Cyclin D1 as a proliferative marker regulating retinoblastoma phosphorylation in mouse lung epithelial cells

Elevations in cyclin DI content increase the phosphorylation status of reti noblastoma (Rb) protein to encourage cell cycle transit. We sought to deter mine if cyclin D1 content could be used as an index of cell proliferation i n mouse lung epithelia following growth manipulations in vitro and in vivo. Rb protein concentration was high in 82-132 and LM2, two fast-growing neop lastic mouse lung epithelial cell lines. The hyperphosphorylated form of Rb predominated in these two cell lines, while Rb in slower-growing cell line s was predominantly hypophosphorylated. Consistent with this, more cyclin D 1 protein was expressed in the fast-growing cell lines than in slower-growi ng cells. We therefore tested whether cyclin D1 content varied with growth status. The amount of cyclin D1 decreased upon serum removal coincident wit h growth inhibition and then increased upon serum re-addition which stimula ted resumption of proliferation. This correlation between cyclin D1 content and growth status also occurred in vivo. Cyclin D1 content increased when lungs underwent compensatory hyperplasia following damage caused by butylat ed hydroxytoluene administration to mice and in lung tumor extracts as comp ared with extracts prepared from uninvolved tissue or control lungs. We con clude that elevated cyclin D1 levels account, at least in part, for the hyp erphosphorylation of Rb in neoplastic lung cells, and are associated with e nhanced lung growth in vitro and in vivo. (C) 2001 Elsevier Science ireland Ltd. All rights reserved.