Nucleotide biosynthesis arrest by silencing SHMT1 function via vitamin B₆-coupled vector and effects on tumor growth inhibition

Serine hydroxymethyltransferase isoforms (SHMT1 & SHMT2α), which serve as scaffold protein for the formation of a multi-enzyme complex and generate one-carbon unit for the de novo thymidylate biosynthesis pathway during DNA synthesis, are vitamin B₆ (VB₆)-dependent enzyme. Cancer cells with high proliferation intensity need increased SHMT activation which enforces the facilitated-diffusion of VB₆ for the continuous functioning of thymidylate synthase cycle. Therefore, SHMT knockdown presents an alternative approach to prevent DNA synthesis in cancer cells; however, its potential to inhibit cancer growth remains unknown so far. Here we demonstrated that VB₆ coupled to poly(ester amine) (VBPEA) enforces a high level of VTC (VB₆-transporting membrane carriers)-mediated endocytosis of the complexed SHMT1 siRNA (siSHMT1) to interrupt the thymidylate biosynthesis pathway of cancer cells. The detrimental effect of SHMT1 knockdown on the disintegration of multi-enzyme complex resulted in cell cycle arrest and a decrease in cell's genomic DNA content, leading to enhanced apoptotic events in cancer cells. A reduction in tumor size was observed with constant SHMT1 suppression in xenograft mice. This study illustrates how silencing the SHMT1 expression inhibits cancer growth and the increased VB₆ channeling for sustenance of cancer cells promotes VB₆-coupled vector to elicit enhanced delivery of siSHMT1.