Background: The JAB1 (cJun activation domain binding protein1), initially discovered as a cJun coactivator, represents the fifth component of an evolutionary highly conserved 8 subunit protein complex named COP9 signalosome (CSN5). Accumulating evidence suggests that the CSN5/JAB1 gene operates as an oncogene in cancer through multiple mechanisms including cell cycle control via downregulation of CDK inhibitors. Targeting CSN5/JAB1 activity in cancer is now possible since a novel inhibitor has been developed for clinical use (CSN5i-3, Novartis). The potential role of CSN5/JAB1 in modulation of anti-tumor responses in cHL is unknown to date.
Methods: The study group included 118 previously untreated cHL patients with available tissue and clinical data. Expression of CSN5/JAB1 was assessed by immunohistochemistry and a previously validated monoclonal antibody. The in vitro system included 6 cHL cell lines (MDA-V, L1236, L428, L540, KMH2, HDLM2). Expression of proteins were analysed by Western blot and gene expression (mRNA) of type 1 interferons (IFNs), including IFN-β, CXCL10 and IFN-γ, with RT-qPCR. The cHL cell lines were treated with the CSN5i-3 at different concentrations. Silencing of CSN5/JAB1 and NFkB (p65) gene was performed using transient transfection with siRNA constructs.
Results: Using a 10% cutoff, CSN5/JAB1 was positive in the neoplastic HRS cells of cHL in 106 of 118 (90%) patients with a predominantly nuclear and weaker cytoplasmic pattern. Treatment of cHL cell lines with increasing concentrations of CSN5i-3 resulted in significant decrease in the cell growth and viability associated with upregulation of the CDK inhibitors p21, p27 and p57. Similarly, CSN5/JAB1 gene silencing resulted in decreased cell growth associated with increased expression of similar cell cycle inhibiting proteins. Inhibition of CSN5/JAB1 activity resulted in significantly increased gene expression of IFN-β, CXCL10 and IFN-γ at a variable degree among the cHL cell lines. Similarly, knocking down CSN5/JAB1 gene led to upregulation of type 1 IFNs. Silencing NFkB gene resulted in decreased levels of CSN5/JAB1 protein suggesting positive regulation of CSN5/JAB1 by NFkB.
Conclusion: The CSN5/JAB1 is overexpressed in HRS cells of most cHL patients, promotes cell growth through cell cycle control and downregulates gene expression of type 1 IFNs in vitro. These functions are sufficiently inhibited by the CSN5i-3, designed for clinical use, in preclinical models of cHL.