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

Research highlights: → Rosiglitazone rapidly (30 min) inhibited microsomal Ca2+ATPase activity (IC50 ∼2 μM). → After 4 h rosiglitazone exposure, the UPR transcription factor XBP-1 was activated. → Within 24-72 h, UPR target genes were upregulated, enhancing ER Ca2+ sequestration. → Replenishment of ER Ca2+ stores appeared to restore normal cell physiology. → Monocyte/VSMC viability was not decreased during 2 weeks' rosiglitazone treatment. -- Abstract: 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 (30 min) brought about concentration-dependent PPARγ-independent inhibition of Ca2+ATPase activity (IC50 ∼2 μM). Fluo-3 fluorimetric data demonstrated in intact cells that 1 h treatment with 1 or 10 μM rosiglitazone caused Ca2+ ions to leak into the cytoplasm. Gene expression analysis showed that within 4 h 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 24-72 h. After 72 h 1 or 10 μM 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 1-10 μM 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.