The vitamin, folate, is an essential cofactor required for one-carbon transfers in DNA synthesis. Folate receptors (FR) account for the majority of folate transport into a select number of normal and malignant cells and are involved in control of cell proliferation. Three isoforms of the FR have been identified. While it is known that some hematopoietic progenitors express FR, it is unclear which phenotypic populations of hematopoietic cells express FR and the isoforms expressed on each cell type. We hypothesized that specific isoforms of FR are expressed on primitive hematopoietic progenitor cells which contribute to their high proliferative potential. To address this hypothesis we performed a detailed characterization of normal hematopoietic cells using fluorescence activated cell sorting (FACS) and Northern blot analysis of FACS sorted cells. Adult bone marrow and term cord blood CD34+ cells, a marker for primitive and committed progenitors, were incubated with a polyclonal rabbit anti-human FR antibody. 92-100% of CD34+ cells expressed FR, while the receptor was detected on only 15-40% of nonadherent, low density cells. To determine which isoforms of the FR were expressed on individual purified populations of cells, RT-PCR was performed on monocytes/granulocytes (CD15), B-cells (CD19), T-cells (CD3), and primitive and committed progenitors (CD34). CD34+ cells were the only cells analyzed that expressed the β isoform. The γ isoform was also expressed on CD34+ cells as well as CD15+ and CD19+ cells. The differential expression of the isoforms may be involved in the increased proliferative capacity of CD34+ cells. We are currently constructing recombinant retroviruses to overexpress these FR isoforms in purified hematopoietic cell populations to further test this hypothesis.
