NF-??B is not directly responsible for photo-resistance 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. Here we present closer study on events invoked by sub-lethal light dose (1 J cm- 2) during the period of 6 hours that is sufficient to invoke resistance to second lethal dose (11J cm-2). First we proved that the dark pause of 6 hours, but not 1 hour, resulted in better cell survival with suppressed phosphatidylserine externalization, decreased ROS production and hypericin content as well as altered expression of HSP70, GRP94, clusterin, NF-??B, I??B-?? or Mcl-1. NF-??B activity assay confirmed activation of this early-response pathway. However inhibition of IKK kinase by parthenolide stopping NF-??B release from complex with I??B did not prevented onset of the resistance, but it invoked some resistance even in groups with shorter, 1 h dark pause. Therefore we predict involvement of another signaling pathway, located up-stream from NF-??B, responsible for onset of resistance to PDT with hypericin in colon adenocarcinoma cells HT-29.