E3 Ubiquitin Ligases Siah1/2 Regulate Hypoxia-Inducible Factor-1 (Hif-1)-Mediated T-H 17 Cell Differentiation

IL-17 is known to be a cytokine mainly secreted from T-h 17 cells, which well associate with autoimmune inflammatory responses. In the generation of T-h 17 cells, RORc and RORa have pivotal roles in controlling the transcription of Il17. We speculated additional regulation in Il17a transcription and randomly screened a 6344 clone cDNA library to identify specific modulators for Il17a promoter activity. After the screen, the E3 ubiquitin ligases SIAH1 and SIAH2 were investigated further and confirmed to increase Il17a promoter activity in a T-cell line and to promote T-h 17 development ex vivo. This enhancement was a consequence of enhanced expression of hypoxia-inducible factor-1 alpha (HIF-1 alpha) protein, which is reported to directly regulate expression of Il17a and Rorgt at the transcriptional level. In the absence of HIF-1 alpha, both ubiquitin ligases had little effect on T-h 17 ]cell differentiation. These results suggest that the SIAH1 and SIAH2 play a pivotal role to promote T-h 17 cell differentiation through maintaining the stability of HIF-1 alpha protein.