NF-κB is Not Directly Responsible for Photoresistance Induced by Fractionated Light Delivery in HT-29 Colon Adenocarcinoma Cells

Our recent study follows up an earlier one which demonstrated hypericin-mediated photocytotoxic effects on HT-29 adenocarcinoma cells by light fractionation with a longer dark pause between two unequal light doses (Sackova, A. [2005] Photochem. Photobiol.81, 1411-1416). Here, we present closer study on events invoked by sublethal light dose (1 J cm-2) during the period of 6 h that is sufficient to invoke resistance to second lethal dose (11 J cm-2). First, we proved that the dark pause of 6 h, but not 1 h, resulted in better cell survival with suppressed phosphatidylserine externalization, decreased reactive oxygen species production and hypericin content as well as altered expression of HSP70, GRP94, clusterin, nuclear factor (NF)-kappa B, I kappa B-alpha or Mcl-1. NF-kappa B activity assay confirmed activation of this early-response pathway. However, inhibition of I kappa B (IKK) kinase by parthenolide by stopping NF-kappa B release from the complex with I kappa B did not prevent onset of resistance, but it invoked some resistance even in groups with shorter, 1 h dark pause. Therefore, we predict involvement of another signaling pathway, located upstream from NF-kappa B, responsible for onset of resistance to photodynamic therapy with hypericin in colon adenocarcinoma cells HT-29.