A class of less toxic
retinoids, called heteroarotinoids, was evaluated for their molecular mechanism of growth inhibition of two
head and neck squamous cell carcinoma (
HNSCC) cell lines SCC-2 and SCC-38. A series of 14 heteroarotinoids were screened for growth inhibition activity in vitro. The two most active compounds, one that contained an
oxygen heteroatom (6) and the other a
sulfur heteroatom (16), were evaluated in a xenograph model of
tumor establishment in nude mice. Five days after
subcutaneous injection of 10(7) SCC-38 cells, groups of 5 nu/nu mice were gavaged daily (5 days/week for 4 weeks) with 20 mg/kg/day of
all-trans-retinoic acid (t-RA, 1), 10 mg/kg/day of 6, 10 mg/kg/day of 16, or
sesame oil. After a few days, the dose of t-RA (1) was decreased to 10 mg/kg/day to alleviate the side effects of
eczema and
bone fracture. No significant toxic effects were observed in the heteroarotinoid groups. All three
retinoids caused a statistically significant reduction in
tumor size as determined by the Student t-test (P < 0. 05). Complete
tumor regression was noted in 3 of 5 mice treated with t-RA (1), 4 of 5 mice treated with 16, 1 of 5 mice treated with 6, and 1 of 5 mice treated with
sesame oil.
Reverse transcriptase polymerase chain reaction (RT-PCR) was used to determine that the expression levels of RARalpha, RXRalpha, and RXRbeta were similar in the two cell lines, while RARbeta expression was higher in SCC-2 over SCC-38, and RARgamma expression was higher in SCC-38 over SCC-2. Receptor cotransfection assays in
CV-1 cells demonstrated that 16 was a potent activator of both RAR and RXR receptors, while 6 was selective for the RXR receptors. Transient cotransfection assays in
CV-1 cells using an
AP-1 responsive reporter plasmid demonstrated that t-RA (1), 6, and 16 each inhibited AP-1-driven transcription in this cell line. In conclusion, the growth inhibition activity of the RXR-selective 6 and the more potent growth inhibition activity of the RAR/RXR pan-agonist 16 implicate both RARs and RXRs in the molecular mechanism of
retinoid growth inhibition. Moreover, the
chemoprevention activity and the lack of toxicity of heteroarotinoids demonstrate their clinical potential in
head and neck cancer chemoprevention.