Inhibition of STAT5A promotes osteogenesis by DLX5 regulation

S100 calcium‑binding protein A16 suppresses the osteogenic differentiation of rat bone marrow mesenchymal stem cells by inhibiting SMAD family member 4 signaling

Osteogenesis is a complex process of bone formation regulated by various factors, yet its underlying molecular mechanisms remain incompletely understood. The present study aimed to investigate the role of S100A16, a novel member of the S100 protein family, in the osteogenic differentiation of rat bone marrow mesenchymal stem cells (BMSCs) and uncover a novel Smad4-mitogen-activated protein kinase (MAPK)/Jun N-terminal kinase (JNK) signaling axis. In the present study, the expression level of S100A16 in bone tissues and BMSCs from ovariectomized rats was evaluated and then the impact of S100A16 silencing on osteogenic differentiation was examined. Increased S100A16 expression was observed in bone tissues and BMSCs from ovariectomized rats, and S100A16 silencing promoted osteogenic differentiation. Further transcriptomic sequencing revealed that the Smad4 pathway was involved in S100A16 silencing-induced osteogenesis. The results of western blot analysis revealed that S100A16 overexpression not only downregulated Smad4 but also activated MAPK/JNK signaling, which was validated by treatment with MAPK and JNK inhibitors U0126 and SP600125. Overall, in the present study, the novel regulatory factors influencing osteogenic differentiation were elucidated and mechanistic insights that could aid in the development of targeted therapeutic strategies for patients with osteoporosis were provided.

RUNX transcription factors: biological functions and implications in cancer

Runt-related transcription factors (RUNX) are a family of transcription factors that are essential for normal and malignant hematopoietic processes. Their most widely recognized role in malignancy is to promote the occurrence and development of acute myeloid leukemia. However, it is worth noting that during the last decade, studies of RUNX proteins in solid tumors have made considerable progress, suggesting that these proteins are directly involved in different stages of tumor development, including tumor initiation, progression, and invasion. RUNX proteins also play a role in tumor angiogenesis, the maintenance of tumor cell stemness, and resistance to antitumor drugs. These findings have led to the consideration of RUNX as a tumor biomarker. All RUNX proteins are involved in the occurrence and development of solid tumors, but the role of each RUNX protein in different tumors and the major signaling pathways involved are complicated by tumor heterogeneity and the interacting tumor microenvironment. Understanding how the dysregulation of RUNX in tumors affects normal biological processes is important to elucidate the molecular mechanisms by which RUNX affects malignant tumors.

Impact of STAT5A-CNVs on growth traits in Chinese beef cattle breeds

CNVs, which are a type of structural variation, make a substantial impact on diverse characteristics in multiple species. Q-PCR and data association analysis were used for STAT5A gene copy in this study. This study aimed to investigate the copy number variation (CNV) of the STAT5A gene in seven Chinese cattle breeds, namely Qinchuan cattle, Xianan cattle, Yunling cattle, Ji'an cattle, Jiaxian Red cattle, Qaidam cattle, and Guyuan yellow cattle. Blood samples were collected for CNV typing, and the correlation between CNV type and growth traits was analyzed using SPSS 23.0 software and ANOVA. The findings revealed variations in the distribution of different copy number types among the different cattle breeds. Furthermore, association analysis demonstrated a positive impact of CNV in the STAT5A gene on cattle growth: in the JX, individuals with duplication types exhibited superior performance in terms of rump length (P < 0.05). Conversely, normal GY cattle demonstrated better body height and abdomen circumference (P < 0.05), while QD cattle exhibited a significant correlation between weight and body length with normal individuals (P < 0.05). Moreover, QC bovine duplication individuals outperformed other types, with copy number variation significantly associated with chest depth, chest width, and body length (P < 0.05). The results validate the correlation between copy number variation (CNV) of the STAT5A gene and growth characteristics in five different cattle breeds, providing a reliable benchmark for the purpose of cattle breeding.

