Regulation of p21/ras protein expression by diallyl sulfide in DMBA induced neoplastic changes in mouse skin

Diallyl sulfide (DAS), a naturally occurring organosulfide, present in garlic, is known to possess pleiotropic biological effects. DAS is known to inhibit chemically induced tumors in a number of animal models. The chemopreventive properties of DAS seem to occur through a number of mechanisms, but its role on primary events on oncogenic activation is not well understood. In the present study, we demonstrated the modulatory effect of DAS on the expression of H-ras gene product, p21/ras protein as one of the mechanisms of its chemopreventive action in chemically induced mouse skin tumors. Our results showed that DAS administration leads to modulation of the DMBA-induced levels of p21/ras oncoprotein as early as 24h after the DMBA application, suggesting down-regulation of the p21/ras by DAS. Furthermore, the modulatory effects of DAS were also evident in DMBA-induced mouse skin tumors. DAS administration led to increase in the levels of cytosolic p21/ras and decrease in the levels of p21/ras in membrane fractions. DAS administration was also found to down regulate the DMBA-induced H-ras mRNA level in mouse skin tumors. The immunohistochemical staining of the skin/tumor showed 55.82 and 46.86% decrease in the area positive for p21/ras expression levels in DAS pre- and post-supplemented groups, respectively. Flow-cytometric analysis, further confirms our results as indicated by a shift in the mean fluorescence intensity (MFI) towards lower fluorescence in DAS administered groups in comparison to the DMBA treated group. Thus, one mechanism of the growth inhibitory properties of DAS is through the suppression of development of tumors that harbor ras mutations by inhibiting the membrane association of oncogenic p21/ras protein.