AP‐1 and Mitf interact with NFATc1 to stimulate cathepsin K promoter activity in osteoclast precursors

Cathepsin K (CTSK) is a secreted protease that plays an essential role in osteoclastic bone resorption and osteoporotic bone loss. We have previously shown that activator protein 1 (AP‐1) stimulates CTSK promoter activity and that proximal nuclear factor of activated T cells cytoplasmic 1 (NFATc1)‐binding sites play a major role in the stimulation of CTSK gene expression by receptor activator of NFκB ligand (RANKL). In the present study, we have extended these observations and further dissected the effects of transcription factors involved in the regulation of CTSK gene expression. Our aim was to investigate the cooperative interplay among transcription factors AP‐1, microphthalmia‐associated transcription factor (Mitf), and NFATc1, and the consequent regulatory effects on CTSK transcription. Experiments were carried out in RAW 264.7 cells, which can be readily differentiated to osteoclasts upon RANKL stimulation. Our data show that AP‐1, Mitf, and NFATc1 are capable of independently stimulating CTSK promoter activity. A combination of any two factors further enhances CTSK promoter activity, with the combination of AP‐1 (c‐fos/c‐jun) and NFATc1 inducing the largest increase. We further identify a synergistic effect when all three factors cooperate intimately at the proximal promoter region, yielding maximal transcriptional upregulation of the CTSK promoter. RANKL induces temporal localization of AP‐1 and NFATc1 to the CTSK promoter. These results suggest that the interaction of multiple transcription factors mediate a maximal response to RANKL‐induced CTSK gene expression. Activator protein 1 (AP‐1), nuclear factor of activated T cells cytoplasmic 1 (NFATc1), and microphthalmia‐associated transcription factor (Mitf) cooperate synergistically to stimulate cathepsin K (CTSK) promoter activity. While a combination of any two factors stimulates CTSK promoter activity, all three together are necessary to induce maximal activity. In addition, abrogation of NFATc1 binding sites significantly diminishes binding of and response to NFATc1, yet both AP‐1 and Mitf still stimulate CTSK promoter activity. Finally, receptor activator of NFκB ligand induces temporal localization of AP‐1 and NFATc1 to the CTSK promoter.