GROWTH FACTORS. Various polypeptide growth factors are secreted by tumors in order to regulate cell proliferation as well as an extremely wide range of cell functions which include the deposition and resolution of matrix protein. This secretion works to rejuvenate cell viability, effectively stimulating cell growth. These secretions work through cell receptors or internally to stimulate growth, but in some cases, the cell type cause the secretion to act as an growth inhibitor instead.