SLC29A1 and SLC29A2 are human nicotinamide cell membrane transporters
Nicotinamide, a main precursor of NAD+, is essential for cellular fuel respiration, energy production, and other cellular processes. Transporters for other precursors of  NAD+ such as nicotinic acid and nicotinamide mononucleotide have been identified, but the cellular transporter of nicotinamide has not been elucidated. Here, we demonstrated that equilibrative nucleoside transporters 1 and 2 (ENT1 and 2, encoded by SLC29A1/2) drove cellular nicotinamide uptake and set nicotinamide metabolism homeostasis. In addition, ENT1/2 exhibited a strong capacity to change the composition of cellular metabolites and the profile of transcripts, especially those related to nicotinamide. Then, we further found that ENT1/2 could regulate cellular respiration and senescence, which are universally acknowledged as nicotinamide biological functions, to which the NAD+ pool level and mitochondrial status influenced by ENT1/2 made important contributions. Together, ENT1 and ENT2 act as both cellular nicotinamide-level keepers and nicotinamide biological regulators through their NAM transport functions. Overall design: To investigate the effects of SLC29A1 and SLC29A2 to cellualr  behavior and fate, the SLC29A1 and SLC29A2 was knocked down, respectively, in AC16 cell line and MSC  with specific siRNA. We then performed expression profiling analysis using data obtained from RNA-seq of above cell groups. Comparative gene expression profiling analysis of RNA-seq data for negtive control and KD group(siNC, siSLC29A1 and siSLC29A2) in AC16 cell and MSC.