Cyclin D1 overexpression vs. retinoblastoma inactivation: implications for growth control evasion in non-small cell...

The cyclin-dependent kinases and their associated regulatory cyclins control cell cycle progression and cell growth. Antibodies against these proteins were used to determine their levels in several lung tumor-derived cell lines and a "normal" immortalized bronchoepithelial cell line in order to investigate their potential roles in the etiology of lung cancer. All the cell lines expressed roughly equal levels of cdk-1; cdk-2; PSTAIRE-sequence containing kinases; proliferating cell nuclear antigen; and cyclins A, B1, and E. Cyclin D1, however, was present at 4- to 100-fold higher levels in 11 of 12 non-small cell lung cancer cell lines than in the bronchoepithelial line and all but one of the small cell lung cancer lines. Furthermore, immunoblots of the retinoblastoma gene product, pRB, revealed a perfect correlation between pRB levels and tumor type with normal levels of phosphorylation-competent pRB in all of the non-small cell lung cancer lines and undetectable levels of pRB in all of the small cell lung cancer lines. These data suggest the possibility that small cell and non-small cell lung cancer may evade normal growth controls by different mechanisms: loss of the proliferation inhibitor pRB in small cell lung cancer and overexpression of the growth promoting cyclin D1 in non-small cell lung cancer.