Imaging glycogenesis as a novel biomarker of drug-induced senescence/quiescence

Abstract

Cellular senescence, which is a state of irreversible growth arrest in response to diverse stimuli, has been implicated in many age-related diseases and shown to play complex roles in tumorigenesis. Since the first description of senescence by Hayflick and colleagues in 1961, significant progress has been made in understanding the characteristics and functions of cellular senescence. However, efforts to identify senescence remain limited due to a lack of robust and universal biomarkers for this cell fate. As enhanced glycogenesis has been associated with cell growth arrest, it has been hypothesized in this thesis that PET imaging agents that measure glycogenesis could be a potential tool for assessment of senescence-directed anticancer therapies. The profile of glycogenesis of a panel of cancer cell lines characterised by various genetic backgrounds was first assessed. Cancer cell lines showed distinct patterns of glycogenesis, among which an AKT-GSK3β-dependent regulation of glycogen synthase (GYS1) through PI3K pathway was observed in PTEN-null tumour cells. A cell growth-related regulation of glycogenesis was found in the tested cells, in which cells when cultured under the G0/G1 phase arrest increased glycogen synthesis via a GYS1-dependent manner and thereby led to an increase in glycogen accumulation. Moreover, tumour cells under hypoxic condition were more susceptible to GYS1 knockdown as glycogenesis was more significantly attenuated under hypoxia than normoxic condition. Additionally, one of the four tested anticancer drugs, namely palbociclib, was shown to dose- and time-dependently induce senescence in ER-positive breast cancer cell lines in vitro, as shown by the presence of senescence-like morphology, increased SA-β-gal activity and upregulation of p21 expression. An increase in glycogenesis was found in response to palbociclib treatment via a G6P-dependent elevation in GYS1 activity. Correspondingly, 18F-NFTG, a glycogenesis-based PET radiotracer previously developed by our group, was largely accumulated in palbociclib-induced quiescent/senescent cells. To extend the exploration, the attempts were made to develop and evaluate a novel senescence-specific probe based on detection of lipofuscin formation. Not only did the non-radioactive reference compound [19F]FET-SBB show specific staining of lipofuscin aggregates in palbociclib-treated cell, but also the radiolabelled [18F]FET-SBB was significantly retained, suggesting potential application of [18F]FET-SBB in targeting senescence. In summary, while senescence mechanisms can be varied, drug-induced senescence is detectable in vitro as increased glycogenesis and lipofuscin formation in some contexts. These methods described here have potential for translation in vivo.Open Acces