Endocrine Abstracts

Although glucocorticoids (Gc) are used to treat a range of pathologies, an individuals response to therapy varies considerably. There is a distinct correlation between certain cell types and steroid sensitivity. Aberrant glucocorticoid receptor (GR) function is well-characterised in lymphoid malignant cells but not small cell lung cancer (SCLC) (DMS79) or osteosarcoma (U2OS) cell lines, which are both Gc resistant.

Previously we have shown reduced GR content in DMS79 cells, and that expression of a functional GR is sufficient to restore Gc sensitivity and induce apoptosis in vitro and in vivo.

The aim of this study was to determine (i) whether expression of a functional GR is sufficient to induce apoptosis in U20S cells and (ii) to identify the mechanism silencing endogenous GR expression in DMS79 cells.

Overexpression of GR in U20S cells decreased the number of viable cells (propidium iodide exclusion), in the absence of ligand. This effect was further potentiated following treatment with dexamethasone. GR deletion studies showed no difference in ligand-independent cell death with abrogation of dimerisation, disruption of ligand-binding, or deletion of AF1. Intact GR was required for Gc dependent cell loss. As observed in DMS79 cells, cell death occurred by apoptosis in U2OS cells.

To determine if loss of GR expression in DMS79 cells was due to GR methylation, 5′ aza-deoxycytidine was used. This increased GR expression, which is compatible with GR gene silencing by methylation.

These results show that in SCLC and osteosarcoma cells, GR expression is pro-apoptotic, and the effect is ligand-independent. GR expression may be switched off in these cells as a defence mechanism against cell death. At least in DMS79 cells, loss of GR expression is reversible with epigenetic modification. Fully understanding how GR expression is regulated; and the underlying mechanism inducing apoptosis in these cancer cells may uncover new therapeutic strategies.