Tuning of alternative splicing--switch from proto-oncogene to tumor suppressor.

STAT5B, a specific member of the STAT family, is intimately associated with prostate tumor progression. While the full form of STAT5B is thought to promote tumor progression, a naturally occurring truncated isoform acts as a tumor suppressor. We previously demonstrated that truncated STAT5 is generated by insertion of an alternatively spliced exon and results in the introduction of an early termination codon. Present approaches targeting STAT proteins based on inhibition of functional domains of STAT's, such as DNA-binding, cooperative binding (protein-protein interaction), dimerization and phosphorylation will halt the action of the entire gene, both the proto-oncogenic and tumor suppressor functions of Stat5B. In this report we develop a new approach aimed at inhibiting the expression of full-length STAT5B (a proto-oncogene) while simultaneously enhancing the expression of STAT5∆B (a tumor suppressor). We have demonstrated the feasibility of using steric-blocking splice-switching oligonucleotides (SSOs) with a complimentary sequence to the targeted exon-intron boundary to enhance alternative intron/exon retention (up to 10%). The functional effect of the intron/exon proportional tuning was validated by cell proliferation and clonogenic assays. The new scheme applies specific steric-blocking splice-switching oligonucleotides and opens an opportunity for anti-tumor treatment as well as for the alteration of functional abilities of other STAT proteins.

Activation of signal transducer and activator of transcription 5 is required for progression of autochthonous prostate cancer: evidence from the transgenic adenocarcinoma of the mouse prostate system.

The constitutive activation of signal transducer and activator of transcription (STAT) proteins has been demonstrated in many diverse human cancer cell lines and clinical tumors including prostate cancer. The STAT family has at least seven members, and the two forms of the STAT5 protein, STAT5A and STAT5B, exhibit a high degree of sequence similarity. We have reported previously that expression of STAT5B, but not STAT5A, could enhance the transforming potential of v-src and induces cell cycle progression and motility in fibroblasts. In the current study we demonstrate specific activation of STAT5B in epithelial cells representing invasive and metastatic prostate cancer. We also demonstrate that the naturally occurring dominant-negative isoform STAT5DeltaB can block cell cycle progression through G(1) and inhibit the growth, invasive potential, and clonogenic ability of these prostate cancer cell lines. Furthermore, we report that the dominant-negative isoform STAT5DeltaB can inhibit the growth of prostate cancer cells in grafting studies. These results support our hypothesis that specific activation of STAT5, in particular STAT5B, facilitates the progression of prostate cancer.