STAT3 inhibitor Stattic and its analogues inhibit STAT3 phosphorylation and modulate cytokine secretion in senescent tumour cells

Signal transducer and activator of transcription 3 (STAT3) signalling serves an important role in carcinogenesis and cellular senescence, and its inhibition in tumour cells represents an attractive therapeutic target. Premature cellular senescence, a process of permanent proliferative arrest of cells in response to various inducers, such as cytostatic drugs or ionizing radiation, is accompanied by morphological and secretory changes, and by altered susceptibility to chemotherapeutic agents, which can thereby complicate their eradication by cancer therapies. In the present study, the responsiveness of proliferating and docetaxel (DTX)‑induced senescent cancer cells to small molecule STAT3 inhibitor Stattic and its analogues was evaluated using tumour cell lines. These agents displayed cytotoxic effects in cell viability assays on both proliferating and senescent murine TRAMP‑C2 and TC‑1 cells; however, senescent cells were markedly more resistant. Western blot analysis revealed that Stattic and its analogues effectively inhibited constitutive STAT3 phosphorylation in both proliferating and senescent cells. Furthermore, whether the Stattic‑derived inhibitor K1836 could affect senescence induction or modulate the phenotype of senescent cells was evaluated. K1836 treatment demonstrated no effect on senescence induction by DTX. However, the K1836 compound significantly modulated secretion of certain cytokines (interleukin‑6, growth‑regulated oncogene α and monocyte chemoattractant protein‑1). In summary, the present study demonstrated differences between proliferating and senescent tumour cells in terms of their susceptibility to STAT3 inhibitors and demonstrated the ability of the new STAT3 inhibitor K1836 to affect the secretion of essential components of the senescence‑associated secretory phenotype. The present study may be useful for further development of STAT3 inhibitor‑based therapy of cancer or age‑related diseases.

Senescent RAW264.7 cells exhibit increased production of nitric oxide and release inducible nitric oxide synthase in exosomes

Aging cells not only cease growing, but also secrete various proteins such as inflammatory cytokines. This secretory phenomenon is known as the senescence‑associated secretory phenotype (SASP). The aim of the present study was to elucidate the effects of senescence on the differentiation of osteoclast precursors (OCPs) and corresponding SASP. RAW264.7 cells were used as OCPs and were cultured to passage (P)5, P10 and P20. Cell proliferation assays, senescence‑associated β‑galactosidase staining and telomere length quantification were subsequently performed, and it was revealed that replicative senescence was induced at P20. In addition, the level of tartrate‑resistant acid phosphatase activity in P20 cells treated with receptor activator of nuclear factor‑κB ligand was significantly lower than that in P5 and P10 cells. The SASP factors interleukin‑6, tumour necrosis factor‑α and nitric oxide were significantly increased in P20 culture supernatants compared with those in P5 and P10 supernatants. Furthermore, the number of exosomes at P20 was increased compared with that at P5 and P10, and inducible nitric oxide synthase (iNOS) was expressed in exosomes at P20, but not in exosomes at P5. In conclusion, the present study revealed that senescent RAW264.7 cells exhibit increased expression of SASP factors and release iNOS in exosomes.

Radiation induces primary osteocyte senescence phenotype and affects osteoclastogenesis in vitro

Irradiation-induced bone remodeling imbalances arise as a consequence of the dysregulation of bone formation and resorption. Due to the abundance of osteocytes, their long life and their dual-regulatory effects on both osteoblast and osteoclast function, they serve as critical coordinators of bone remolding. In the present study, femur and tibia-derived primary osteocytes were cultured and irradiated to observe the functional changes and the cellular senescence phenotype in vitro. Irradiation directly reduced cell viability, affected the crucial dendritic morphology and altered the expression of functional proteins, including upregulation of receptor activator of nuclear factor-κB ligand and sclerostin, and downregulation of osteoprotegerin. Irradiated osteocytes were shown to exhibit notable DNA damage, which resulted in the initiation of a typical cellular senescence phenotype. Furthermore, it was found that irradiation-induced prematurely senescent osteocytes stimulate molecular secretion, referred to as senescence-associated secretory phenotype (SASP), which may be involved in modulation of the bone microenvironment, including the promotion of osteoclastogenesis. Taken together, the results showed that irradiation triggered osteocyte senescence and the acquisition of an associated secretory phenotype. This further resulted in an imbalance of bone remodeling through senescent influence on proliferation, morphology and marker protein production, but also indirectly via a paracrine pathway through SASP secretion. The results of the present study may highlight the potential of SASP-targeted interventions for the management of radiation-induced bone loss.