Rosiglitazone induces the unfolded protein response, but has no significant effect on cell viability, in monocytic and vascular smooth muscle cells

Given the safety concerns expressed over negative cardiovascular outcomes resulting from the clinical use of rosiglitazone, and the view that rosiglitazone exerts PPARγ-independent effects alongside its insulin-sensitising PPARγ-dependent effects, we hypothesised that rosiglitazone may trigger Unfolded Protein Responses (UPRs) due to disruptions in [Ca²⁺]_i homeostasis within two cardiovascular cell types: monocytic (MM6) and vascular smooth muscle (A7r5) cells.In microsomal samples derived from both cell types, pre-incubation with rosiglitazone rapidly (30min) brought about concentration-dependent PPARγ-independent inhibition of Ca²⁺ATPase activity (IC₅₀ ∼2μM). Fluo-3 fluorimetric data demonstrated in intact cells that 1h treatment with 1 or 10μM rosiglitazone caused Ca²⁺ ions to leak into the cytoplasm. Gene expression analysis showed that within 4h of rosiglitazone exposure, the UPR transcription factor XBP-1 was activated (likely due to corresponding ER Ca²⁺ depletion), and the UPR target genes BiP and SERCA2b were subsequently upregulated within 24-72h.After 72h 1 or 10μM rosiglitazone treatment, microsomal Ca²⁺ATPase activity increased to >2-fold of that seen in control microsomes, while [Ca²⁺]_i returned to basal, indicating that UPR-triggered SERCA2b upregulation was responsible for enhanced enzymatic Ca²⁺ sequestration within the ER. This appeared to be sufficient to replenish ER Ca²⁺ stores and restore normal cell physiology, as cell viability levels were not decreased due to rosiglitazone treatment throughout a 2-week study.Thus, incubation with 1-10μM rosiglitazone triggers the UPR, but does not prove cytotoxic, in cells of the cardiovascular system. This observation provides an important contribution to the current debate over the use of rosiglitazone in the clinical treatment of Type-2 Diabetes.