AbstractHuman leucine zipper protein (LZIP) associates with CC chemokine receptor 1 (CCR1) and this protein–protein interaction should play an important role in leukocyte cell mobility. LZIP is known to regulate leukotactin‐1 (Lkn‐1)‐dependent cell migration without affecting the chemotactic activities of other CC chemokines that bind to CCR1. Since Lkn‐1 is engaged in the transcriptional activation of nuclear factor κB (NF‐κB) and subsequent activation of the chemoattractant ability of leukocytes, we investigated the regulatory role of LZIP in the NF‐κB pathway that is induced by CCR1‐dependent chemokines. LZIP increased NF‐κB‐dependent luciferase activity in response to Lkn‐1 in HOS/CCR1 cells and THP‐1 cells. However, the NF‐κB‐dependent luciferase activities induced by other CCR1‐dependent chemokines were not affected by LZIP overexpression. LZIP also increased Lkn‐1‐induced chemotactic activity through activation of the NF‐κB pathway, whereas LZIP did not affect either the transactivation of NF‐κB or the chemotactic activities induced by other CCR1‐dependent chemokines. Western blot analysis showed that LZIP increased the degradation of IκBα induced by Lkn‐1 but not by other CCR1‐dependent chemokines. Results from electrophoretic mobility shift assay (EMSA) showed that LZIP enhanced the Lkn‐1‐induced DNA‐binding activity of NF‐κB. These data indicate that LZIP functions as a positive regulator in the NF‐κB activation pathway that is triggered by Lkn‐1 without affecting the transcriptional activation of NF‐κB induced by other CCR1‐dependent chemokines. J. Cell. Physiol. 211: 630–637, 2007. © 2006 Wiley‐Liss, Inc.