Molecular mechanisms of constitutive NF-κB/Rel activation in Hodgkin/Reed-Sternberg cells

A common characteristic of malignant cells derived from patients wifh Hodgkin's disease (HD) is a high level of constitutive nuclear NF-kappa B/Rel activity, which stimulates proliferation and confers resistance to apoptosis, We have analysed the mechanisms that account for NF-kappa B activation in a panel of Hodgkin/Reed-Sternberg (H-RS) cell lines. Whereas two cell lines (L428 and KMH-2) expressed inactive I kappa B alpha, no significant changes in NF-kappa B or I kappa B expression were seen in other H-RS cells (L591, L1236 and HDLM-2), Constitutive NF-kappa-B was susceptible to inhibition by recombinant I kappa B alpha, suggesting that neither mutations in the NF-KB genes nor posttranslational modifications of NF-kappa B were involved. Endogenous I kappa B alpha was bound to p65 and displayed a very short half-life. I kappa B alpha. degradation could be blocked by inhibitors of the NF-kappa B activating pathway. Proteasomal inhibition caused an accumulation of phosphorylated I kappa B alpha and a reduction of NF-kappa B activity in HDLM-2 and L1236 cells. By in vitro kinase assays we demonstrate constitutive I kappa B kinase (IKK) activity in H-RS cells, indicating ongoing signal transduction, Furthermore, HRS cells secrete one or more factor(s) that were able to trigger NF-kappa B activation. We conclude that aberrant activation of IKK's, and in some cases defective I kappa Bs, lead to constitutive nuclear NF-kappa B activity, which in turn results in a growth advantage of Hodgkin's disease tumor cells.