Structural analysis of human SEPHS2 protein, a selenocysteine machinery component, over-expressed in triple negative breast cancer

Abstract

Selenophosphate synthetase 2 (SEPHS2) synthesizes selenide and ATP into selenophosphate, the selenium donor for selenocysteine (Sec), which is cotranslationally incorporated into selenoproteins. The action and regulatory mechanisms of SEPHS2 as well as its role in carcinogenesis (especially breast cancer) remain ambiguous and need further clarification. Therefore, lacking an experimentally determined structure for SEPHS2, we first analyzed the physicochemical properties of its sequence, modeled its three-dimensional structure and studied its conformational behavior to identify the key residues (named HUB nodes) responsible for protein stability and to clarify the molecular mechanisms by which it induced its function. Bioinformatics analysis evidenced higher amplification frequencies of SEPHS2 in breast cancer than in other cancer types. Therefore, because triple negative breast cancer (TNBC) is biologically the most aggressive breast cancer subtype and its treatment represents a challenge due to the absence of well-defined molecular targets, we evaluated SEPHS2 expression in two TNBC cell lines and patient samples. We demonstrated mRNA and protein overexpression to be correlated with aggressiveness and malignant tumor grade, suggesting that this protein could potentially be considered a prognostic marker and/or therapeutic target for TNBC. The selenoproteins include selenocysteine (Sec), which is a nonstandard amino acid in the UGA codon. These proteins are found in all organisms throughout the tree of life. The incorporation of this 21st amino acid into proteins in mammalian cells is guided by the Sec biosynthesis machinery 1 , of which selenophosphate synthetase 2 (SEPHS2) is an important component 2. SEPHS2 catalyzes the synthesis of selenophosphate from selenide, adenosine triphosphate (ATP), and water and produces adenosine monophosphate (AMP) and inorganic phosphate. Selenophosphate is the selenium donor for Sec synthesis, which, in contrast to other amino acids, takes place on its own tRNA, tRNA Sec 3,4. The selenocysteine incorporation machinery requires certain protein-protein and protein-RNA interactions to function and is guided by stem-loop structures localized in the 3′ untranslated regions of selenoprotein-encoding genes. Recently, SEPHS2 has been demonstrated to interact with selenocysteine synthase (SepSecS) and SEPHS1, which plays a nonessential role in selenoprotein metabolism 5. Although some papers have reported that a decrease or increase in selenoprotein expression can induce a cancer phenotype 6 , the role of selenoproteins in carcinogenesis and their mechanisms of action and regulation remain ambiguous and need further clarification. In fact, few data are available regarding changes in SEPHS2

Integrated Analysis to Study the Relationship between Tumor-Associated Selenoproteins: Focus on Prostate Cancer

“…Moreover, we identified human miR-544a as able to modulate SELENOK expression in the two HepG2 and Huh7 cell lines [19], and also analyzed the selenoproteins' transcriptomes in MCF-7 and MDA-MB231 human breast cancer cell lines, compared to MCF10A normal epithelial breast cells [20]. Recently, we reported that elevated tumor tissue expression of the selenoprotein SEPHS2 in triple-negative breast cancer (TNBC) patients was correlated with the malignancy grade [21].…”

Selenium as a Bioactive Micronutrient in the Human Diet and Its Cancer Chemopreventive Activity

“…Selenophosphate synthetase 2 (SPS2), much like other representatives described in this section of this review, belongs to the group of selenoproteins. This enzyme is important for the existence and synthesis of selenoproteins [ 120 ]. SPS2 is responsible for the formation of selenophosphate in the selenide, adenosine triphosphate (ATP), and H 2 O reaction, in which it is a catalyst [ 120 , 121 , 122 ].…”

“…This enzyme is important for the existence and synthesis of selenoproteins [ 120 ]. SPS2 is responsible for the formation of selenophosphate in the selenide, adenosine triphosphate (ATP), and H 2 O reaction, in which it is a catalyst [ 120 , 121 , 122 ]. The product of this reaction (selenophosphate) plays the role of an active Se donor [ 122 ], which is crucial during the biosynthesis of Sec.…”

Are trace element concentrations suitable biomarkers for the diagnosis of cancer?

“…Especially breast but also other cancer cells are described to be selenophilic, catalyzed by the cystine/glutamate antiporter solute carrier family 7 member 11 (SLC7A11) that promotes Se uptake and selenoprotein synthesis. In addition, selenophosphate synthase 2 (SEPHS2) essential for selenoprotein synthesis was upregulated in BC compared with normal breast tissue, positively associated with poor survival of BC patients [ 30 ]. Thus, several types of tumors actively accumulate Se.…”