CXCL7 is an ERL+ chemokine that is secreted by activated platelets, neutrophils, lymphocytes, and macrophages as a precursor. Pro-platelet basic protein or leukocyte derived grow factor (PPBP/LDGF) (a protein of ~14 kD) is modified by post-transcriptional cleavage to give rise to connective tissue activating peptide (CTAP-III), CXCL7, beta-thromboglobulin (beta-TG), and two variants of thrombocidin (TC-1 and TC-2). CTAP-III is activated by enzymatic proteolysis performed by cell surface-bound cathepsin G (CathG), a chymotryptic serine protease found in primary neutrophil granules. In this way, neutrophils are the mayor cells that convert CTAP-III into CXCL7. In addition, monocytes and mast cells can proteolytically modify CTAP-III to produce CXCL7. In fact, CathG has been detected in mast cells in addition to chymase, another chymotryptic enzyme. Further C-terminal truncations, that eliminate the last four and seven amino acids in CXCL7, generate two additional natural isoforms; these variants are more potent neutrophils activators. High levels of CTAP-III have been detected in plasma of patients with lung cancer. In fact, it has been suggested as a biomarker for early lung cancer detection. An increase in positive immunostained cells has been detected in the bronchial submucosa of patients with stable severe chronic obstructive pulmonary disease. Also, studies with premalignant breast cancer cells transfected with CXCL7 showed that these cells became as invasive as malignant breast cancer cells. In addition, CXCL7 induces stimulation of the lymphangiogenic factors VEGF-C and VEGF-D in human breast cancer cells, suggesting an important role for CXCL7 in tumor invasion.
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