Unphosphorylated STAT5A stabilizes heterochromatin and suppresses tumor growth

Abstract

Tumor suppressors known to date impede cancer growth by arresting the cell cycle or promoting apoptosis. Here we show that unphosphorylated human STAT5A functions as a tumor suppressor capable of repressing multiple oncogenes via heterochromatin formation. Unphosphorylated STAT5A binds to heterochromatin protein 1α (HP1α) and stabilizes heterochromatin. Expressing unphosphorylated STAT5A or HP1α inhibits colon cancer growth in mouse xenograft models. Transcriptome profiling shows that expressing an unphosphorylatable STAT5A has similar effects to overexpressing HP1α in global gene expression. Notably, the majority of the genes commonly repressed by unphosphorylated STAT5A and HP1α have been implicated in cancer development. Finally, down-regulation, somatic mutations, and deletions of STAT5 genes are found in certain human cancers. These results suggest that unphosphorylated STAT5A may epigenetically suppress tumor growth by promoting heterochromatin formation.JAK/STAT | Drosophila | fluorescence recovery after photobleaching (FRAP) | transcription H eterochromatin plays a role in chromosomal compaction and transcriptional silencing and is emerging as a mechanism of tumor suppression. Certain tumor suppressors, such as breast cancer type 1 susceptibility protein (BRCA1) and the retinoblastoma protein (RB), have been shown to promote heterochromatin formation at specific loci or maintain the stability of constitutive heterochromatin (1-4). On the other hand, oncogenic JAK disrupts heterochromatin formation in both Drosophila and human cells (5, 6). In addition, a reduction in the levels of heterochromatin protein 1 (HP1), the major component and a determinant of heterochromatin (7), is associated with human cancer progression, including colon and breast cancers and leukemia (8). The target genes normally repressed by heterochromatin, however, have not been systematically investigated.Previous studies using a Drosophila leukemia model have demonstrated a noncanonical mode of JAK/STAT signaling, in which the unphosphorylated form of STAT is localized in heterochromatin in association with HP1; STAT activation (by phosphorylation) causes its dispersal from heterochromatin, leading to HP1 delocalization and heterochromatin loss (9-11). Moreover, unphosphorylated STAT and heterochromatin are essential for maintaining genomic stability and counteracting aging in Drosophila (12, 13). To investigate whether unphosphorylated STAT in mammals also promotes heterochromatin formation, and whether this constitutes a mechanism of tumor suppression, we examined the interaction between human STAT5A and HP1α (also known as "chromobox protein homolog 5," or CBX5) in human cells, tested their effects on tumorigenesis in mouse xenograft models, and examined their effects on global gene transcription by means of expression profiling. Results STAT5A and HP1αPhysically Interact. To test whether human STAT5A physically interacts with human HP1α, we used a series of plasmids to express HP1α-Flag and wild-type or mutant GFP-tagged STAT5A (...

A new role for STAT3 as a regulator of chromatin topology

“…31,32 To date, there is little evidence for this mechanism in mammalian cells, although a role for STAT family members in heterochromatin maintenance in mammalian cells has been suggested. 33 Our data also underline an important role of the ND in STAT signaling and suggest that inhibition of the ND may affect U-STAT dimerization and P-STAT tetramerization and prevent/ inhibit DNA looping/bending leading to changes in 3D chromatin structure and gene expression. Indeed, when using inhibitor of the STAT3 ND, we observed changes in heterochromatin foci localization in the nucleus, 30 further suggesting that STAT3 may be involved in the formation of higher order protein-DNA or protein-protein complexes responsible for regulation of 3D chromatin structure in the nucleus.…”

Co-operating STAT5 and AKT signaling pathways in chronic myeloid leukemia and mastocytosis: possible new targets of therapy

“…76 Likewise, essential tyrosine and serine/threonine phosphorylation sites related to the transforming function of STATs have been mapped ( Figure 4B). 77 Although non-phosphorylated STAT5 (non-pSTAT5) may epigenetically suppress tumor growth by promoting heterochromatin formation, acting thus as a "tumor-suppressor", 78 the major role of STAT5 (like other STATs) is to promote the transcription of different genes. To fulfill this role, STAT5: i) undergoes an activation consisting in a tyrosine-phosphorylation step; ii) dimerizes through reciprocal interaction mediated by the phosphor-tyrosyl residue and the SH2 domain of the STAT monomers; iii) is internalized into the cell nucleus via associating with importins; iv) binds a specific DNA sequence; and v) activates the transcription through recruitment of protein partners.…”

Requirement for CRIF1 in RNA interference and Dicer-2 stability

“…36 For Western blot studies, adult flies were allowed to lay eggs on apple juice/agar plates. Larvae were collected for preparation of cell lysates.…”