The green staining represents the CDK4 protein, and the red staining represents -Actin in the cellular cytoplasm. in sarcomas patients and the clinical stage and the TNM grade in synovial sarcoma patients. Knockdown of CDK4 with specific small interference RNAs inhibits cell proliferation and enhances apoptotic effects in synovial sarcoma cells. CDK4 inhibitor palbociclib suppresses synovial sarcoma cell proliferation and growth in a dose and time-dependent manner. Palbociclib also inhibits the CDK4/6-Rb signaling pathway and promotes cell apoptosis without changing CDK4/6 protein levels, suggesting that palbociclib only represses the hyper-activation, not the expression of CDK4/6. Flow cytometry analysis reveals that palbociclib induces G1 cell-cycle arrest and apoptotic effects by targeting the CDK4/6-Rb pathway in synovial sarcoma cells. Furthermore, wound healing assays demonstrate that TRX 818 inhibition of the CDK4/6-Rb pathway by palbociclib significantly decreases synovial sarcoma cell migration in vitro. Our study highlights the importance of the CDK4/6-Rb pathway in human synovial sarcoma pathogenesis, and the role of the current selective CDK4/6 inhibitor, palbociclib, as a potential promising targeted therapeutic agent in the treatment of human synovial sarcoma. Introduction Synovial sarcoma (SS) is a high-grade subtype of soft tissue sarcoma that occurs mainly in children and TRX 818 young adults, characterized by the chromosomal translocation t(X;18) (p11.2; q11.2)1C3. The current treatment for localized synovial sarcoma is surgery, occasionally with the combination of additional radiotherapy and chemotherapy, and the published five-year survival rate varies from 40% to 60%4,5. However, once the primary disease advances with pulmonary metastasis and relapse, the prognosis is poor, even if under the intensive multi-agent chemotherapy. The limited availability of effective therapeutic measures indicates an urgent clinical need for novel alternative treatment strategies for patients with synovial sarcoma. Aberrations in cell cycle control is defined as one of the hallmarks of TRX 818 cancer, and may be a favorable target for the improvement of new therapeutic options for the treatment of sarcoma6,7. As one of the essential signaling pathways involved in cell cycle progression, the cyclin-dependent kinase (CDK) 4/6-retinoblastoma protein (Rb) pathway (CDK4/6-Rb pathway) is frequently found to be aberrant in cancer8. CDK4 is one of the serine/threonine (Ser/Thr) protein kinases that mediates cell cycle progression through the G1-S phase, in preparation for DNA synthesis9. The heterodimers formed by CDK4, or its close homolog CDK6, with D-type cyclins (cyclin D) are critical for cell cycle progression. In human malignancies, CDK4 associates with cyclin D and regulates the cell cycle through hyperphosphorylation and deactivation of the tumor suppressor retinoblastoma protein (Rb)10,11. Specifically, in response to pro-proliferative stimuli, cyclin D1 associates with CDK4 and gains access to the nuclear cyclin D1-CDK4 complex12. These active cyclin D/CDK4 complexes induce the phosphorylation of Rb, and thereby switch off the tumor suppressing function of Rb13. The hyperphosphorylated form of Rb is no longer able to bind with the transcription factor E2F1, leading to cancer cell cycle progression Rabbit Polyclonal to FRS3 through activated transcription of various cell-cycle and anti-apoptotic genes14,15. Activation and amplification of the cyclin D/CDK4/Rb pathway has been shown to correlate with uncontrolled tumor cell growth and proliferation in various types of malignancies, including in sarcoma16. CDK4/6 specific inhibitors are the most clinically advanced type of CDK inhibitor, and notably, a dual CDK4/6 inhibitor, palbociclib (IBRANCE?). Although palbociclib was initially developed to target the ATP-binding site of CDK4, due to the high homologous and structural similarities between CDK4 and CDK6, palbociclib also targets CDK6. Palbociclib was the first drug in this class to receive Food and Drug Administration (FDA) approval as initial endocrine-based therapy for the treatment of postmenopausal women with hormone receptor (HR)-positive/human epidermal growth factor receptor 2 (HER2)-negative advanced or metastatic breast cancer in combination with an aromatase inhibitor, letrozole, or the selective estrogen receptor downregulator, fulvestrant17C21. The FDA have since also approved the CDK4/6 inhibitors, ribociclib (KISQALI?) and abemaciclib (VerzenioTM), for a similar application22. These agents have also been investigated in other solid tumors, TRX 818 ranging from melanoma to non-small cell lung cancer23,24. Although the field of targeted-therapy for carcinomas is growing rapidly, trials with targeted treatment for rare cancers, such as sarcomas, remain scarce. Interestingly, palbociclib.