Dual scaffold delivery of miR-210 mimic and miR-16 inhibitor enhances angiogenesis and osteogenesis to accelerate bone healing

Angiogenesis is critical for successful bone repair, and interestingly, miR-210 and miR-16 possess counter-active targets involved in both angiogenesis and osteogenesis: miR-210 acts as an activator by silencing EFNA3 & AcvR1b, while miR-16 inhibits both pathways by silencing VEGF & Smad5. It was thus hypothesized that dual delivery of both a miR-210 mimic and a miR-16 inhibitor from a collagen-nanohydroxyapatite scaffold system may hold significant potential for bone repair. Therefore, this systems potential to rapidly accelerate bone repair by directing enhanced angiogenic-osteogenic coupling in host cells in a rat calvarial defect model at a very early 4 week timepoint was assessed. In vitro, the treatment significantly enhanced angiogenic-osteogenic coupling of human mesenchymal stem cells, with enhanced calcium deposition after just 10 days in 2D and 14 days on scaffolds. In vivo, these dual-miRNA loaded scaffolds showed more than double bone volume and vessel recruitment increased 2.3 fold over the miRNA-free scaffolds. Overall, this study demonstrates the successful development of a dual-miRNA mimic/inhibitor scaffold for enhanced in vivo bone repair for the first time, and the possibility of extending this 'off-the-shelf' platform system to applications beyond bone offers immense potential to impact a myriad of other tissue engineering areas. STATEMENT OF SIGNIFICANCE: miRNAs have potential as a new class of bone healing therapeutics as they can enhance the regenerative capacity of bone-forming cells. However, angiogenic-osteogenic coupling is critical for successful bone repair. Therefore, this study harnesses the delivery of miR-210, known to be an activator of both angiogenesis and osteogenesis, and miR-16 inhibitor, as miR-16 is known to inhibit both pathways, from a collagen-nanohydroxyapatite scaffold system to rapidly enhance osteogenesis in vitro and bone repair in vivo in a rat calvarial defect model. Overall, it describes the successful development of the first dual-miRNA mimic/inhibitor scaffold for enhanced in vivo bone repair. This 'off-the-shelf' platform system offers immense potential to extend beyond bone applications and impact a myriad of other tissue engineering areas.

Metformin potentiates immunosuppressant activity and adipogenic differentiation of human umbilical cord-mesenchymal stem cells

Metformin, a first-line drug for type-2 diabetes, displays pleiotropic effects on inflammation, aging, and cancer. Obesity triggers a low-grade chronic inflammation leading to insulin resistance, characterized by increased pro-inflammatory cytokines produced by adipocytes and infiltrated immune cells, which contributes to metabolic syndrome. We investigated metformin's differentiation and immunoregulatory properties of human umbilical cord-mesenchymal stem cells (UC-MSC), as cellular basis of its beneficial role in metabolic dysfunctions. Isolation, characterization and multilineage differentiation of UC-MSC were performed using standard protocols and flow-cytometry. Metformin effects on UC-MSC growth was assessed by colony formation and MTT assay, gene and protein expression by qRT-PCR, and western blot analysis. Proliferation of peripheral blood mononuclear cells (PBMCs) co-cultured with metformin-treated UC-MSC-conditioned media was evaluated by dye dilution assay. We show that metformin decreases proliferation and colony formation of UC-MSCs and enhances their adipogenic lineage commitment. Metformin (3 mM) increases PPARγ and downregulates FABP4 mRNA both in basal and in adipogenic culture conditions; however, the modulation of PPARγ expression is unrelated to the antiproliferative effects. Moreover, metformin inhibits UC-MSC inflammatory activity reducing the expression of IL-6, MCP-1, and COX-2. Conditioned media, collected from metformin-treated UC-MSCs, down-regulate CD3+ T lymphocyte growth in stimulated PBMCs and, in particular, reduce the CD8+ T cell population. These results indicate that metformin may favor new adipocyte formation and potentiate immune suppressive properties of UC-MSCs. Thus, adipose tissue regeneration and anti-inflammatory activity may represent possible mechanisms by which metformin exerts its positive effect on lipid metabolism.

