Phosphorylation
Confirmed phosphorylation sites on YBX1
The first phosphorylation site identified was serine 102 (S102) in the CSD region of YBX1 (Table 1).27 This is also the most extensively studied site in YBX1. Sutherland et al showed that mutating S102 of YBX1 to alanine (A) decreased tumor growth in breast cancer, strongly suggesting that S102 phosphorylation is critical for the oncogenic activity of YBX in breast cancer. Phosphorylation at S102 may promote the YBX1 oncogenic ability by facilitating nuclear translocation of YBX1 and enhancing its binding to DNA. Furthermore, it was reported that the level of phosphorylated S102 positively correlates with EGFR (epidermal growth factor receptor) and HER2 (human epidermal growth factor receptor 2) expression: both are upregulated in primary breast cancer specimens. On the other hand, S102A-YBX1 mutant prevented induction of both EGFR and HER2 and showed decreased tumor growth in breast cancer.28
In addition to phosphorylation of S102 in breast cancer, a recent study by our lab underlines the significance of the novel phosphorylation of S165 in the CSD region of YBX1 (Table 1).29 We provided the first evidence regarding the important connection between YBX1 and the nuclear factor kappa B (NF-κB). We suggest that YBX1 functions as a potential NF-κB activator. Using mass spectrometry (MS) analysis, we identified novel phosphorylation of S165 on YBX1. Overexpression of the S165A-YBX1 mutant in either 293 cells or colon cancer HT29 cells showed dramatically reduced NF-κB activating ability as compared to that of wt YBX1, confirming that S165 phosphorylation is critical for the activation of NF-κB by YBX1. We also showed that expression of the S165A-YBX1 mutant significantly decreased expression of NF-κB-inducible genes, reduced cell growth, and compromised tumorigenic ability as compared to wt YBX1. Taken together, we proved that YBX1 functions as a tumor promoter via NF-κB activation, and that phosphorylation of S165 of YBX1 is critical for this function. Therefore, our important discovery may lead to blocking S165 phosphorylation as a potential therapeutic strategy to treat colon cancer.
Other YBX1 phosphorylation sites have also been identified. Using deletion constructs of YBX1, von Roeyen et al identified two additional phosphorylation sites, tyrosine 188 (Y188) and Y281 in the CTD domain of YBX1 (Table 1). Using site-specific antibodies, they proved that phosphorylation of Y281, but not Y188 in YBX1 takes place and appears to correlate with nuclear protein shuttling of YBX1 in undifferentiated human monocytic THP-1 cells.30 Kasyapa et al reported that fibroblast growth factor receptor 2 (FGFR2), a receptor tyrosine kinase, can interact with YBX1, leading to the phosphorylation of tyrosine 162 (Y162) in the CTD of YBX1 in atypical myeloproliferative disorders (Table 1).31 This finding suggests that tyrosine phosphorylation of YBX1 may play an important role in the development of this devastating disease, in which FGFR2 has been widely implicated.