Make it STING: nanotechnological approaches for activating cGAS/STING as an immunomodulatory node in osteosarcoma

Abstract

Osteosarcoma is a highly aggressive bone cancer primarily affecting children, adolescents, and young adults. The current gold standard for treatment of osteosarcoma patients consists of two to three rounds of chemotherapy, followed by extensive surgical intervention from total limb reconstruction to amputation, followed by additional rounds of chemotherapy. Although chemotherapy has advanced the treatment of osteosarcoma significantly, the overall 5-year survival rate in resistant forms of osteosarcoma is still below 20%. The interaction between cancer and the immune system has long been recognized as a critical aspect of tumour growth. Tumour cells within the tumour microenvironment (TME) suppress antitumour immunity, and immunosuppressive cells and cytokines provide the extrinsic factors of tumour drug resistance. Emerging research demonstrates an immunostimulatory role for the cGAS/STING pathway in osteosarcoma, typically considered an immune-cold or immunosuppressed cancer type. cGAS/STING signalling appears to drive an innate immune response against tumours and potentiates the efficacy of other common therapies including chemo and radiotherapy. Nanotechnological delivery systems for improved therapy delivery for osteosarcoma have also been under investigation in recent years. This review provides an overview of cGAS/STING signalling, its divergent roles in the context of cancer, and collates current research which activates cGAS/STING as an adjuvant immunomodulatory target for the treatment of osteosarcoma. It will also discuss current nanotechnological delivery approaches that have been developed to stimulate cGAS/STING. Finally, it will highlight the future directions that we believe will be central to the development of this transformative field.

Keywords: STING; cGAS; drug delivery; immunotherapy; innate immunity; nanotechnology; osteosarcoma.

Figure 1

Diagrammatic overview of cGAS/STING signal transduction. Release of double stranded DNA (dsDNA) from cells, due to apoptosis or cellular stress, leads to cGAS homodimerization and subsequent activation of its catalytic activity. cGAS utilises cytosolic ATP and GTP as substrates for the formation of cGAMP. Cells may also internalise cGAMP present in the extracellular milieu which is released in a similar fashion to that of dsNDA. Inactive STING, located at the endoplasmic reticulum, binds cGAMP and undergoes translocation to the Golgi apparatus wherein it recruits Tank-binding kinase 1 (TBK1). Active TBK1 in-turn phosphorylates Interferon regulatory factor 3 (IRF3) and IkB kinase (IKK). These events culminate in the expression of numerous Interferon stimulated genes (ISGs) through the synergistic activity of IRF3 and NF-kB. Created with BioRender.com.

Figure 2

Depiction of the central cGAS/STING paradigm in the TME. Cellular stressors, namely chromosomal instability and replicative stress can result in the release of dsDNA into the cytosol. cGAS may subsequently recognise this dsDNA and induce cGAMP synthesis. Ultimately this may activate tumour-cell intrinsic cGAS/STING signalling and ISG expression. Alternatively, tumour cell stress can lead to the release of dsDNA, mtDNA and cGAMP into the extracellular space. These molecules can be internalised by dendritic cells present in the TME leading to tumour-cell extrinsic cGAS/STING activation. Cumulatively, both forms of cGAS/STING activation potentiate CD8+ cytotoxic T-lymphocyte recruitment to, and activation within, the TME for enhanced tumour cell elimination. Created with BioRender.com. Adapted from (19).

Figure 3

Divergent outcomes of cGAS/STING signalling. Active STING can drive two antithetical transcriptional profiles in a context dependent fashion with clear repercussions for tumorigenesis. Central to an anti-tumour inflammatory profile is the production of pro-inflammatory cytokines (such as IL-6), pro-inflammatory chemokines (such as CXCL10), and type 1 interferons (IFN-α/IFN-β). These inflammatory mediators promote tumour cell elimination via CD8+ T-cell mediated cytotoxicity. Contrastingly, a tumorigenic anti-inflammatory profile is underpinned by the secretion of anti-inflammatory cytokines (namely IL-10 and TGF-β), and angiogenic factors (VEGF). These anti-inflammatory proteins hamper the activity of tumoricidal immune cells while simultaneously potentiating tumour cell metastasis. Created with BioRender.com.

Figure 4

Avenues of immune activation by cyclic dinucleotide nanoparticles. Cyclic dinucleotide (CDN) nanoparticles primarily elicit an anti-tumour immune response through 3 main avenues. Firstly, nanoparticles may specifically enter the tumour microenvironment and support a tumour-intrinsic immune response by bolstering the activity of tumour-intrinsic immune cells. Secondly, nanoparticles may enter lymphoid organs wherein they are internalised and degraded by resident immune cells, namely dendritic cells, thus providing a priming signal to the immune system which may evoke a more potent anti-tumour immune response, while also supporting the elimination of circulating and metastatic tumour cells. Lastly, sustained release of CDNs from the nanoparticles supports sustained immune priming. Collectively, these three mechanisms of action may drive a more robust anti-tumour immune response. Created with BioRender.com.