Bcl-3, induced by Tax and HTLV-1, inhibits NF-κB activation and promotes autophagy.

The human T cell leukemia virus type 1 (HTLV-1) is a complex human retrovirus that causes an aggressive leukemia known as adult T cell leukemia (ATL). The HTLV-1-encoded oncoprotein Tax induces persistent activation of the nuclear factor-κB (NF-κB) pathway, which is perceived as the primary cause of ATL. Bcl-3, a member of the NF-κB inhibitor (IκB) family, is highly expressed in many HTLV-1-infected T cell lines and ATL cells. However, the role of Bcl-3 in Tax-induced NF-κB activation has not been fully elucidated. Here, we show that Tax induces Bcl-3 expression, which in turn negatively regulates the Tax-induced NF-κB activation. Interestingly, both Bcl-3 up-regulation and NF-κB inhibition promote the autophagy process in HTLV-1-infected cells. Consistent with this, over-expression of Bcl-3 also results in enhancement of rapamycin-, pifithrin-α- or starvation-induced autophagy in control cells. Together, these data demonstrate that Bcl-3 acts as a negative regulator of NF-κB activation and promotes autophagy in HTLV-1-infected cells.