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

Abstract

Serine hydroxymethyltransferase isoforms (SHMT1 & SHMT2alpha), 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.