Collectively, our study suggests that (P)RR plays a key role in energy homeostasis and regulation of plasma lipids by integrating hepatic glucose and lipid metabolism.

NAD+-dependent SIRT7 deacylase plays essential roles in ribosome biogenesis, stress response, genome integrity, metabolism and aging, while how it is transcriptionally regulated is still largely unclear. TGF-β signaling is highly conserved in multicellular organisms, regulating cell growth, cancer stemness, migration and invasion. Here, we demonstrate that histone deacetylase HDAC8 forms complex with SMAD3/4 heterotrimer and occupies SIRT7 promoter, wherein it deacetylates H4 and thus suppresses SIRT7 transcription. Treatment with HDAC8 inhibitor compromises TGF-β signaling via SIRT7-SMAD4 axis and consequently, inhibits lung metastasis and improves chemotherapy efficacy in breast cancer. Our data establish a regulatory feedback loop of TGF-β signaling, wherein HDAC8 as a novel cofactor of SMAD3/4 complex, transcriptionally suppresses SIRT7 via local chromatin remodeling and thus further activates TGF-β signaling. Targeting HDAC8 exhibits therapeutic potential for TGF-β signaling related diseases.

Dietary interventions such as intermittent fasting (IF) have emerged as an attractive strategy for cancer therapies; therefore, understanding the underlying molecular mechanisms is pivotal. Here, we find SIRT7 decline markedly attenuates the anti-tumor effect of IF. Mechanistically, AMP-activated protein kinase (AMPK) phosphorylating SIRT7 at T263 triggers further phosphorylation at T255/S259 by glycogen synthase kinase 3β (GSK3β), which stabilizes SIRT7 by decoupling E3 ligase UBR5. SIRT7 hyperphosphorylation achieves anti-tumor activity by disrupting the SKP2-SCF E3 ligase, thus preventing SKP2-mediated K63-linked AKT polyubiquitination and subsequent activation. In contrast, GSK3β-SIRT7 axis is inhibited by EGF/ERK2 signaling, with ERK2 inactivating GSK3β, thus accelerating SIRT7 degradation. Unfavorably, glucose deprivation or chemotherapy hijacks the GSK3β-SIRT7 axis via ERK2, thus activating AKT and ensuring survival. Notably, Trametinib, an FDA-approved MEK inhibitor, enhances the efficacy of combination therapy with doxorubicin and IF. Overall, we have revealed the GSK3β-SIRT7 axis that must be fine-tuned in the face of the energetic and oncogenic stresses in malignancy.

SIRT7 plays critical roles in tumorigenesis and tumor progression; however, the underlying mechanisms are poorly understood. Here, we aimed to identify downstream targets of SIRT7 to help delineate its precise function. In this study, we demonstrate that SIRT7 is essential to regulate IDH1 expression in various cancer cell types. Interestingly, both SIRT7 and IDH1 levels are downregulated in breast cancer lung metastases and are useful for predicting disease progression and prognosis. Mechanistically, SIRT7 enhances IDH1 transcription, and this process is mediated by SREBP1. SIRT7 insufficiency reduces cellular α-ketoglutarate, a metabolite product of IDH1, and suppresses lipogenesis and gluconeogenesis. Moreover, α-ketoglutarate decline increases HIF1α protein levels and, thus, promotes glycolysis. This effect permits cancer cells to facilitate Warburg effect and undergo fast proliferation. Overall, the SIRT7–IDH1 axis regulates cancer cell metabolic reprogramming and, thus, might serve as a point of therapeutic intervention.