. In particular, miR-17, miR-210, miR-7-5p, and miR-1304-3p have shown significant effects on HSD17B1 expression in breast cancer cell lines . Treatment with miR-17 resulted in substantial upregulation of HSD17B1 to 203%, 556%, and 554% in ZR-75-1, MCF7, and T-47D cells respectively, compared to control . In contrast, miR-210 showed variable effects, resulting in 140% expression in ZR-75-1 cells but decreasing expression to 45% and 40% in MCF7 and T-47D cells respectively .

Similarly, miR-7-5p treatment led to 135% HSD17B1 expression in ZR-75-1 cells but reduced expression to 77% in MCF10A cells, with no significant change in T-47D cells . These findings highlight the complex and cell type-specific regulation of HSD17B1 by miRNAs, which may contribute to the differential expression patterns observed in normal versus cancerous tissues.

What signaling pathways influence HSD17B1 transcriptional regulation?

The transcriptional regulation of HSD17B1 involves multiple signaling pathways, particularly those related to steroid hormone receptors. The androgen receptor (AR) pathway appears to play a significant role, as demonstrated by the effects of dihydrotestosterone on HSD17B1 expression . These effects were diminished when cells were treated with the AR antagonist hydroxyflutamide, confirming AR involvement .

In addition to classical hormone receptor signaling, inflammatory pathways may also influence HSD17B1 expression. Research with transgenic mice has shown that HSD17B1 expression can induce inflammation-aided rupture of mammary ducts . This suggests potential feedback mechanisms where inflammatory signals might further modulate HSD17B1 expression, creating a cycle that could contribute to disease progression in contexts such as breast cancer.

What is the mechanism of substrate inhibition observed in HSD17B1?

A fascinating aspect of HSD17B1 biochemistry is its substrate inhibition mechanism, which has been elucidated through crystallographic studies. Unlike other 17β-HSD members, 17β-HSD1 undergoes significant substrate-induced inhibition . Crystal structures of human 17β-HSD1 in complex with estrone (E1) and the cofactor analog NADP+ have revealed the structural basis for this phenomenon .

These studies demonstrated that estrone can bind in a reverse orientation in the enzyme active site, leading to the formation of a non-productive or "dead-end" complex . Histidine 221 (His221) was identified as the key residue responsible for reorganizing and stabilizing this reversely bound estrone . This mechanism represents a form of enzyme self-regulation that exists widely in NADP(H)-preferred enzymes .

This discovery has significant implications for understanding the physiological regulation of estrogen metabolism and opens new avenues for inhibitor design. Specifically, compounds that promote the formation of dead-end complexes could serve as novel inhibitors of 17β-HSD1 activity, potentially offering more selective approaches for targeting estrogen-dependent diseases .

How does HSD17B1 contribute to inflammation in hormone-responsive tissues?

Research using transgenic mouse models has revealed an intriguing relationship between HSD17B1 expression and inflammation in mammary tissue. Studies with mammary-restricted HSD17B1 expression demonstrated that the enzyme induced lesions at the sites of ducts and alveoli, which were accompanied by peri- and intraductal inflammation .

These inflammatory changes were associated with disruption of the myoepithelial cell layer, suggesting that HSD17B1-mediated estrogen production may influence tissue integrity and inflammatory responses . The lesions were shown to be estrogen-dependent, as treatment with the antiestrogen ICI 182,780 reversed the phenotype even after lesions were established .

The mechanism by which HSD17B1 and increased local estradiol production promote inflammation remains an active area of research. Possibilities include direct effects of estradiol on inflammatory signaling pathways, alterations in the local microenvironment that promote inflammatory cell recruitment, or indirect effects through changes in ductal epithelial cell behavior. Understanding these mechanisms could provide insights into the early stages of estrogen-dependent disease development and potentially identify new targets for preventive interventions.

What are the implications of HSD17B1 inhibition for therapeutic development?

The central role of HSD17B1 in estrogen activation makes it an attractive target for therapeutic intervention in estrogen-dependent diseases. Research has demonstrated the potential efficacy of specific HSD17B1 inhibitors in reducing estrogen-dependent tumor growth in preclinical models .

In studies with MCF-7 xenografts expressing HSD17B1, treatment with a specific inhibitor significantly reduced tumor growth by nearly 60% compared to untreated controls . Importantly, this inhibition was selective for the tumor tissue and did not affect uterine growth, suggesting a favorable tissue-specific profile that could minimize side effects associated with systemic estrogen suppression .

The discovery of the substrate inhibition mechanism and dead-end complex formation in HSD17B1 opens new avenues for inhibitor design. Compounds that promote the formation of these non-productive enzyme-substrate complexes could provide novel approaches to inhibiting enzyme activity . This strategy represents a potentially more selective approach than traditional competitive inhibitors.

Future research directions include the development of more selective and potent HSD17B1 inhibitors, investigation of combination therapies with existing anti-estrogen treatments, and identification of patient populations most likely to benefit from HSD17B1-targeted therapies based on expression profiles or genetic variants. Additionally, understanding the potential roles of HSD17B1 beyond estrogen metabolism could reveal new applications for HSD17B1 inhibitors in diverse disease contexts.