Threonine appears to be essential for proliferation of human as well as mouse embryonic stem cells.

Abstract

Mouse embryonic stem (mES) cell proliferation depends exclusively on the nutritionally essential amino acid, L-threonine, in the medium. Other essential and non-essential amino acids need not be added to the medium for mES cell proliferation. Furthermore, the threonine analog, 3-hydroxynorvaline (3-HNV), selectively inhibits mES cell proliferation (Wang et al., 2009). HeLa, MEF and 3T3 cell growth all are not affected by 3-HNV. Selective inhibition of ES cell proliferation by 3-HNV is expected to have teratogenic and embryotoxic effects on development, and these effects have been observed in chicken and mouse embryos (Louw et al., 2005). It has been proposed that rapid catabolism via threonine dehydrogenase (TDH) accounts for how threonine supplementation is needed to support mES cell proliferation and that TDH is the site of 3-HNV inhibition of mES cell proliferation (Wang et al., 2009; Han et al., 2013; Chen and Wang, 2014). In support of this possibility, TDH induction enhances reprogramming of mouse somatic cells into induced pluripotent stem (iPS) cells, and 3-HNV inhibits this induction (Han et al., 2013). The concentrations of threonine or 3-HNV needed for mES cell proliferation or its inhibition are, however, over an order of magnitude lower than the apparent Km or Ki values for interaction of threonine or 3-HNV with TDH (Wang et al., 2009). Hence, another mechanism may account for the influences of threonine and 3-HNV on mES cell proliferation. To test this hypothesis, we determined whether 3-HNV inhibits human embryonic stem (hES) cell proliferation. Humans produce truncated and apparently inactive TDH proteins that cannot make appropriate contact with 3-HNV, threonine or NAD+, the cofactor needed for TDH activity (Edgar, 2002).