Abstract
Tylophorine, the biologically active component of herb tylophora indica from India, was first isolated, discovered and further described their pharmacological actions in 1935. The extract of herb tylophora indica contains up to 30 % tylophorine of total alkaloids. However, tylocrebrine, a tylophorine analog, failed phase I clinical trials of anti-cancer agent due to neurotoxic side effects in the 1960s. According to structure–activity relationship (SAR), we have successfully synthesized orally active tylophorine-derived dibenzoquinoline-33b without central neurotoxicity in our previous studies. Recently, we also reported that tylophorine downregulated cyclin A2 expression through c-Jun accumulation and consequently exerted G1 arrest in carcinoma cells. Tylophorine compounds have been the focus of drug development for decades. Tylophorine derivatives exhibit anti-cancer activities but their cellular targets remain unknown. We used a biotinylated tylophorine derivative to probe for the interacting cellular target(s) of tylophorine. Tylophorine directly binds to caprin-1 and consequently enhances the recruitment of G3BP1, c-Myc mRNA, and cyclin D2 mRNA to form a ribonucleoprotein complex. Subsequently, this tylophorine targeted ribonucleoprotein complex is sequestered to the polysomal fractions and the protein expressions of the associated mRNA-transcripts are repressed. Caprin-1 depleted carcinoma cells become more resistant to tylophorine, associated with decreased formation of the ribonucleoprotein complex targeted by tylophorine. Consequently, tylophorine downregulates c-Myc and cyclins D1/D2, causing hypophosphorylation of Rb and suppression of both processing-body formation and the Warburg effect. Gene expression profiling and gain-of-c-Myc-function experiments also revealed that the downregulated c-Myc contributes to the anti-oncogenic effects of tylophorine compounds. Furthermore, the potent tylophorine derivative dibenzoquinoline-33b elicited a similar effect, as c-Myc protein levels were also decreased in xenograft tumors treated with dibenzoquinoline-33b. Thus, tylophorine compounds exert anti-cancer activity predominantly by targeting and sequestering the caprin-1 protein and c-Myc mRNA associated ribonucleoprotein complex. Our studies may provide useful information on the direct anti-tumor mechanisms of tylophorine to modify and optimize functional group, and further improve the drug development of tylophorine compounds as anti-cancer agents.