Apoptotic resistance exhibited by dexamethasone-resistant murine 7TD1 cells is controlled independently of interleukin-6 triggered signaling

Abstract

Interleukin-6 (IL6)-mediated signaling is known to play a role in pathogenesis and resistance in several cancers like multiple myeloma (MM). In this report we used the IL6-dependent 7TD1 murine B-cell hybridoma as an in vitro model to study the interactions between IL6-signaling pathways and the development of dexamethasone resistance. Though in initial stages, 7TD1 cells grew IL6-dependent and were sensitive to dexamethasone-induced apoptosis, chronic exposure to dexamethasone led to a dexamethasone-resistant phenotype (7TD1-Dxm) that grew independent of exogenous IL6. While IL6-mediated JAK/ STAT3 and PI3K/AKT signaling was important for proliferation of both cell lines, as shown in proliferation assays using the respective pathway inhibitors, AG490 and LY294002, the resistant cells were insensitive to induction of apoptosis using the same. STAT3 was constitutively phosphorylated in resistant cells and inhibition of its dimerization induced apoptosis but did not alter their insensitivity to dexamethasone. Our results suggest a role of entities downstream of IL6-mediated JAK/STAT3 signaling in development of dexamethasone resistance by 7TD1-Dxm cells.

Autocrine Production of Interleukin-6: A Mechanism of Interleukin-6 Independence in Dexamethasone-Resistant 7TD1 Murine Myeloma Cells

“…Previous studies in our laboratory have established a dexamethasone-resistant 7TD1 sub-line (7TD1-Dxm) and have observed that 7TD1-Dxm cells show IL6 independence for proliferation and dexamethasone resistance concomitantly [15]. We have also shown that dexamethasone resistance observed in 7TD1-Dxm cells is due to inhibition of cytochrome c release [16].…”

“…7TD1-Dxm (Dexamethasone-resistant 7TD1) cells were generated as described previously [15]- [19]. Both 7TD1 and 7TD1-Dxm cells were cultured and maintained in complete culture medium as noted previously [15]- [19].…”

“…Both 7TD1 and 7TD1-Dxm cells were cultured and maintained in complete culture medium as noted previously [15]- [19]. Appropriate vehicle controls were included in experiments.…”

“…Anti-IL6 antibody (Santa Cruz Biotechnology, Inc., Dallas, TX) was used at a concentration of 2 µg/ml. Cellular proliferation was determined using the MTT assay [15]- [20]. All experimental procedures were carried out with a prior wash-out period of 24 hrs during which both the cell lines were incubated without IL6 or Dxm.…”

“…Briefly, 7TD1 and 7TD1-Dxm cells were subjected to various treatments, incubated for 48 hrs and processed as described previously [15]- [17] [19] before they were analyzed using a flow cytometer (Becton Dickinson, East Rutherford, NJ).…”