NFIB-MLL1 complex is required for the stemness and Dlx5-dependent osteogenic differentiation of C3H10T1/2 mesenchymal stem cells

Despite significant progress in our understanding of the molecular mechanism of mesenchymal stem cell (MSC) differentiation, less is known about the factors maintaining the stemness and plasticity of MSCs. Here, we show that the NFIB-MLL1 complex plays key roles in osteogenic differentiation and stemness of C3H10T1/2 MSCs. We find that depletion of either NFIB or MLL1 results in a severely hampered osteogenic potential and failed activation of key osteogenic transcription factors, such as Dlx5, Runx2, and Osx, following osteogenic stimuli. In addition, the NFIB-MLL1 complex binds directly to the promoter of Dlx5, and exogenous expression of Myc-Dlx5, but not the activation of either the BMP- or the Wnt-signaling pathway, is sufficient to restore the osteogenic potential of cells depleted of NFIB or MLL1. Moreover, chromatin immunoprecipitation (ChIP) and ChIP-sequencing analysis showed that the NFIB-MLL1 complex mediates the deposition of trimethylated histone H3K4 at both Dlx5 and Cebpa, key regulator genes that function at the early stages of osteogenic and adipogenic differentiation, respectively, in uncommitted C3H10T1/2 MSCs. Surprisingly, the depletion of either NFIB or MLL1 leads to decreased trimethylated histone H3K4 and results in elevated trimethylated histone H3K9 at those developmental genes. Furthermore, gene expression profiling and ChIP-sequencing analysis revealed lineage-specific changes in chromatin landscape and gene expression in response to osteogenic stimuli. Taken together, these data provide evidence for the hitherto unknown role of the NFIB-MLL1 complex in the maintenance and lineage-specific differentiation of C3H10T1/2 MSCs and support the epigenetic regulatory mechanism underlying the stemness and plasticity of MSCs.

Downregulation of the RNA-binding protein PUM2 facilitates MSC-driven bone regeneration and prevents OVX-induced bone loss

BACKGROUND

Although mRNA dysregulation can induce changes in mesenchymal stem cell (MSC) homeostasis, the mechanisms by which post-transcriptional regulation influences MSC differentiation potential remain understudied. PUMILIO2 (PUM2) represses translation by binding target mRNAs in a sequence-specific manner.

METHODS

In vitro osteogenic differentiation assays were conducted using human bone marrow-derived MSCs. Alkaline phosphatase and alizarin red S staining were used to evaluate the osteogenic potential of MSCs. A rat xenograft model featuring a calvarial defect to examine effects of MSC-driven bone regeneration. RNA-immunoprecipitation (RNA-IP) assay was used to determine the interaction between PUM2 protein and Distal-Less Homeobox 5 (DLX5) mRNA. Ovariectomized (OVX) mice were employed to evaluate the effect of gene therapy for postmenopausal osteoporosis.

RESULTS

Here, we elucidated the molecular mechanism of PUM2 in MSC osteogenesis and evaluated the applicability of PUM2 knockdown (KD) as a potential cell-based or gene therapy. PUM2 level was downregulated during MSC osteogenic differentiation, and PUM2 KD enhanced MSC osteogenic potential. Following PUM2 KD, MSCs were transplanted onto calvarial defects in 12-week-old rats; after 8 weeks, transplanted MSCs promoted bone regeneration. PUM2 KD upregulated the expression of DLX5 mRNA and protein and the reporter activity of its 3'-untranslated region. RNA-IP revealed direct binding of PUM2 to DLX5 mRNA. We then evaluated the potential of adeno-associated virus serotype 9 (AAV9)-siPum2 as a gene therapy for osteoporosis in OVX mice.

CONCLUSION

Our findings suggest a novel role for PUM2 in MSC osteogenesis and highlight the potential of PUM2 KD-MSCs in bone regeneration. Additionally, we showed that AAV9-siPum2 is a potential gene therapy for osteoporosis.

Advance in bone destruction participated by JAK/STAT in rheumatoid arthritis and therapeutic effect of JAK/STAT inhibitors

