Inhibition of Balb/c 3T3 cell transformation by synthetic acyclic retinoid NIK-333; possible involvement of enhanced gap junctional intercellular communication

Background: In an attempt to develop effective and non-cytotoxic cancer chemopreventive derivatives of retinoids, a novel acyclic retinoid has previously been synthesized. This acyclic retinoid, NIK-333, had been reported to suppress recurrence of primary hepatocellular carcinomas. We explored the molecular mechanisms by which NIK-333 exerts anti-proliferative effects. Methods: We employed Balb/c 3T3 cells, since they can be used as a quantitative transformation assay and since we can study a possible involvement of gap-junctional intercellular communication (GJIC) in their growth control. We included all-trans-retinoic acid (ATRA) for comparison. Results: We first confirmed that these cells express the retinoid receptors, RAR alpha, gamma and RXR alpha, suggesting that they respond to NIK-333 and ATRA. When NIK-333 or ATRA was added to Balb/c 3T3 cells during the induction of cell transformation by a standard (3-methylcholanthrene (MCA) alone) or two-stage (low dose of MCA plus 12-O-tetradecanoylphorbol-13-acetate (TPA)) protocol, there was a significant decrease in the number of transformed foci. The extent of inhibition of transformation by NIK-333 was similar to that exerted by ATRA. Employing the microinjection dye-transfer assay, we found that both retinoids increase GJIC level when measured 24 h after treatment. The extent of GJIC increase by NIK-333 was similar to that of ATRA. These retinoids also increased the amount of connexin 43 (Cx43) on the plasma membrane as revealed by immunostaining. Conclusion: These data indicate that NIK-333 suppresses chemical carcinogenesis in vitro and support the hypothesis that enhancement of GJIC is involved in this process. (c) 2007 Published by Elsevier Ltd on behalf of International Society for Preventive Oncology.