We have previously reported that the proanthocyanidin (PAC) fraction of blueberry leaf extract (BB‐PAC) inhibits the proliferation of HTLV‐1‐infected adult T‐cell leukemia (ATL) by inducing apoptosis. In the present study, we further analyzed the structure of BB‐PAC and elucidated the molecular mechanism underlying the inhibitory function of HTLV‐1‐infected and ATL cells. After hot water extraction with fractionation with methanol‐acetone, BB‐PAC was found to be concentrated in fractions 4 to 7 (Fr7). The strongest inhibition of ATL cell growth was observed with Fr7, which contained the highest BB‐PAC polymerization degree of 14. The basic structure of BB‐PAC is mainly B‐type bonds, with A‐type bonds (7.1%) and cinchonain I units as the terminal unit (6.1%). The molecular mechanism of cytotoxicity observed around Fr7 against ATL cells was the degradation of JAK1 to 3 and the dephosphorylation of STAT3/5, which occurs by proteasome‐dependent proteolysis, confirming that PAC directly binds to heat shock protein 90 (HSP90). JAK degradation was caused by proteasome‐dependent proteolysis, and we identified the direct binding of PAC to HSP90. In addition, the binding of cochaperone ATPase homolog 1 (AHA1) to HSP90, which is required for activation of the cofactor HSP90, was inhibited by BB‐PAC treatment. Therefore, BB‐PAC inhibited the formation of the HSP90/AHA1 complex and promoted the degradation of JAK protein due to HSP90 dysfunction. These results suggest that the highly polymerized PAC component from blueberry leaves has great potential as a preventive and therapeutic agent against HTLV‐1‐infected and ATL cells.

N-myc downstream-regulated gene 2 (NDRG2), which is a tumour suppressor, is frequently lost in many types of tumours, including adult T-cell leukaemia/lymphoma (ATL). The downregulation of NDRG2 expression is involved in tumour progression through the aberrant phosphorylation of several important signalling molecules. We observed that the downregulation of NDRG2 induced the translocation of protein arginine methyltransferase 5 (PRMT5) from the nucleus to the cytoplasm via the increased phosphorylation of PRMT5 at Serine 335. In NDRG2^low ATL, cytoplasmic PRMT5 enhanced HSP90A chaperone activity via arginine methylation, leading to tumour progression and the maintenance of oncogenic client proteins. Therefore, we examined whether the inhibition of PRMT5 activity is a drug target in NDRG2^low tumours. The knockdown of PRMT5 and binding partner methylsome protein 50 (MEP50) expression significantly demonstrated the suppression of cell proliferation via the degradation of AKT and NEMO in NDRG2^low ATL cells, whereas NDRG2-expressing cells did not impair the stability of client proteins. We suggest that the relationship between PRMT5/MEP50 and the downregulation of NDRG2 may exhibit a novel vulnerability and a therapeutic target. Treatment with the PRMT5-specific inhibitors CMP5 and HLCL61 was more sensitive in NDRG2^low cancer cells than in NDRG2-expressing cells via the inhibition of HSP90 arginine methylation, along with the degradation of client proteins. Thus, interference with PRMT5 activity has become a feasible and effective strategy for promoting cancer vulnerability in NDRG2^low ATL.

Inactivation of PTEN by post-translational modifications causes aberrant amplification of the PI3K/AKT signaling pathway in many tumors. PTEN is a tumor suppressor phosphatase that is frequently phosphorylated at conserved serine/threonine residues (S380, T382, and T383 clusters) in the C-terminal tail of ATL and various solid cancer cells. Here, we identify SCY1-like protein 2 (SCYL2), with a protein kinase-like domain, as a novel PTEN-binding protein; however, the mechanism by which SCYL2 regulates PTEN phosphorylation remains unclear. SCYL2-associated complex phosphorylates PTEN at STT, and SCYL2 downregulation has anti-tumor effects in ATL via inhibition of the PI3K/AKT signaling pathway by dephosphorylating PTEN at STT. SCYL2 reportedly binds to the clathrin heavy chain (CHC), which regulates cytoplasmic vesicle formation, trafficking, and signaling pathways. Our results indicate that SCYL2 expression induces the binding of CHC to PTEN. Furthermore, the inhibition of clathrin-coated vesicles (CCVs) by CHC downregulation or inhibition suppresses cell survival by reducing phosphorylated PTEN at the STT, suggesting that SCYL2 enhances PTEN phosphorylation through CCVs as a signaling platform. Our results indicate that SCYL2/CHC complex plays a pivotal role in regulating the PI3K/AKT signaling pathway through PTEN phosphorylation, thus leading to tumor development and may be a promising novel target for treating tumors.

Adult T-cell leukemia/lymphoma (ATL) is an aggressive T-cell malignancy with a poor prognosis that is caused by human T-cell leukemia virus type 1 infection. We previously demonstrated that N-myc downstream-regulated gene 2 (NDRG2) is significantly downregulated in ATL, resulting in aberrant activation of the signal transduction pathways through the dissociation of serine/threonine protein phosphatase 2A. To identify potential targets of NDRG2, we performed comprehensive mass spectrometry of differentially phosphorylated peptides in ATL cells with overexpression of NDRG2 using a TiO₂-based enrichment method. Kyoto Encyclopedia of Genes and Genomes and gene ontology analysis revealed that the downregulated phosphopeptides correlated with signaling pathways, T-cell differentiation, and proliferation. Our results identified signal transducer and activator of transcription 5B as a novel NDRG2-regulated protein that is dephosphorylated at serine 193 and tyrosine 699. Although enforced expression of NDRG2 in ATL cell lines does not change the phosphorylation of Janus kinase 3, an upstream regulator of STAT5, phosphorylated STAT5 at tyrosine and serine is significantly suppressed by the direct binding to STAT5 with NDRG2 leading to the inhibition of STAT5 downstream gene expression. Furthermore, NDRG2 binds to STAT5B with alanine replacement of Y699 (Y699A), but only weakly associates with S193A, suggesting that NDRG2 is directly involved in serine phosphorylation through the recruitment of serine/threonine protein phosphatase 2A to STAT5. Because S193 A remarkably induces reduced phosphorylation of Y699 and subsequent transcriptional activity, the induction of serine phosphorylation through the loss of NDRG2 expression is dispensable for STAT5 tyrosine phosphorylation and activity. Since the loss of NDRG2 expression is essential factor to maintenance of ATL cells by STAT5 activity through phosphorylation of serine and tyrosine, targeting STAT5 becomes a feasible and effective strategy in NDRG2-deficient ATL.