STAT3 Stabilizes IKK alpha Protein through Direct Interaction in Transformed and Cancerous Human Breast Epithelial Cells

Abstract

Simple Summary Signal transducer and activator of transcription 3 (STAT3) and nuclear factor-kappa B (NF-kappa B) play a cooperative role in inflammation-associated tumorigenesis at multi-levels. The alpha subunit of the inhibitor of the kappa B kinase (IKK) complex, IKK alpha, is involved in both classical and non-classical activation of NF-kappa B. However, the interplay between STAT3 and IKK alpha has not been clarified yet. Here, we report overexpression and co-localization of IKK alpha and STAT3 in human breast cancer tissues as well as in human breast cancer cells, which promotes breast cancer promotion and progression. IKK alpha was stabilized and upregulated by STAT3. We identified the lysine 44 residue of IKK alpha as a putative binding site for STAT3. Taken together, these findings propose a novel mechanism responsible for NF-kappa B activation by STAT3 through stabilization of IKK alpha. Thus, STAT3 and IKK alpha could integrate, and coordinately mediate the growth and progression of human breast cancer. Signal transducer and activator of transcription 3 (STAT3) and nuclear factor-kappa B (NF-kappa B) are two representative transcription factors that play a critical role in inflammation-associated tumorigenesis through multi-level cooperation. Unlike other types of tumors, breast carcinomas have shown a significant dependency on the non-classical NF-kappa B pathway as well as the classical one. The alpha subunit of the inhibitor of the kappa B kinase (IKK) complex, IKK alpha, is involved in both classical and non-classical activation of NF-kappa B. Although the cross-talk between STAT3 and NF-kappa B has been suggested in several studies, the interplay between STAT3 and the regulators of NF-kappa B including IKK alpha has not been fully clarified yet. In this study, we observed overexpression and co-localization of IKK alpha and STAT3 in human breast cancer tissues as well as in H-Ras transformed human breast epithelial (H-Ras MCF-10A) and breast cancer (MDA-MB-231) cells. By utilizing small interfering RNA (siRNA) technology, we were able to demonstrate that STAT3 up-regulated IKK alpha, but not IKK beta or IKK gamma, in these cells. This was attributable to direct binding to and subsequent stabilization of IKK alpha protein by blocking the ubiquitin-proteasome system. Notably, we identified the lysine 44 residue of IKK alpha as a putative binding site for STAT3. Moreover, siRNA knockdown of IKK alpha attenuated viability, anchorage-independent growth and migratory capabilities of H-Ras MCF-10A cells. Taken together, these findings propose a novel mechanism responsible for NF-kappa B activation by STAT3 through stabilization of IKK alpha, which contributes to breast cancer promotion and progression. Thus, breaking the STAT3-IKK alpha alliance can be an alternative therapeutic strategy for the treatment of breast cancer.