TSP50 encodes a testis-specific protease and is negatively regulated by p53.

Earlier studies suggested that TSP50 is a testis-specific gene that encodes a protein, which is homologous to serine proteases but differs in that threonine replaces serine in its catalytic triad. Most importantly, it was abnormally reactivated in many breast cancer biopsies tested. While further investigating its biochemical and cell biological natures, we found that TSP50 exhibited enzyme activity and was located in the endoplasmic reticulum and cytosol membrane. During our studies to elucidate the regulatory mechanisms related to its differential expression, we discovered a putative p53-binding site and several Sp1-binding sites in the TSP50 promoter, which led us to test if it was regulated by the p53 gene. We found that the p53 transgene negatively regulated the TSP50 promoter in diverse types of cell lines. This result was consistent with other observations: (a) p53 overexpression reduced endogenous TSP50 expression; and (b) breast cancer cell lines containing mutated p53, such as MCF7/Adr, or normal p53, such as MCF7, produced high or low levels of TSP50 transcripts, which was consistent with the fact that TSP50 promoter activity was much higher in MCF7/Adr than that in MCF7 cells. We also found that the quantity of Sp1 transcription factor was lower in MCF7/Adr than in MCF7 cells, which suggested that another mechanism (i.e., transcription factor modulation) was also involved in TSP50 differential expression.