The half-life of the transcript encoding the folate receptor alpha in KB cells is reduced by cytosolic proteins expressed in folate-replete and not in folate-depleted cells.

The KB cell, a transformed human cell line, constitutively expresses a high level of the glycosylphosphatidylinositol (GPI) anchored folate receptor protein alpha (FR alpha) and thereby can grow in medium containing <1 nM folate. When transferred from a folate-replete (FR) medium to one folate-deficient (FD), intracellular folate diminishes about 50-fold and expression of the FR alpha increases 6-fold. This up-regulation is mediated by a 4.5-fold increase in the initial transcription rate and a 2.4-fold prolongation of the mRNA half-life that together provide a higher level of the steady-state mRNA abundance. An RNA gel -shift assay of a 5' region of the mRNA that includes all of the non-coding and 24 nt of coding sequence, and a 3' region comprised only of coding sequence, identified unique complexes with cytosolic proteins from the FR-KB cells that were not observed with the cytosol from FD-KB cells. It appears, therefore, that expression of these folate-dependent cytosolic trans-active proteins function to maintain a shorter half-life of the mRNA in the FR-KB cells by binding to 5' and 3' cis elements, reducing the stability of this transcript.