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic joint inflammation and bone erosion. The bones in the human body are constantly undergoing bone remodeling throughout their lives, which is the process of bone resorption by osteoclasts to damaged bone tissue and new bone formation by osteoblasts. Osteoblasts (OBs) are the main functional cells in bone formation, responsible for the synthesis, secretion and mineralization of the bone matrix. On the contrary, osteoclasts (OCs) mediate bone breakdown during natural bone turnover, but excessive breakdown occurs in RA. Under the condition of RA inflammation, many molecules, such as IL-1β, IL-6, TNF-α, IL-17 and hypoxia-inducible factor-1α (HIF-1α) are produced that could mediate bone loss. Studies have shown that cytokines mainly promote the formation of OCs and play a role in bone resorption by stimulating OBs to express receptor activator of NF-κB ligand (RANKL). JAK/STAT plays a crucial role in the process of bone destruction. And JAK/STAT pathway mediates the RANKL/receptor activator of NF-κB (RANK)/osteoprotegerin (OPG) axis. Tofacitinib, Baricitinib, Peficitinib and Filgotinib are now being used in patients with moderate to severe RA, as well as in patients with RA who have an inadequate response to methotrexate therapy and bone destruction. Currently, Tofacitiniband Baritinib areapprovedfor thetreatmentof moderate-to-severely active RA. JAK inhibitors have been reported to have better efficacy and lower adverse effects compared with methotrexate and adalimumab. In addition, two JAK inhibitors are currently in development: the JAK1 selective Upadacitinib, and the JAK3 selective inhibitor Decernotinib. In addition to the above JAK inhibitors, some small molecular compounds inhibit bone destruction by inhibiting the Phosphorylation of STAT3. In this paper, the research progress of bone destruction participated by JAK/ STAT in rheumatoid arthritis and therapeutic effect of JAK/STAT inhibitors were reviewed.

The long non-coding RNA T cell leukemia homeobox 1 neighbor enhances signal transducer and activator of transcription 5A phosphorylation to promote colon cancer cell invasion, migration, and metastasis

Colon cancer is among the most prevalent gastrointestinal tumor types. The long noncoding RNA (lncRNA) T cell leukemia homeobox 1 neighbor (TLX1NB) is up-regulated in colorectal cancer (CRC). However, the functional role of this lncRNA in colon cancer remains unknown. In our study, we investigated the clinical significance of TLX1NB in colon cancer through bioinformatics analysis and explored its role in migration, invasion and metastasis of colon cancer cell with a series of experiments. Firstly, TLX1NB was up-regulated in colon cancer tissues and increased TLX1NB expression was significantly associated with advanced N stages. In wound healing assays and transwell assays, TLX1NB overexpression promoted HCT116 cell migration and invasion while TLX1NB knockdown inhibited SW620 cell migration and invasion. In vivo, TLX1NB knockdown suppressed pulmonary metastasis of SW620 cell and vimentin expression but increased E-cadherin expression. Then, TLX1NB overexpression enhanced signal transducer and activator of transcription 5A (STAT5A) phosphorylation and TLX1NB knockdown suppressed STAT5A phosphorylation. Moreover, the inhibition of STAT5A phosphorylation reversed TLX1NB overexpression-associated increase in HCT116 cell migratory and invasive activity. In conclusion, TLX1NB enhances STAT5A phosphorylation to promote colon cancer cell invasion, migration, and metastasis.

The Effect of Bone Marrow Mesenchymal Stem Cells (BMSCs) on Brain Injury Repair and Synapse Regeneration in Mice Under Different Conditions of Intrauterine Ischemia and Hypoxia Through Wnt Pathway

Bone marrow mesenchymal stem cells (BMSCs) can be differentiated into a variety of cells and repair damaged cells. We explore whether BMSCs can repair brain damage and synapses regeneration in mice under intrauterine ischemia and hypoxia. Twenty-five pregnant mice were assigned into control group, 6% hypoxic injury group, 8% hypoxic injury group, 6% treatment group, 8% treatment group followed by analysis of the expression of MBP, MAG, CSPGs, IGF-1, NCAN, COLIV, SynD1G1, GFAP, GSK-3β, and β-actin by RT-PCR and Western blot. Our results showed that the expression of MBP, MAG, COL IV, SynD1G1, IGF-1 in the treatment group were significantly higher than those in hypoxic injury group with significant differences between the 8% treatment group and 6% treatment group (P < 0.05). In conclusion, BMSCs can repair brain damage and synapse regeneration in mice under different intrauterine ischemia and hypoxia conditions which might be through Wnt signaling pathway.

Asymmetrically Substituted m-Terphenyl Phosphates Inhibit the Transcription Factor STAT5a

We recently presented Stafia‐1 as the first chemical entity that inhibits the transcription factor STAT5a with selectivity over the highly homologous STAT5b. Stafia‐1, which was identified from a series of symmetrically substituted m‐terphenyl phosphates, binds to the interface between the SH2 domain and the linker domain of STAT5a. Here, we outline a synthetic strategy for the synthesis of asymmetrically substituted m‐terphenyl phosphates, which can be tailored to address their asymmetric STAT5a binding site in a more specific manner. The asymmetrically substituted m‐terphenyl phosphate with the highest activity against STAT5a was converted to a phosphatase‐stable monofluoromethylene phosphonate. The synthetic methodology and activity analysis described here provide first insights into the structure‐activity relationships of m‐terphenyl phosphates for use as selective STAT5a inhibitors.

