Six-transmembrane epithelial antigen of prostate 1 (STEAP1)

STEAP1 is a cell surface protein that belongs to the STEAP (Six-transmembrane epithelial antigen of prostate) family, which consists of four members (STEAP1-4)[1][7]. It was first identified in advanced prostate cancer and has since been recognized as a potential therapeutic target due to its high expression in various cancer types[7]. ## Structure and Molecular Characteristics STEAP1 is a 339 amino acid integral membrane protein with six transmembrane domains[8]. Its structure includes: - Intracellular N and C termini located in the cytosol[7] - Six transmembrane helices spanning the cell membrane[1] - A single heme B prosthetic group bound in the transmembrane domain[1] Unlike other STEAP family members (STEAP2-4), STEAP1 lacks the N-terminal NADPH-binding F420H2:NADP+ oxidoreductase domain, which initially led researchers to believe it couldn't function as a metalloreductase independently[5][7]. However, recent studies have shown that STEAP1 can exhibit cellular ferric reductase activity when fused to the intracellular NADPH-binding domain of STEAP4[7][9]. ## Expression and Function STEAP1 is predominantly expressed in prostate epithelial cells but shows limited expression in normal tissues outside of secretory tissues of the bladder and prostate[3][10]. It is highly upregulated in various cancer types, including: - Prostate cancer (especially metastatic castration-resistant prostate cancer) - Bladder cancer - Colorectal cancer - Breast cancer - Non-small cell lung cancer - Ovarian cancer - Ewing's sarcoma family of tumors (EFT)[5][7][10] Functionally, STEAP1 appears to serve as: - An ion channel or transporter protein - A mediator of cell adhesion, proliferation, and invasiveness - A facilitator of intracellular communication - A participant in iron metabolism[8] In cancer cells, STEAP1 has been shown to: - Inhibit c-Myc expression when knocked down, causing tumor cells to arrest in G1 phase - Promote cell migration and angiogenesis through the JAK2/STAT3 signaling pathway - Facilitate epithelial-mesenchymal transition (EMT), accelerating cell invasion and metastasis[5] ## Clinical Significance and Therapeutic Potential STEAP1's high expression in multiple cancer types and limited presence in normal tissues makes it an attractive target for cancer therapy[5][10]. Several therapeutic approaches targeting STEAP1 are being developed: 1. **Antibody-Drug Conjugates (ADCs)**: Such as DSTP3086S for metastatic castration-resistant prostate cancer[6][10] 2. **Chimeric Antigen Receptor (CAR) T-cell therapy**: STEAP1-directed CAR T cells have shown substantial antitumor activity in metastatic human prostate cancer models[6] 3. **Bispecific Antibodies**: Novel anti-STEAP1 bispecific antibodies like BC261 have demonstrated potent cytotoxicity against cancer cell lines despite low antigen density[10] 4. **Diagnostic Applications**: STEAP1 has potential as a biomarker for cancer diagnosis and prognosis. For example, STEAP1-positive extracellular vesicle levels in plasma have been associated with prostate cancer diagnoses[7] STEAP1 expression is positively correlated with disease progression and poor prognosis in several cancer types, particularly prostate cancer where its expression is elevated in all stages of the disease and associated with higher Gleason scores[7]. ## Molecular Interactions STEAP1 may function in heterooligomeric complexes with other STEAP paralogues, particularly STEAP2[1]. Recent cryo-EM structures of homotrimeric human STEAP4 revealed a domain-swapped architecture that supports a model where the heme in STEAP1 could receive electrons from NADPH bound to an adjacent STEAP2/3/4 subunit[1][9]. This comprehensive understanding of STEAP1's structure, function, and role in cancer progression continues to drive the development of targeted therapies that may improve outcomes for patients with STEAP1-expressing malignancies.

Drugs targeting STEAP1 aim to inhibit its pro-tumor functions, such as promoting cell proliferation, migration, invasion, and angiogenesis, or to directly induce cell death in STEAP1-expressing cancer cells. This is achieved through approaches like Antibody-Drug Conjugates (ADCs), Chimeric Antigen Receptor (CAR) T-cell therapy, and Bispecific Antibodies, by targeting STEAP1's high expression on cancer cell surfaces.