Autonomy in tumor cell proliferation

Autonomous replication of tumor cells seems to be an essential factor in the definition of the malignant tumor itself, although tumor cell proliferation is, in general, controlled by the host response including immunological reactions and microenvironment. The cause of the autonomy can hypothetically be classified into four categories as follow: (a) auto- and paracrine growth stimulation; (b) growth factor receptor abnormalities; (c) abnormal signal transduction; (d) self-incitement of 'initiator-replicon' system in DNA replication. These intracellular mechanisms may play important roles in the autonomy as shown in autocrine growth factors from the data obtained in protein-free cell culture. Hypothetically, negative regulation systems on the initiator-replicon may play roles of cell replication in multicellular organisms. Oncogene products and growth factors may affect this regulation system.