Mechanism of Crassostrea gigas T-box Transcription Factor 2: Regulation by a Transcript Isoform of Microphthalmia-Associated Transcription Factor and Its Role in Cell Proliferation.

T-box transcription factor 2 (TBX2) plays a critical role in various biological processes, including cell cycle regulation, malignant transformation, and regulating melanogenesis. In our previous study, we identified a Crassostrea gigas TBX2 (CgTBX2) and investigated its regulatory role in melanin production in oysters. Here, the mechanism of CgTBX2 in regulating cell proliferation was investigated. First, we found that CgTBX2 promoted the proliferation of mouse melanoma (B16F10) cells. CgMITF-X3, a 1347 bp transcript isoform of MITF from C. gigas, was then cloned and it was also found to promote cell proliferation. Co-transfection of CgTBX2 and CgMITF-X3 into B16F10 cells had a synergistic effect on cell proliferation, suggesting that CgMITF-X3 enhanced the function of CgTBX2 in promoting cell proliferation. CgMITF-X3 promoted the transcriptional activation of CgTBX2 by directly binding to the TCTCACGCGG sequence in the CgTBX2 promoter region. In addition, CgTBX2 and CgMITF-X3 proteins were co-located in the nucleus, indicating that these two proteins may perform a certain function collectively in the nucleus. Taken together, our findings revealed that CgTBX2 is directly activated by CgMITF-X3 at the transcriptional level, and both CgTBX2 and CgMITF-X3 facilitate cell proliferation.