Rb protein in its hypophosphorylated form acts as a cell cycle regulator for G1 arrest. Both cyclin D1 overexpression and P16(INK4) loss of protein produce persistent hyperphosphorylation of Rb with resultant evasion of cell cycle arrest. To better establish the mechanisms of loss of Rb function in neuroendocrine lung tumors, we performed an immunohistochemical analysis of the P16(INK4)/cyclin D1/Rb pathway in the spectrum of neuroendocrine tumors, including 34 typical carcinoids (TCs), 25 atypical carcinoids (ACs), 42 large cell neuroendocrine carcinomas (LCNECs), and 79 small cell lung carcinomas (SCLCs). Absence of Rb expression was not observed in TCs but was seen in 21% of ACs, 68% of LCNECs, and 87% of SCLCs. P16 was expressed in 91% of TCs, 77% of ACs, 78% of LCNECs, and 93% of SCLCs. Cyclin D1 was overexpressed in 6% of TCs, 20% of ACs, 9.5% of LCNECs, and 1.3% of SCLCs. There was an inverse relationship between Rb and P16 in high-grade tumors (P < 0.001) and a direct relationship between cyclin D1 and Rb (P < 0.001) in all tumors, demonstrating that P16 and cyclin D1 act exclusively on the Rb pathway for cell cycle regulation. Overall, the Rb pathway (Rb/P16(INK4)/cyclin D1) was altered more frequently in ACs than in TCs (P = 0.001) and more frequently in LCNECs than in ACs (P = 0.001). Although Rb-negative tumors had shorter survival in the overall group (P < 0.001) as a result of lack of Rb in most SCLCs, cyclin D1 overexpression and P16 loss did not influence survival in any individual category. We conclude that Rb pathway of G1 arrest is consistently compromised in high-grade neuroendocrine lung tumors (92%), primarily through loss of Rb protein, and is intact in low-grade TCs. In ACs an intermediate level of alterations (59%) is seen, consistent with their less-aggressive behavior compared with high-grade tumors. The specific profile of the Rb pathway parameters might provide specific therapeutic targets in neuroendocrine lung tumors.
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