Biologically active neuromedin U (NmU) is a 25-amino acid peptide expressed at moderate to high levels in tissues of the brain, gut, and bone marrow. To date, NmU has been reported to stimulate smooth muscle contraction, increase blood pressure, alter ion transport in the gut, regulate local blood flow and modulate adrenocortical function. We have found that c-Myb, a transcription factor required for normal and malignant hematopoiesis, and NmU gene expression are both elevated in primary acute myeloid leukemia cells and demonstrated that NmU promotes the growth of human myeloid leukemia cells (Shetzline S, Rallapalli R, Dowd K, Zou S, Nakata Y, Swider C, Kalota A, Choi J, Gewirtz A. 2004. Blood. 104(6): 1833-1840). Since myeloid leukemia cells express the cognate receptor of NmU, NMU1R, we hypothesized that NmU stimulates myeloid leukemia cell proliferation via a novel autocrine loop. To understand the mechanism by which NmU stimulates myeloid leukemia cell proliferation, we examined the ability of NmU to induce intracellular calcium ion (Ca+2) flux and produce inositol phosphates in K562 cells, a myeloid leukemia cell line, because signaling through NMU1R has been shown to induce these secondary messengers in Cos-7, HEK-293, and CHO cells. First, we measured the intracellular accumulation of Ca+2 in Indo-1 labeled K562 cells following NmU challenge. We observed Ca+2 mobilization within 100 seconds of adding NmU. Next, we determined the ability of NmU to induce the production of inositol phosphates in K562 cells that were labeled with myo-[3H]-inositiol. A dose-dependent increase was observed in inositol phosphate production following NmU challenge with an EC50 of 10 μM. Taken together, these data demonstrate that the NMU1R expressed on K562 cells is functional and indicate that NmU initiates its proliferation signal through a G-protein couple receptor. Studies are underway in our laboratory to define the molecular mechanism of NmU-mediated mobilization of Ca+2 and production of inositol phosophates, the results of which will hopefully provide new insights into the physiological function of NmU in myeloid leukemias.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]