The relative endogenous expression levels of the IFNAR2 isoforms influence the cytostatic and pro-apoptotic effect of IFN alpha on pleomorphic sarcoma cells

Based on our previous studies where we found that IFNAR2-1, the short IFNalpha/beta receptor variant, was expressed in pleomorphic sarcoma cells, we decided to determine the relative levels of expression of IFNAR2.1 versus the longer form, named IFNAR2.2, in different pleomorphic sarcoma cells in relation to their response to interferon alpha treatment. When examining a panel of PS cells isolated from surgical specimens, we found that IFNAR2.1 prevailed in 6 out 7 lines analysed and that these generally showed cell cycle arrest and low levels of apoptosis upon IFNalpha treatment. The reverse ratio, i.e. higher constitutive levels of IFNAR2.2 than IFNAR2.1, was associated with an irreversible inhibition of cell growth and pronounced apoptosis. Impairment of tumour growth by low- and high-dose IFNalpha treatment of nude mice inoculated with PS cells expressing predominantly IFNAR2.1 further asserted the effect of the cytokine also in vivo. A proteomic analysis of 120 signalling components in growth arrested, apoptotic PS cells harbouring higher levels of IFNAR2.2 revealed engagement of the canonical Jak/Stat/ISGF3-pathway, the activation of the mitochodrial apoptotic pathway and a potentially novel mechanism of cell cycle blockade unrelated to down-regulation of cyclin A/B and their interacting/regulating kinases. Our results confirm the dominant negative role of IFNAR2.1, but also suggest that the relative endogenous levels of the two IFNalpha/beta receptor isoforms may dictate the signalling pathways triggered by the ligand, such as to cause exclusively cell cycle arrest or induce programmed cell death. This parameter may be of importance for the clinical outcome of IFNalpha treatment of PS.