Absence of p16^INK4 protein is restricted to the subset of lung cancer lines that retains wildtype RB

Cell cycle dependent phosphorylation of the RB tumor suppressor protein is mediated by a family of G1 cyclin dependent kinases (cdks) and cyclins including the activated cdk4:cyclin D complex. The identification of a cdk4 inhibitor, p16^INK4, as a target for mutations in cultured tumor lines and primary tumors suggested that RB activity may be affected in these cells. We have examined 88 lung cancer lines for p16^INK4 protein expression and have observed a striking inverse correlation between the presence of p16^INK4 and wildtype RB. We demonstrated that only 6/55 (11%) of small cell lung cancer (SCLC) samples had absent p16^INK4 protein, and all 6 belonged to the rare subset of SCLC with wildtype RB expression. Conversely of 48 SCLC samples with absent or mutant RB, all showed detectable levels of p16^INK4 protein. In contrast, we observed that 23/33 (70%) of non-SCLC samples had loss of p16^INK4. Twenty-two of 26 non-SCLC lines with wildtype RB had absent p16^INK4 while 6 of 7 non-SCLC lines with absent or mutant RB had detectable p16^INK4. The inverse correlation of RB and p16^INK4 expression and the absence of p16^INK4 inactivation in RB (-/-) SCLC lines (0/48) confirms a common p16^INK4/RB growth suppressor pathway in human cancers and provides evidence that p16^INK4, and not an adjacent gene on chromosome 9p, is a specific target for mutational events.