Introduction:

Bladder cancer (BC) is the most expensive malignancy to treat in the US, has limited treatment options and a low 5-year survival after treatment for the more aggressive muscle-invasive and metastatic disease. Our lab has identified secreted protein acidic and rich in cysteine (SPARC) as a tumor suppressor in BC whose expression significantly decreases in advanced stage disease. SPARC exerts multi-faceted tumor suppressor effects on cancer cells. It mitigates hallmarks of BC including proliferation, survival, and invasiveness, while also exerting antioxidant and anti-inflammatory effects through inhibition of multiple oncogenic pathways orchestrating oncogenesis, inflammation and metabolic programming. In this study, we sought to elucidate the role of SPARC in regulating the metabolic programming in BC and identify SPARC-regulated pathways that could be used not only as diagnostic and prognostic biomarkers, but can also be exploited as therapeutic targets to improve disease outcomes and patient quality of life.

Methods:

We used human BC cell lines UMUC3, T24, and its isogenic tumorigenic SPARC-low cell line T24T. SPARC’s effect on cellular bioenergetics was assessed by knocking down SPARC in SPARC-proficient cells and treating cells with recombinant SPARC. We performed comparative transcriptomic profiling of T24, T24T, and T24 depleted of SPARC to identify SPARC-signatures associated with aggressive malignant and metabolic phenotypes using Gene Signature Enrichment Analysis (GSEA). We compared SPARC-signatures with patient survival data from multiple bladder cancer cohorts from the Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA). To determine the effect of SPARC on the canonical oncogenic, inflammatory and metabolic pathways, we performed in vitro mechanistic studies.

Results:

SPARC exerted a significant inhibitory effect on BC cell bioenergetics with significant inhibition of mitochondrial ATP production, basal and maximal respiration as well as spare respiratory capacity. The effect was more profound on the more aggressive, SPARC-low T24T cell compared to its isogenic SPARC-high T24 cells. Consistently, depletion of SPARC in T24 cells not only restored their aggressive invasive phenotype, but also increased their ATP production and mitochondrial respiration. Integrated transcriptomic profiling indicated that loss of SPARC is associated with enrichment of metabolic pathways that take place in the mitochondria such as glycolysis, fatty acid synthesis and oxidation. In addition, loss of SPARC is associated with oncogenic, pro-angiogenic and inflammatory signatures. Importantly, in patient samples, SPARC transcript expression negatively correlated with the enzymes involved in energy generating metabolic pathways such as glycolysis, fatty acid synthesis, and fatty acid oxidation that were also significantly associated with lower survival.

Conclusion:

Our data reveals a novel function of tumor suppressor SPARC: inhibiting mitochondrial bioenergetics and ATP production that fuel mitogenic and invasive phenotypes in BC cells. We also show that loss of SPARC in BC cells is associated with enrichment of oncogenic signatures. SPARC loss is also linked to activation of key pathways that are not only implicated in cell proliferation and invasiveness but are also associated with metabolic programming of BC cells. This programming supports increased energy production to support demands for highly proliferating invasive tumors. The SPARC-treated metabolic profile reveals cancer cell vulnerabilities, suggestive of a promising target for future translational research: BC cell metabolism through oncogenic metabolic pathways that are activated in SPARC-low tumors.  

Funding: NIH R01 CA193437 to Neveen Said, MD PhD