The proto-oncogene cellular myelocytomatosis (c-Myc) is a
transcription factor that is upregulated in several human
cancers. Therapeutic targeting of c-Myc remains a challenge because of a disordered
protein tertiary structure. The basic helical structure and zipper
protein of c-Myc forms an obligate heterodimer with its partner
MYC-associated factor X (MAX) to function as a
transcription factor. An attractive strategy is to inhibit MYC/MAX dimerization to decrease c-Myc transcriptional function. Several methods have been described to inhibit MYC/MAX dimerization including small molecular inhibitors and proteomimetics. We studied the effect of a second-generation small molecular inhibitor 3JC48-3 on
prostate cancer growth and viability. In our experimental studies, we found 3JC48-3 decreases
prostate cancer cells' growth and viability in a dose-dependent fashion in vitro. We confirmed inhibition of MYC/MAX dimerization by 3JC48-3 using immunoprecipitation experiments. We have previously shown that the MYC/MAX heterodimer is a transcriptional repressor of a novel
kinase protein kinase D1 (PrKD1). Treatment with 3JC48-3 upregulated PrKD1 expression and phosphorylation of known PrKD1 substrates: the
threonine 120 (Thr-120) residue in
beta-catenin and the
serine 216 (Ser-216) in Cell Division Cycle 25 (CDC25C). The mining of gene expression in human metastatic
prostate cancer samples demonstrated an inverse correlation between PrKD1 and c-Myc expression. Normal mice and mice with patient-derived
prostate cancer xenografts (PDX) tolerated
intraperitoneal injections of 3JC48-3 up to 100 mg/kg
body weight without dose-limiting toxicity. Preliminary results in these PDX mouse models suggest that 3JC48-3 may be effective in decreasing the rate of
tumor growth. In conclusion, our study demonstrates that 3JC48-3 is a potent MYC/MAX heterodimerization inhibitor that decreases
prostate cancer growth and viability associated with upregulation of PrKD1 expression and
kinase activity.