Long non-coding RNA RP11-84C13.1 promotes osteogenic differentiation of bone mesenchymal stem cells and alleviates osteoporosis progression via the miR-23b-3p/RUNX2 axis

The objective of the present study was to determine the role of RP11-84C13.1 in osteoporosis (OP) and its molecular mechanism. First, clinical samples were collected from OP patients and normal control patients. Human bone marrow stromal cells (hBMSCs) were extracted from femoral head tissues. Runt-related transcription factor 2 (RUNX2) and RP11-84C13.1 serum levels were assessed by reverse transcription-quantitative (RT-q)PCR. Following transfection of pcDNA-RP11-84C13.1, si-RP11-84C13.1, microRNA (miRNA)-23b-3p mimic and miRNA-23b-3p inhibitor, the expression levels of RUNX2 and RP11-84C13.1 were determined by RT-qPCR. In addition, the osteogenic ability of hBMSCs was assessed by Alizarin Red staining. The binding of RP11-84C13.1 to miRNA-23b-3p and the binding of miRNA-23b-3p to RUNX2 was confirmed by dual-luciferase reporter gene assay. Long non-coding RNA (lncRNA) RP11-84C13.1 was significantly downregulated in the serum of OP patients. The osteogenic differentiation-related genes RUNX2 and RP11-84C13.1 were markedly upregulated in a time-dependent manner, while the miRNA-23b-3p level gradually decreased in hBMSCs with the prolongation of osteogenesis. RP11-84C13.1 knockdown inhibited the osteogenic differentiation of hBMSCs. Furthermore, RP11-84C13.1 regulated RUNX2 expression by targeting miRNA-23b-3p. Overexpression of miRNA-23b-3p partially reversed the promoting effect of RP11-84C13.1 on the osteogenesis of hBMSCs. In conclusion, lncRNA RP11-84C13.1 upregulated RUNX2 by absorbing miRNA-23b-3p, and thus induced hBMSC osteogenesis to alleviate osteoporosis.

Signaling Pathway and Transcriptional Regulation in Osteoblasts during Bone Healing: Direct Involvement of Hydroxyapatite as a Biomaterial

Bone defects and periodontal disease are pathological conditions that may become neglected diseases if not treated properly. Hydroxyapatite (HA), along with tricalcium phosphate and bioglass ceramic, is a biomaterial widely applied to orthopedic and dental uses. The in vivo performance of HA is determined by the interaction between HA particles with bone cells, particularly the bone mineralizing cells osteoblasts. It has been reported that HA-induced osteoblastic differentiation by increasing the expression of osteogenic transcription factors. However, the pathway involved and the events that occur in the cell membrane have not been well understood and remain controversial. Advances in gene editing and the discovery of pharmacologic inhibitors assist researchers to better understand osteoblastic differentiation. This review summarizes the involvement of extracellular signal-regulated kinase (ERK), p38, Wnt, and bone morphogenetic protein 2 (BMP2) in osteoblastic cellular regulation induced by HA. These advances enhance the current understanding of the molecular mechanism of HA as a biomaterial. Moreover, they provide a better strategy for the design of HA to be utilized in bone engineering.

Effect of Inducible BMP-7 Expression on the Osteogenic Differentiation of Human Dental Pulp Stem Cells

BMP-7 has shown inductive potential for in vitro osteogenic differentiation of mesenchymal stem cells, which are an ideal resource for regenerative medicine. Externally applied, recombinant BMP-7 was able to induce the osteogenic differentiation of DPSCs but based on our previous results with BMP-2, we aimed to study the effect of the tetracyclin-inducible BMP-7 expression on these cells. DPSC, mock, and DPSC-BMP-7 cell lines were cultured in the presence or absence of doxycycline, then alkaline phosphatase (ALP) activity, mineralization, and mRNA levels of different osteogenic marker genes were measured. In the DPSC-BMP-7 cell line, the level of BMP-7 mRNA significantly increased in the media supplemented with doxycycline, however, the expression of Runx2 and noggin genes was upregulated only after 21 days of incubation in the osteogenic medium with doxycycline. Moreover, while the examination of ALP activity showed reduced activity in the control medium containing doxycycline, the accumulation of minerals remained unchanged in the cultures. We have found that the induced BMP-7 expression failed to induce osteogenic differentiation of DPSCs. We propose three different mechanisms that may worth investigating for the engineering of expression systems that can be used for the induction of differentiation of mesenchymal stem cells.

