Signal transducers and activators of transcription (STAT) were originally discovered as components of
cytokine signal transduction pathways. Persistent activation of one STAT, STAT3, is a common feature of
prostate cancer. Activated STAT3 was found in pathology specimens obtained from
prostatectomy in the cancerous areas but not in the normal margins. Because the activation of STAT3 is mediated by the action of an upstream
Janus kinase (JAK)
kinase, usually JAK1 or JAK2, the activation step for STAT3 might itself be a target for
therapy in
prostate cancer. However, the redundancy of upstream
kinases may make this strategy unreliable for
therapy. To develop molecular targets for
prostate cancer treatment, JAK
kinase and STAT3 inhibition of two
prostate cancer lines were compared. DU145 and NRP-154 cells were treated with JAK
kinase inhibitors, analyzed for onset of apoptosis, and measured by
annexin V binding and
propidium iodide uptake. Activation of
caspases in the cells was determined by measuring cleaved
caspase-3 following treatment. For determining the effect on mitochondrial membrane depolarization that accompanies apoptosis, the
fluorescent dye JC-1 was used. STAT3 was specifically inhibited by transfecting either a dominant-negative (DN) STAT3 plasmid or antisense STAT3
oligonucleotides into the cells. To look for reduction in STAT3 levels within cells, fixed and permeabilized
prostate cancer cells were stained with antibody to STAT3. We found that more than one JAK
kinase is involved in STAT3 activation in
prostate cancer lines.
AG490 (JAK2 specific) induced apoptosis in DU145 but not in NRP-154
prostate cancer lines, whereas
piceatannol (JAK1 specific) induced apoptosis in NRP-154 but not in DU145 cells. Next, we demonstrated efficacy of specific STAT3 inhibitors in
prostate cancer lines. Both induction of apoptosis and reduction in intracellular
STAT3 protein were observed following treatment with antisense STAT3
oligonucleotides, while transfection of a DN-STAT3 plasmid into both
prostate cancer cell lines resulted in loss of viability and onset of apoptosis. We conclude that STAT3-specific inhibitors, rather than JAK
kinase-specific inhibitors, should be more useful therapeutically in treating
androgen-resistant
prostate cancer and that STAT3 is an appropriate target in the treatment of
prostate cancer.