Induction of HEXIM1 activities by HMBA derivative 4a1: Functional consequences and mechanism

Abstract

We have been studying the role of Hexamethylene bisacetamide (HMBA) Induced Protein 1 (HEXIM1) as a tumor suppressor whose expression is decreased in tamoxifen resistant and metastatic breast cancer. HMBA was considered the most potent and specific inducer for HMBA inducible protein 1 (HEXIM1) prior to our studies. Moreover, the ability of HMBA to induce differentiation is advantageous for its therapeutic use when compared to cytotoxic agents. However, HMBA induced HEXIM1 expression required at mM concentrations and induced dose limiting toxicity, thrombocytopenia. Thus we structurally optimized HMBA and identified a more potent inducer of HEXIM1 expression, 4a1. The studies reported herein tested the ability of 4a1 to induce HEXIM1 activities using a combination of biochemical, cell phenotypic, and in vivo assays. 4a1 induced breast cell differentiation, including the stem cell fraction in triple negative breast cancer cells. Clinically relevant HEXIM1 activities that are also induced by 4a1 include enhancement of the inhibitory effects of tamoxifen and inhibition of breast tumor metastasis. We also provide mechanistic basis for the phenotypic effects of 4a1. Our results support the potential of an unsymmetrical HMBA derivative, such as 4a1, as lead compound for further drug development.

HMBA is a putative HSP70 activator stimulating HEXIM1 expression that is down-regulated by estrogen

“…P21 protein expression is a biomarker for cell differentiation, and all three compounds significantly increased p21 expression compared to the control (Fig. 7A), which is consistent with the study of HMBA and 4a1 in breast cancer cells [19]. The results suggest that an HSP70 activator could induce cell differentiation, which is different from what has been reported for HSP70 inhibitors.…”

“…2) [18]. The finding provides a unique unsymmetrical molecular scaffold that can induce HEXIM1 expression in prostate and breast cancer cells, and has opened a new lead optimization direction for HMBA [18,19]. Our results support the potential of both symmetrical and unsymmetrical HMBA derivatives as new drug candidates.…”

“…At this stage, estrogen receptor still plays a critical role in cancer cell proliferation, but does not respond to these agents [28,29]. HEXIM1 interferes with estrogen receptor in a different mechanism compared to tamoxifen and aromatase inhibitors, and may be able to solve the endocrine therapy resistant breast cancer [19]. HMBA is a well-used tool drug to induce HEXIM1 expression, but its toxicity limits the clinical application.…”

Phosphorylation of the histone demethylase KDM5B and regulation of the phenotype of triple negative breast cancer

“…We previously reported that the tumor suppressor HEXIM1 is a mediator of KDM5B recruitment to its target genes, and HEXIM1 is required for the inhibition of nuclear hormone receptor activity by KDM5B 25 . We also reported that induction of HEXIM1 expression by a small molecule inducer resulted in downregulation of Nanog expression and CSC fraction in MDA-MB-231 cells 41 . We determined if HEXIM1 is also a mediator of the regulation of the expression of pluripotency genes by KDM5B.…”

Hexim1, an RNA-controlled protein hub

“…Other names are Cardiac Lineage-associated Protein (CLP-1) [2], M enage A Quatre (MAQ-1) [3] or Estrogen Down-regulated Gene (EDG-1) [4]. Hexim1 synthesis is induced by HEXamethylene-bis-acetamide (HMBA) and analogs [5], histone deacetylase inhibitors [6] and nucleotide depletion [7]. Hexim1 is degraded by autophagy in some circumstances [8].…”