Osteomodulin positively regulates osteogenesis through interaction with BMP2

Osteomodulin (OMD), a member of the small leucine-rich proteoglycan family, distributes in mineralized tissues and is positively regulated by bone morphogenetic protein 2 (BMP2). However, the exact function of OMD during mineralization and its association with BMP2 remain poorly understood. Herein, the expression pattern of OMD during osteogenesis was investigated in human dental pulp stem cells. Silencing OMD gene significantly suppressed the alkaline phosphatase activity, mineralized nodule formation and osteogenesis-associated gene transcription. Besides, OMD could enhance BMP2-induced expression of SP7 and RUNX2 with concentration dependence in vitro. Rat mandibular bone defect model revealed that scaffolds injected with the combination of OMD and suboptimal BMP2 exhibited more mature and abundant mineralized bone than that treated with OMD or suboptimal BMP2 alone. Mechanistically, OMD could bind to BMP2 via its terminal leucine-rich repeats and formed complexes with BMP2 and its membrane receptors, thus promoting BMP/SMAD signal transduction. In addition, OMD was a putative target gene of SMAD4, which plays a pivotal role in this pathway. Collectively, these data elucidate that OMD may act as a positive coordinator in osteogenesis through BMP2/SMADs signaling.

JAK/STAT Activation: A General Mechanism for Bone Development, Homeostasis, and Regeneration

The Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathway serves as an important downstream mediator for a variety of cytokines, hormones, and growth factors. Emerging evidence suggests JAK/STAT signaling pathway plays an important role in bone development, metabolism, and healing. In this light, pro-inflammatory cytokines are now clearly implicated in these processes as they can perturb normal bone remodeling through their action on osteoclasts and osteoblasts at both intra- and extra-articular skeletal sites. Here, we summarize the role of JAK/STAT pathway on development, homeostasis, and regeneration based on skeletal phenotype of individual JAK and STAT gene knockout models and selective inhibition of components of the JAK/STAT signaling including influences of JAK inhibition in osteoclasts, osteoblasts, and osteocytes.

The Selectivity of Fosfosal for STAT5b over STAT5a is Mediated by Arg566 in the Linker Domain

Fosfosal is the O-phosphorylated derivative of salicylic acid, with documented clinical use as a prodrug for the treatment of inflammatory diseases. We recently discovered that fosfosal itself inhibits the protein-protein interaction domain, the SH2 domain, of the tumor-related transcription factor STAT5b. Here, we demonstrate that fosfosal is selective for STAT5b over its close homologue STAT5a. This selectivity is mediated by the STAT5b residue Arg566, located in the SH2 domain-adjacent linker domain. Our data provide further evidence for the role of the STAT linker domain in determining the activity of small molecules against the SH2 domain. We present a refined binding model for fosfosal and STAT5b, which can serve as the basis for the development of fosfosal-based STAT5b inhibitors.

Stafia-1: a STAT5a-Selective Inhibitor Developed via Docking-Based Screening of in Silico O-Phosphorylated Fragments

We present a new approach for the identification of inhibitors of phosphorylation-dependent protein-protein interaction domains, in which phenolic fragments are adapted by in silico O-phosphorylation before docking-based screening. From a database of 10 369 180 compounds, we identified 85 021 natural product-derived phenolic fragments, which were virtually O-phosphorylated and screened for in silico binding to the STAT3 SH2 domain. Nine screening hits were then synthesized, eight of which showed a degree of in vitro inhibition of STAT3. After analysis of its selectivity profile, the most potent inhibitor was then developed to Stafia-1, the first small molecule shown to preferentially inhibit the STAT family member STAT5a over the close homologue STAT5b. A phosphonate prodrug based on Stafia-1 inhibited STAT5a with selectivity over STAT5b in human leukemia cells, providing the first demonstration of selective in vitro and intracellular inhibition of STAT5a by a small-molecule inhibitor.