ABSTRACT: Estrogen receptor α (ER-α) is a major driver of breast cancer (BC), being expressed in 75% of all BC cases. Agents such as tamoxifen are used to block ER-α activity and interfere with its down-stream oncogenic singling. However, a major problem associated with tamoxifen is the emergence of resistance. Resistant BC is often associated with aggressive relapse, metastasis, and high mortality rates of 80%. A key mechanism for development of resistance is activation of EGFR/HER2/HER3 signaling pathways and other hormone receptors (androgen receptor (AR), progesterone receptor (PR), prolactin receptors (PRL-R)) that can intrinsically activate ER-α. This study investigated a novel class of thiosemicarbazone anti-cancer agents, namely Dp44mT and DpC, on the expression and activation between major BC hormonal receptors, their co-factors and key pathways involved in resistance in ER-α-positive BC using RNA sequencing, immunoblotting and confocal microscopy and an in vivo orthotopic BC model. For the first time, we demonstrate that DpC and Dp44mT markedly reduce the expression of ER-α, AR, PR and PRL-R by inducing their proteasomal degradation. Further, these agents also inhibited EGFR, HER2 and HER3 activation in ER-α-positive BC cells, suggesting their potential ability to overcome development of resistance. Importantly, expression of key co-factors that promote ER-α-transcriptional activity, including SRC3, c-Myc, SP1, and c-Src, were also decreased by these agents. This study demonstrates for the first time the multi-axial inhibitory effects of the novel DpT agents in ER-positive BC invitro and in vivo. We demonstrate that Dp44mT and DpC decrease ER-α expression, transcriptional activity, and promote its proteasomal degradation. This study also demonstrated the ability of the DpT agents in inhibiting RTKs and key oncogenic pathways that are well established to promote endocrine resistance. Hence, for the first time we demonstrate that Dp44mT and DpC may be a promising therapeutic approach to treat ER-positive BC and to overcome resistance to conventional BC therapies.
