Schlafen12 Reduces the Aggressiveness of Triple Negative Breast Cancer through Post-Transcriptional Regulation of ZEB1 That Drives Stem Cell Differentiation

Research progress of the SLFN family in malignant tumors

The Schlafen (SLFN) gene family has emerged as a critical subject of study in recent years, given its involvement in an array of cellular functions such as proliferation, differentiation, immune responses, viral infection inhibition, and DNA replication. Additionally, SLFN genes are linked to chemosensitivity, playing a pivotal role in treating malignant tumors. Human SLFNs comprise three domains: the N-terminal, middle (M), and C-terminal. The N- and C-terminal domains demonstrate nuclease and helicase/ATPase activities, respectively. Meanwhile, the M-domain likely functions as a linker that connects the enzymatic domains of the N- and C-terminals and may engage in interactions with other proteins. This paper aims to present a comprehensive overview of the SLFN family's structure and sequence, examine its significance in various tumors, and explore its connection with immune infiltrating cells and immune checkpoints. The objective is to assess the potential of SLFNs as vital targets in cancer therapy and propose novel strategies for combined treatment approaches.

Structural, molecular, and functional insights into Schlafen proteins

Schlafen (SLFN) genes belong to a vertebrate gene family encoding proteins with high sequence homology. However, each SLFN is functionally divergent and differentially expressed in various tissues and species, showing a wide range of expression in cancer and normal cells. SLFNs are involved in various cellular and tissue-specific processes, including DNA replication, proliferation, immune and interferon responses, viral infections, and sensitivity to DNA-targeted anticancer agents. The fundamental molecular characteristics of SLFNs and their structures are beginning to be elucidated. Here, we review recent structural insights into the N-terminal, middle and C-terminal domains (N-, M-, and C-domains, respectively) of human SLFNs and discuss the current understanding of their biological roles. We review the distinct molecular activities of SLFN11, SLFN5, and SLFN12 and the relevance of SLFN11 as a predictive biomarker in oncology.

The diverse roles that Schlafen family proteins play in cell proliferation, immune modulation, and other biological processes make them promising targets for treating and tracking diseases, especially cancer. Ukhyun Jo and Yves Pommier from the National Cancer Institute in Bethesda, USA, review the molecular characteristics and structural features of Schlafen proteins. These proteins take their name from the German word for “sleep”, as the first described Schlafen proteins caused cells to stop dividing, although later reports found that related members of the same protein family serve myriad cellular functions, including in the regulation of DNA replication. A better understanding of Schlafen proteins could open up new avenues in cancer management, for instance, diagnostics that monitor activity levels of one such protein, SLFN11, could help oncologists predict how well patients might respond to anti-cancer therapies.

SLFN12 Over-expression Sensitizes Triple Negative Breast Cancer Cells to Chemotherapy Drugs and Radiotherapy

BACKGROUND/AIM

Schlafen 12 (SLFN12) expression correlates with survival in triple negative breast cancer (TNBC). SLFN12 slows TNBC proliferation and induces TNBC differentiation, but whether SLFN12 affects the tumoral response to chemotherapy or radiation is unknown.

MATERIALS AND METHODS

We over-expressed SLFN12 in MDA-MB-231 cells using two different lentiviral vectors. We assessed viable cell numbers via crystal violet assay after treatment with carboplatin, paclitaxel, olaparib, zoledronic acid, camptothecin, or cesium irradiation. CHK1 and CHK2 phosphorylation was assessed by western blot and the effects of inhibiting CHK1/CHK2 by AZD7762 were examined. Key findings were confirmed in Hs578t and BT549 TNBC cells after adenoviral SLFN12 over-expression.

RESULTS

SLFN12 over-expression increased TNBC sensitivity to radiation, carboplatin, paclitaxel, zoledronic acid, and camptothecin, but not to olaparib. SLFN12 over-expression decreased CHK1 and CHK2 phosphorylation after treatment with the DNA damaging agent camptothecin (CPT). The CHK1/CHK2 inhibitor diminished the significant cytotoxicity difference between over-expression and baseline SLFN12 levels in response to carboplatin.

CONCLUSION

SLFN12 increases TNBC sensitivity to DNA-damaging agents at least in part by reducing CHK1/2 phosphorylation. This may contribute to improved survival in patients whose TNBC over-expresses SLFN12. Therefore, SLFN12 levels may be used to customize or predict radiotherapy and chemotherapy effects in TNBC.

Schlafens Can Put Viruses to Sleep

The Schlafen gene family encodes for proteins involved in various biological tasks, including cell proliferation, differentiation, and T cell development. Schlafens were initially discovered in mice, and have been studied in the context of cancer biology, as well as their role in protecting cells during viral infection. This protein family provides antiviral barriers via direct and indirect effects on virus infection. Schlafens can inhibit the replication of viruses with both RNA and DNA genomes. In this review, we summarize the cellular functions and the emerging relationship between Schlafens and innate immunity. We also discuss the functions and distinctions of this emerging family of proteins as host restriction factors against viral infection. Further research into Schlafen protein function will provide insight into their mechanisms that contribute to intrinsic and innate host immunity.

