The Non-Genomic Effects Of The PPAR-γ Ligand Rosiglitazone On Intracellular Calcium Concentrations In Mammalian Monocytic And Smooth Muscle Cells

Thiazolidinediones   such  as  rosiglitazone  are  used   in  the  treatment  of  Type-2 Diabetes,  and  are  ligands  for  peroxisome  proliferator-activated  receptor-gamma (PPARγ), a ligand-activated  transcription  factor  that  regulates  expression  of  genes involved in glucose and lipid metabolism.  However, rosiglitazone is known to exert PPARγ-independent  effects  alongside  its  classical  receptor-dependent  effects.  This study  investigated the PPARγ-independent  effects  of  rosiglitazone  on  intracellular calcium (Ca2+i) signalling in cultured monocytic and vascular smooth muscle cellsRosiglitazone rapidly  (5-30min)inhibited the  Ca2+sequestration  activity  of the  ER-resident  Ca2+pump  enzyme SERCA2bin  a  dose-dependent  manner  (IC50~2μM). 10μM  Rosiglitazone  triggeredrapid  increases  in [Ca2+]i;however,  restoration  to basal  levels  occurred  within  72h.  Consequently,  cell  viability  was  not  adversely affected by rosiglitazone treatment. Initiation  of  the  unfolded  protein  response  (UPR)  was  identified  as  the  mechanism underpinning rosiglitazone‟s Ca2+homeostatic  restorative  properties.  Rosiglitazone induced alternate  splicing  of the  UPR  transcription  factor XBP-1, which  led  to increased  mRNA  and protein  expressionof  SERCA2b  (shown  via  bioinformatics analysis  to  bea  UPR  target  gene),and  increased ER Ca2+-ATPase  activity  in rosiglitazone-treated cells.  In  tissue  (rabbit  aortic  ring)  samples,  10μM  rosiglitazone  induced  transient  (within 1h)  non-significant  losses  in  vasorelaxatory  sensitivity  to  sodium  nitroprusside,  but extended  treatment  (4-24h) increased  sensitivity  beyond  that  of  vehicle-treated samples. Thus, at cell  and tissue  levels, thisdata suggests that rosiglitazone initially causes  increased[Ca2+]idue  to  inhibition  of  SERCA2b,but  extended  incubation induces-via upregulation of UPR target genes -the restoration of Ca2+homeostasis, and possibly even improvements in function in some contexts. Clearly, the data presented in this in vitrostudy must be treated tentatively, and more research is needed before these potentially beneficial effects can be firmly identified as  being  clinically  significant.  Nevertheless,  the  data  obtained  here  may  constitute preliminary evidence that, alongside its PPAR-dependent effects, rosiglitazone may exert functional improvements on the vasculature.