Oncogenic Collaboration of the Cyclin D1 (PRAD1, bcl‐1) Gene with a Mutated p53 and an Activated ras Oncogene in Neoplastic Transformation

Cyclin D1 is one of the key regulators in G1 progression in the cell cycle and is also a candidate oncogene (termed PRAD1 or bcl‐1) in several types of human tumors. We report a collaboration of the cyclin D1 gene with ras and a mutated form of p53 (p53‐mt) in neoplastic transformation. Transfection of cyclin D1 alone or in combination with ras or with p53‐mt was not sufficient for focus formation of rat embryonic fibroblasts. However, focus formation induced by co‐transfection of ras and p53‐mt was enhanced in the presence of the cyclin D1‐expression plasmid. Co‐transfection of ras‐ and p53‐mt‐transformants with the cyclin D1‐expression plasmid resulted in reduced serum dependency in vitro, Furthermore, the transformants expressing exogenous cyclin D1 grew faster than those without the cyclin D1 plasmid when injected into nude mice. These observations strengthen the significance of cyclin D1 overexpression through gene rearrangement or gene amplification observed in human tumors as a step in multistep oncogenesis; deregulated expression of cyclin D1 may reduce the requirement for growth factors and may stimulate in vivo growth.