Vil-Cre specific Schlafen 3 knockout mice exhibit sex-specific differences in intestinal differentiation markers and Schlafen family members expression levels

The intestinal epithelium requires self-renewal and differentiation in order to function and adapt to pathological diseases such as inflammatory bowel disease, short gut syndrome, and ulcers. The rodent Slfn3 protein and the human Slfn12 analog are known to regulate intestinal epithelial differentiation. Previous work utilizing a pan-Slfn3 knockout (KO) mouse model revealed sex-dependent gene expression disturbances in intestinal differentiation markers, metabolic pathways, Slfn family member mRNA expression, adaptive immune cell proliferation/functioning genes, and phenotypically less weight gain and sex-dependent changes in villus length and crypt depth. We have now created a Vil-Cre specific Slfn3KO (VC-Slfn3KO) mouse to further evaluate its role in intestinal differentiation. There were increases in Slfn1, Slfn2, Slfn4, and Slfn8 and decreases in Slfn5 and Slfn9 mRNA expression that were intestinal region and sex-specific. Differentiation markers, sucrase isomaltase (SI), villin 1, and dipeptidyl peptidase 4 and glucose transporters, glucose transporter 1 (Glut1), Glut2, and sodium glucose transporter 1 (SGLT1), were increased in expression in VC-Slfn3KO mice based on intestinal region and were also highly female sex-biased, except for SI in the ileum was also increased for male VC-Slfn3KO mice and SGLT1 was decreased for both sexes. Overall, the variations that we observed in these VC-Slfn3KO mice indicate a complex regulation of intestinal gene expression that is sex-dependent.

Schlafens: Emerging Proteins in Cancer Cell Biology

Schlafens (SLFN) are a family of genes widely expressed in mammals, including humans and rodents. These intriguing proteins play different roles in regulating cell proliferation, cell differentiation, immune cell growth and maturation, and inhibiting viral replication. The emerging evidence is implicating Schlafens in cancer biology and chemosensitivity. Although Schlafens share common domains and a high degree of homology, different Schlafens act differently. In particular, they show specific and occasionally opposing effects in some cancer types. This review will briefly summarize the history, structure, and non-malignant biological functions of Schlafens. The roles of human and mouse Schlafens in different cancer types will then be outlined. Finally, we will discuss the implication of Schlafens in the anti-tumor effect of interferons and the use of Schlafens as predictors of chemosensitivity.

Structure of PDE3A-SLFN12 complex reveals requirements for activation of SLFN12 RNase

DNMDP and related compounds, or velcrins, induce complex formation between the phosphodiesterase PDE3A and the SLFN12 protein, leading to a cytotoxic response in cancer cells that express elevated levels of both proteins. The mechanisms by which velcrins induce complex formation, and how the PDE3A-SLFN12 complex causes cancer cell death, are not fully understood. Here, we show that PDE3A and SLFN12 form a heterotetramer stabilized by binding of DNMDP. Interactions between the C-terminal alpha helix of SLFN12 and residues near the active site of PDE3A are required for complex formation, and are further stabilized by interactions between SLFN12 and DNMDP. Moreover, we demonstrate that SLFN12 is an RNase, that PDE3A binding increases SLFN12 RNase activity, and that SLFN12 RNase activity is required for DNMDP response. This new mechanistic understanding will facilitate development of velcrin compounds into new cancer therapies.

The small molecule DNMDP acts as a velcrin by inducing complex formation between phosphodiesterase PDE3A and SLFN12, which kills cancer cells that express sufficient levels of both proteins. Here, the authors present the cryo-EM structure of the DNMDP-stabilized PDE3A-SLFN12 complex and show that SLFN12 is an RNase. PDE3A binding increases SLFN12 RNase activity, and SLFN12 RNase activity is required for DNMDP-mediated cancer cell killing.

Schlafen 12 Is Prognostically Favorable and Reduces C-Myc and Proliferation in Lung Adenocarcinoma but Not in Lung Squamous Cell Carcinoma

Schlafen 12 (SLFN12) is an intermediate human Schlafen that induces differentiation in enterocytes, prostate, and breast cancer. We hypothesized that SLFN12 influences lung cancer biology. We investigated survival differences in high versus low SLFN12-expressing tumors in two databases. We then adenovirally overexpressed SLFN12 (AdSLFN12) in HCC827, H23, and H1975 cells to model lung adenocarcinoma (LUAD), and in H2170 and HTB-182 cells representing lung squamous cell carcinoma (LUSC). We analyzed proliferation using a colorimetric assay, mRNA expression by RT-qPCR, and protein by Western blot. To further explore the functional relevance of SLFN12, we correlated SLFN12 with seventeen functional oncogenic gene signatures in human tumors. Low tumoral SLFN12 expression predicted worse survival in LUAD patients, but not in LUSC. AdSLFN12 modulated expression of SCGB1A1, SFTPC, HOPX, CK-5, CDH1, and P63 in a complex fashion in these cells. AdSLFN12 reduced proliferation in all LUAD cell lines, but not in LUSC cells. SLFN12 expression inversely correlated with expression of a myc-associated gene signature in LUAD, but not LUSC tumors. SLFN12 overexpression reduced c-myc protein in LUAD cell lines but not in LUSC, by inhibiting c-myc translation. Our results suggest SLFN12 improves prognosis in LUAD in part via a c-myc-dependent slowing of proliferation.