Decrease in RelA phosphorylation by inhibiting protein kinase A induces cell death in NF-κB-expressing and drug-resistant tumor cells

The RelA (p65) is a subunit of nuclear transcription factor kappa B (NF-κB) and actively participates in expression of NF-κB-dependent genes involved in inflammation and tumorigenesis. Hence, the regulation of p65 is an important strategy to regulate those responses. In this study, we provide data that the dichlorophenyl derivative of 1,2,4-thiadiazolidine (known as P3-25) induced cell death in NF-κB-expressing and doxorubicin-resistant cells. P3-25 inhibited NF-κB DNA binding activity partially, but inhibited NF-κB-dependent genes expression completely. It inhibited phosphorylation of Rel A (p65) by inhibiting activity of protein kinase A (PKA). The PKA inhibition was independent of adenylate cyclase activity or cAMP level. The PKA activity decreased due to inhibition of catalytic subunit of PKA. P3-25 inhibited almost 80% PKA activity at 100 nM concentration, having an IC50 at 10.5 nM. P3-25 potentiated different chemotherapeutic agents-mediated cell death. Our results suggest that P3-25 inhibits PKA activity followed by decreased phosphorylation of p65 and transcriptional activity of NF-κB thereby decreasing antiapoptotic proteins resulting in induction of apoptosis in NF-κB-expressing and doxorubicin-resistant cells. The study might help to understand the mechanism of P3-25-mediated apoptosis and to design it as new chemotherapeutic drug for tumor therapy.