Aging is characterized by a gradual functional decline resulting from a complex interaction between genetic, epigenetic and stochastic factors and the rate of aging is recognized as the most important risk factor for the development of the most common age-related diseases (ARDs). Research on aging process is currently focused on understanding the molecular mechanisms underlying age-related features accompanying on one hand, the onset of ARDs and on the other hand, the chances to reach a successful aging. A chronic, systemic, low-grade, age-related pro-inflammatory status, called “Inflammaging” is correlated with ARDs development. The two main culprits of inflammaging are the repeated stimulation of immune system over time and the increased burden of senescent cells. Senescent cells are able to modify the microenvironment acquiring a senescence-associated secretory phenotype (SASP). In patients affected by ADRs, the rate of aging process in increased, as well as the burden of senescent cells with SASP. In this scenario, a better understanding of the molecular mechanisms that promote cellular senescence is of basic and clinical relevance. A number of mechanisms promoting senescence have been well characterized. Increasing evidence suggest that nuclear DNA fragility and a sub-functional DNA damage response (DDR) is associated with an increase release of nucleic acids in cytoplasm, including RNA:DNA hybrids. However, only few studies have tempted to assess the pro- or anti-inflammatory effects of this misplaced nucleic acid pool in different cellular models. In this framework, we have analysed the cytoplasmic pool of misplaced nucleic acids in a model of human endothelial cells (HUVECs), in two prototypical stress conditions related to inflammaging, such as “replicative cellular senescence” and “hyperglycemic condition”. A significant increased amount of misplaced nucleic acids was observed in the cytoplasm of senescent cells compared to the younger ones, and in young cells cultured for 1-week in hyperglycemic condition compared to young cells cultured in normoglycemic medium. The cytosolic pool of misplaced nucleic acids is composed of dsDNA, including micronuclei, bubbles and telomere sequences, dsRNA and hybrids. A reduced expression of RNaseH2, the enzyme involved in the degradation of RNA moiety of RNA:DNA hybrids was observed in senescent cells compared to the younger ones and in young cells treated with hyperglycemic medium. The amounts of mis incorporated ribonucleotides, biomarkers od genome instability, were inversely related to RNaseH2 expression level, suggesting that both “senescence” and “hyperglycemic condition” are associated with increased genomic instability. Since the presence of nucleic acids in cytoplasm can activate a number of cytosolic receptors inducing the antiviral response, characterized by increased release of type 1 Interferon (IFN-1), we analysed cGAS/STING/IRF3 axis and IFN-1 expression in “senescence” and “hyperglycemic condition”. Surprisingly, a strong reduction of cGAS expression was observed in senescent cells in normo and hyperglycemic conditions, in association with the absence of IFN-1 modulation, whereas the expression of proinflammatory cytokines, such as IL-1beta, IL-6 and IL-8 was significantly increased. Overall, our results suggest an imbalance between antiviral and proinflammatory response in senescent cells and in young cells under hyperglycemic conditions. Increasing evidence suggest that senescent cells have an enhanced susceptibility to viral infections. We hypothesized that such imbalance between antiviral and proinflammatory responses could be, almost in part, a direct consequence of their ability “to tolerate” a high cytosolic nucleic acids load. Overall, our results pave the way to explain why old subjects, especially the elderly patients affected by diabetes, are more susceptible to adverse outcomes of infectious diseases.

Cytosolic nucleic acids as new markers of senescence and hyperglycemia-induced inflammation

RAMINI, DEBORAH
2021-05-11

Abstract

Aging is characterized by a gradual functional decline resulting from a complex interaction between genetic, epigenetic and stochastic factors and the rate of aging is recognized as the most important risk factor for the development of the most common age-related diseases (ARDs). Research on aging process is currently focused on understanding the molecular mechanisms underlying age-related features accompanying on one hand, the onset of ARDs and on the other hand, the chances to reach a successful aging. A chronic, systemic, low-grade, age-related pro-inflammatory status, called “Inflammaging” is correlated with ARDs development. The two main culprits of inflammaging are the repeated stimulation of immune system over time and the increased burden of senescent cells. Senescent cells are able to modify the microenvironment acquiring a senescence-associated secretory phenotype (SASP). In patients affected by ADRs, the rate of aging process in increased, as well as the burden of senescent cells with SASP. In this scenario, a better understanding of the molecular mechanisms that promote cellular senescence is of basic and clinical relevance. A number of mechanisms promoting senescence have been well characterized. Increasing evidence suggest that nuclear DNA fragility and a sub-functional DNA damage response (DDR) is associated with an increase release of nucleic acids in cytoplasm, including RNA:DNA hybrids. However, only few studies have tempted to assess the pro- or anti-inflammatory effects of this misplaced nucleic acid pool in different cellular models. In this framework, we have analysed the cytoplasmic pool of misplaced nucleic acids in a model of human endothelial cells (HUVECs), in two prototypical stress conditions related to inflammaging, such as “replicative cellular senescence” and “hyperglycemic condition”. A significant increased amount of misplaced nucleic acids was observed in the cytoplasm of senescent cells compared to the younger ones, and in young cells cultured for 1-week in hyperglycemic condition compared to young cells cultured in normoglycemic medium. The cytosolic pool of misplaced nucleic acids is composed of dsDNA, including micronuclei, bubbles and telomere sequences, dsRNA and hybrids. A reduced expression of RNaseH2, the enzyme involved in the degradation of RNA moiety of RNA:DNA hybrids was observed in senescent cells compared to the younger ones and in young cells treated with hyperglycemic medium. The amounts of mis incorporated ribonucleotides, biomarkers od genome instability, were inversely related to RNaseH2 expression level, suggesting that both “senescence” and “hyperglycemic condition” are associated with increased genomic instability. Since the presence of nucleic acids in cytoplasm can activate a number of cytosolic receptors inducing the antiviral response, characterized by increased release of type 1 Interferon (IFN-1), we analysed cGAS/STING/IRF3 axis and IFN-1 expression in “senescence” and “hyperglycemic condition”. Surprisingly, a strong reduction of cGAS expression was observed in senescent cells in normo and hyperglycemic conditions, in association with the absence of IFN-1 modulation, whereas the expression of proinflammatory cytokines, such as IL-1beta, IL-6 and IL-8 was significantly increased. Overall, our results suggest an imbalance between antiviral and proinflammatory response in senescent cells and in young cells under hyperglycemic conditions. Increasing evidence suggest that senescent cells have an enhanced susceptibility to viral infections. We hypothesized that such imbalance between antiviral and proinflammatory responses could be, almost in part, a direct consequence of their ability “to tolerate” a high cytosolic nucleic acids load. Overall, our results pave the way to explain why old subjects, especially the elderly patients affected by diabetes, are more susceptible to adverse outcomes of infectious diseases.
senescence, inflammation, hyperglycemia, hydrids
senescenza; infiammazione; iperglicemia; ibridi
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11566/289652
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