Symbol STAT5A Entrez 6776 OMIM 601511 | Alt. symbols STAT5 HUGO 11366 RefSeq NM_003152 | |
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Signal transducer and activator of transcription 5 (STAT5) refers to two highly related proteins, STAT5A and STAT5B, which are part of the seven-membered STAT family of proteins. Though STAT5A and STAT5B are encoded by separate genes, the proteins are 90% identical at the amino acid level. STAT5 proteins are involved in cytosolic signalling and in mediating the expression of specific genes. Aberrant STAT5 activity has been shown to be closely connected to a wide range of human cancers, and silencing this aberrant activity is an area of active research in medicinal chemistry.
Contents
Activation and function
In order to be functional, STAT5 proteins must first be activated. This activation is carried out by kinases associated with transmembrane receptors:
In the activation pathway illustrated to the left, the ligand involved is a cytokine and the specific kinase taking part in activation is JAK. The dimerized STAT5 represents the active form of the protein, which is ready for translocation into the nucleus.
Once in the nucleus, the dimers bind to STAT5 response elements, inducing transcription of specific sets of genes. Upregulation of gene expression by STAT5 dimers has been observed for genes dealing with:
Activated STAT5 dimers are, however, short-lived and the dimers are made to undergo rapid deactivation. Deactivation may be carried out by a direct pathway, removing the phosphate groups using phosphatases like PIAS or SHP-2 for example, or by an indirect pathway, which involves reducing cytokine signalling.
STAT5 and cancer
STAT5 has been found to be constitutively phosphorylated in cancer cells, implying that the protein is always present in its active form. This constant activation is brought about either by mutations or by aberrant expressions of cell signalling, resulting in poor regulation, or complete lack of control, of the activation of transcription for genes influenced by STAT5. This leads to constant and increased expression of these genes. For example, mutations may lead to increased expression of anti-apoptotic genes, the products of which actively prevent cell death. The constant presence of these products preserve the cell in spite of it having become cancerous, causing the cell to eventually become malignant.
Treatment approaches
Attempts at treatment for cancer cells with constitutively phosphorylated STAT5 have included both indirect and direct inhibition of STAT5 activity. While more medicinal work has been done in indirect inhibition, this approach can lead to increased toxicity in cells and can also result in non-specific effects, both of which are better handled by direct inhibition.
Indirect inhibition targets kinases associated with STAT5, or targets proteases that carry out terminal truncation of proteins. Different inhibitors have been designed to target different kinases:
Direct inhibition of STAT5 activity makes use of small molecule inhibitors that prevent STAT5 from properly binding to DNA, or prevent proper dimerization. The inhibiting of DNA binding utilizes RNA interference, antisense oligodeoxynucleotide, and short hairpin RNA. The inhibition of proper dimerization, on the other hand, is brought about by the use of small molecules that target the SH2 domain. Recent work on drug development in the latter field have proved particularly effective.