Among the key cell cycle regulators, cyclin D1 has been implicated most strongly in oncogenesis. This G1 cyclin is a putative proto-oncogene whose clonal rearrangement and/or amplification and mRNA overexpression occurs in several types of human neoplasias. We have now raised a series of monoclonal antibodies to human cyclin D1 and analysed its regulation at the protein level in 40 human tumour cell lines. We found that 12 cell lines displayed low or undetectable cyclin D1 protein level, while the remaining lines accumulated the protein to a level comparable to, or moderately higher than, that of four normal diploid non-immortalized cell types. The cell cycle-dependent oscillation and subcellular localization of cyclin D1 were similar in both tumour and normal cells. The protein localized to the nucleus of G1 cells, and it was reduced to immunocytochemically undetectable level in DNA-replicating cells. At the functional level, microinjection and electroporation of anti-D1 antibodies revealed that in most tumour cell lines studied, including those with amplification at the cyclin D1 locus, this cyclin is essential for cell cycle progression in G1. Some tumours, however, seem to have evolved mechanism(s) that enable them to bypass the requirement for functional cyclin D1.

Danish Cancer Society Division for Cancer Biology Copenhagen