SPP1 and cancer: The overall proliferation of the tumor cells are also modulated via paracrine interactions by the cancer associated fibroblasts (CAF) [30,51,67]—a self-proliferating (in the presence of Osteopontin (OPN)) fibroblasts phenotype that is derived from the wild type fibroblasts in the presence of tumor cell and fibroblasts-derived leukemia inhibitory factor (LIF) [41,53,66,71,72,80–82].