G-Quadruplex Mediated c-myc Specific Downregulation: A Unique Pathway of the Anticancer Action of Immunomodulator Drugs

Abstract Hydroxychloroquine (HCQ), and chloroquine (CQ) are in the preclinical trial stage for cancer along with their active application in autoimmune diseases and malaria. One of the critical hallmarks of cancer cells is the elevated expression of various oncogenes which promote cancer progression and contribute to poor prognosis. The upstream of the promoter region of these oncogenes often exhibits a G-quadruplex (G4) DNA structure which regulates the gene expression. Hence, targeting G4 structure has emerged as a promising therapeutic strategy for cancer. In this study, the recognition of HCQ and CQ with the G4 structure of different oncogenes and its effect on gene regulation has been explored by a combination of various biophysical and in-vitro and in-vivo biological methods. This study depicts that HCQ and CQ downregulate the c-myc oncogene transcription significantly in a G4-dependent manner compared to other oncogenes. The different biophysical techniques and molecular dynamics simulation studies illustrate that these drug molecules stack predominately at the terminal of the c-myc G4 and the binding of these molecules stabilizes c-myc G4 significantly higher than the G4 structure of other oncogenes. The in-vitro cell data exhibit a notable reduction in both c-myc mRNA and protein levels in a triple-negative breast cancer cell line following HCQ treatment. The pre-clinical breast cancer mouse model in-vivo data also indicate that HCQ reduces tumor growth through the downregulation of the c-myc oncogene. Simultaneously, HCQ also enhances the therapeutic efficacy of standard chemotherapeutic agents to be a potential candidate for combination therapy. This work demonstrates the alternative strategy of anticancer action of widely used drugs by specifically downregulating the c-myc oncogene in a G4-dependent manner.