Expression and secretion of neuroleukin/phosphohexose isomerase/maturation factor as autocrine motility factor by tumor cells

Abstract

The results obtained from fragmented protein microsequencing have suggested that autocrine motility factor (AMF), a tumor-secreted Mr 55,000 cytokine that regulates cell motility in vitro as well as invasion and metastasis in vivo, is the neuroleukin (NLK)/phosphohexose isomerase (PHI)/maturation factor (MF) polypeptide. Here, we cloned, sequenced, and studied the expression, secretion, and distribution of AMF/NLK/PHI/MF in neoplastic and their normal counterpart cells. Although both normal and neoplastic cells express the gene product, overexpression associated with selective secretion of the protein was observed only in tumor cells. The cDNA sequences of AMF/NLK/PHI/MF found in both human cancer and normal cells were found to be identical, suggesting that its secretion by neoplastic cells is independent of mutation or alternative splicing. Immunohistochemical visualization has depicted AMF/NLK/PHI/MF to be localized into tubular-like vesicles, diffusely distributed throughout the cytoplasm and not colocalized with any particular cytoskeletal network. Confocal microscopic imaging had shown a partial colocalization between AMF and its receptor (Mr 78,000 glycoprotein), especially on the malignant cell surface periphery. The results suggest that extracellular AMF activity may be a result of the product of intracellular cleavage of a precursor polypeptide, which is overexpressed and selectively secreted through a nonclassical secretory mechanism by neoplastic cells.