Transcriptional Regulation of Cox-2 Expression in human Osteosarcoma Cells

Abstract

Prostaglandins (PGs) are important modulators in bone biology and may contribute to tumor formation and progression also in human osteosarcoma. The inducible cyclooxygenase-2 (Cox-2) is a candidate inflammatory marker in human osteosarcoma and a rate-limiting enzyme in PG biosynthesis. 15-deoxy-d12,14-PGJ2 (15d-PGJ2), a metabolite of PGD2 and PPARγ ligand, exerts a panel of different biological activities via receptor-dependent and receptor-independent mechanisms. The present study aimed at investigating the intracellular redox status and signalling cascades leading to Cox-2 induction in human MG-63 osteosarcoma cells. 15d-PGJ2 induced an accumulation of reactive oxygen species (ROS) that in turn led to upregulation of Cox-2 via different routes in a PPARγ- and PGD2-receptor-independent manner. First, phosphorylation of p38 MAPK directly enhanced Cox-2 expression by promoting mRNA stability. Second, 15d-PGJ2 induced activation of EGF receptors and downstream activation of Cox-2 via phosphorylation of p42/44 MAPK. Functional activity of Cox-2 expression was tested by measurements of PGE2 and PGF2α. Glutathione precursor molecules reversed enhanced ROS levels and Cox-2 expression. The synthetic compound 9,10-dihydro-15d-PGJ2 lacking the αβ-unsaturated carbonyl group in the cyclopentenone ring did not exhibit cellular responses observed with 15d-PGJ2. Thus, it is concluded that the electrophilic carbon atom of 15d-PGJ2 is responsible for alterations in the intracellular redox status and Cox-2 expression. The electrophilic character of 15d-PGJ2 was also related to the induction of apoptotic markers such as caspase-3 activation and PARP cleavage. Apoptosis was regulated independently of MAPK activation, PPARγ, and PGD2 receptors. Both, death receptor- and mitochondria-dependent pathways might be involved in 15d-PGJ2-induced programmed cell death, as procaspase-8 cleavage was upregulated and expression levels of the pro-apoptotic Bcl-2 family member Puma were increased. Furthermore, 15d-PGJ2 mediated the elevation of the intracellular GSH content and expression of heme oxygenase-1 (HO-1), both events that might act in a cytoprotective manner preventing cytotoxic effects of 15d-PGJ2. HO-1 expression might be regulated by Nrf-2 or Egr-1, as these candidate transcription factors underwent nuclear translocation upon 15d-PGJ2 treatment.

Datum der Bewilligung	2011
Originalsprache	Englisch
Gradverleihende Hochschule	Medizinische Universität Graz
Betreuer/-in	Ernst Malle (Betreuer*in) & Werner Windischhofer (Betreuer*in)