Cell-Cycle-Dependent Phosphorylation of PRPS1 Fuels Nucleotide Synthesis and Promotes Tumorigenesis.

Nucleotide supply is essential for DNA replication in proliferating cells, including cancer cells. Ribose-phosphate diphosphokinase 1 (PRPS1) is a key enzyme to produce the consensus precursor of nucleotide synthesis. PRPS1 participates in the pentose phosphate pathway (PPP) by catalyzing the phosphoribosylation of D-ribose 5-phosphate (R-5P) to 5-phosphoribosyl-1-pyrophosphate. Therefore, PRPS1 not only controls purine biosynthesis and supplies precursors for DNA and RNA biosynthesis but also regulates PPP through a feedback loop of the PRPS1 substrate R-5P. However, it is still elusive whether PRPS1 enhances nucleotide synthesis during cell-cycle progression. In this study, we explore the role and activation mechanism of PRPS1 in cell-cycle progression of colorectal cancer, and observed a peak in its enzymatic activity during S phase. CDK1 contributes to upregulation of PRPS1 activity by phosphorylating PRPS1 at S103; loss of phosphorylation at S103 delayed the cell cycle and decreased cell proliferation. PRPS1 activity in colorectal cancer samples is higher than in adjacent tissue, and the use of an antibody that specifically detects PRPS1 phosphorylation at S103 showed consistent results in 184 colorectal cancer tissues. In conclusion, compared with upregulation of PRPS1 expression levels, increased PRPS1 activity, which is marked by S103 phosphorylation, is more important in promoting tumorigenesis and is a promising diagnostic indicator for colorectal cancer. SIGNIFICANCE: These findings show that the enzymatic activity of PRPS1 is crucial for cell-cycle regulation and suggest PRPS1 phosphorylation at S103 as a direct therapeutic target and diagnostic biomarker for colorectal cancer.

Derepression of LOXL4 inhibits liver cancer growth by reactivating compromised p53.

TP53 is the most frequently mutated gene in human cancer, whereas tumors with wild-type TP53 develop alternative strategies to survive. Identifying new regulators of p53 reactivation would greatly contribute to the development of cancer therapies. After screening the entire genome in liver cancer cells, we identified lysyl oxidase-like 4 (LOXL4) as a novel regulator for p53 activation. We found that 5-azacytidine (5-aza-CR) induces LOXL4 upregulation, with LOXL4 subsequently binding the basic domain of p53 via its low-isoelectric point region. The interaction between LOXL4 and p53 induces the reactivation of compromised p53, resulting in cell death. Furthermore, the nude mouse xenograft model showed that the 5-aza-CR-dependent LOXL4-p53 axis reduces tumor growth. A positive correlation between LOXL4 expression and overall survival in liver cancer patients with wild-type p53 tumors was observed. In conclusion, we found that 5-aza-CR-induced LOXL4 upregulation reactivates wild-type p53 and triggers cell death, which blocks liver cancer development.

Opposing Roles of Acetylation and Phosphorylation in LIFR-Dependent Self-Renewal Growth Signaling in Mouse Embryonic Stem Cells.

LIF promotes self-renewal of mouse embryonic stem cells (mESCs), and in its absence, the cells differentiate. LIF binds to the LIF receptor (LIFR) and activates the JAK-STAT3 pathway, but it remains unknown how the receptor complex triggers differentiation or self-renewal. Here, we report that the LIFR cytoplasmic domain contains a self-renewal domain within the juxtamembrane region and a differentiation domain within the C-terminal region. The differentiation domain contains four SPXX repeats that are phosphorylated by MAPK to restrict STAT3 activation; the self-renewal domain is characterized by a 3K motif that is acetylated by p300. In mESCs, acetyl-LIFR undergoes homodimerization, leading to STAT3 hypo- or hyper-activation depending on the presence or absence of gp130. LIFR-activated STAT3 restricts differentiation via cytokine induction. Thus, LIFR acetylation and serine phosphorylation differentially promote stem cell self-renewal and differentiation.

Lysyl Oxidase 3 Is a Dual-Specificity Enzyme Involved in STAT3 Deacetylation and Deacetylimination Modulation.

In mammalian cells, histone deacetylase (HDAC) and Sirtuin (SIRT) are two families responsible for removing acetyl groups from acetylated proteins. Here, we describe protein deacetylation coupled with deacetylimination as a function of lysyl oxidase (LOX) family members. LOX-like 3 (Loxl3) associates with Stat3 in the nucleus to deacetylate and deacetyliminate Stat3 on multiple acetyl-lysine sites. Surprisingly, Loxl3 N-terminal scavenger receptor cysteine-rich (SRCR) repeats, rather than the C-terminal oxidase catalytic domain, represent the major deacetylase/deacetyliminase activity. Loxl3-mediated deacetylation/deacetylimination disrupts Stat3 dimerization, abolishes Stat3 transcription activity, and restricts cell proliferation. In Loxl3-/- mice, Stat3 is constitutively acetylated and naive CD4+ T cells are potentiated in Th17/Treg cell differentiation. When overexpressed, the SRCR repeats from other LOX family members can catalyze protein deacetylation/deacetylimination. Thus, our findings delineate a hitherto-unknown mechanism of protein deacetylation and deacetylimination catalyzed by lysyl oxidases.