MOLECULAR MECHANISM OF ACTION OF THE NATURAL POLYPHENOLIC COMPOUND AND THE P300 INHIBITOR “CARNOSOL” AGAINST THE TRIPLE NEGATIVE BREAST CANCE

Carnosol, a naturally occurring Phyto polyphenol found in sage, oregano, and rosemary, has been extensively studied by our laboratory for its anticancer effects in various types of cancer. In human Triple-Negative Breast Cancer (TNBC), carnosol was shown to inhibit cellular viability, colony growth, induced cell cycle arrest, autophagy, and apoptosis. Nonetheless, very little is known about the molecular mechanism of action. In the current study, the ability of carnosol to inhibit metastasis and tumour growth was examined. Wound healing and invasion assays revealed that carnosol inhibited migration and invasion at non-cytotoxic concentrations of MDA-MB-231 cells. Also, carnosol was found to inhibit the activity and downregulated the expression of MMP-9. Activation of STAT3, a transcription factor that regulates MMP-9 expression, was also inhibited via carnosol-mediated ROS-dependent proteasome degradation. In vivo study using chick embryo tumour growth assay has shown that carnosol significantly and markedly suppressed tumour growth and metastasis of breast cancer xenografts. Additionally, we found that carnosol induces ROS-dependent, p38-dependent ER stress and activates UPR via upregulating the ER stress sensors (ATF4/CHOP, ATF6⍺, and IRE1⍺/XBP1). Also, upstream triggers of Unfolded Protein Response (UPR) pathway, β-catenin and ER stress chaperones were upregulated. On the other hand, the cell survival Akt/mTOR signalling pathway was downregulated in a ROS-dependent manner. We also found that carnosol targeted p300 and PCAF Histone Acetyltransferases (HATs) to proteasome degradation through a ROS-dependent mechanism. Interestingly, using a cell-free system, we show for the first time that carnosol efficiently and selectively inhibited histone acetyltransferase activity of p300 while having no effect on the other HATs such as PCAF and GCN5. This work provides further confirmation that carnosol represents a promising anti-breast cancer therapeutic compound and identifies it as a novel natural p300 inhibitor that could be added to the existing panel.