Multiple mechanisms of p16INK4A inactivation in non-small cell lung cancer cell lines.
p16INK4A, a specific inhibitor of cyclin-dependent kinase (cdk)4 and cdk6, is a candidate tumor suppressor in malignancies with wild-type retinoblastoma (Rb). Loss of p16INK4A frees these cdks from inhibition, permitting constitutive phosphorylation of Rb and inactivation of its growth suppressive properties. Consistent with this model, Rb-positive non-small cell lung cancers (NSCLCs) have little or no detectable p16INK4A protein, whereas Rb-negative lung cancers have abundant p16INK4A. However, only some NSCLCs have homozygous deletions or nonsense mutations in a remaining p16INK4A allele, suggesting that other mechanisms must account for absent or low levels of p16INK4A protein. Here, we analyzed 9 Rb-positive NSCLC cell lines for the controls governing p16INK4A activity. Four lines had homozygous deletions of p16INK4A (SK-LU-1, SK-MES-1, A-427, and SW900), and three had a point mutation in a single allele. First, in H520 cells, the previously reported deletion at codon 45 results in a frameshift that produces no detectable protein. Second, in Calu-3 cells, a His to Tyr substitution at codon 83 produced a variant with a shortened half-life that was unable to form complexes with cdk4 or cdk6. Third, in H661 cells, the previously reported point mutation in the second intron splice donor site resulted in a smaller p16INK4A protein. Although this variant formed complexes with cdk4 and cdk6, it had a profoundly reduced half-life, producing low steady-state levels of p16INK4A and abundant levels of free cdks. Finally, Calu-1 and Calu-6 cells transcribed no detectable mRNA encoding authentic p16INK4A. These cell lines displayed methylation of the CpG island surrounding the first exon of p16INK4A and expressed abundant levels of a nontranslated mRNA containing an alternative first exon (E1 beta), as did all other cell lines in which the p16INK4A locus was not deleted. These data indicate that Rb-positive NSCLC cells have evolved a variety of pathways to suppress p16INK4A expression. Reintroduction of p16INK4A into these cell lines by retroviral transfer resulted in a reduced growth rate, increased abundance of hypophosphorylated Rb, accumulation of cells in G1, and a less transformed morphology in Rb-positive, but not Rb-negative cells, suggesting that loss of p16INK4A is essential for maintenance of the transformed phenotype.
Department of Medicine, Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts 02115, USA.
G I
GI
Shapiro
J E
JE
Park
C D
CD
Edwards
L
L
Mao
A
A
Merlo
D
D
Sidransky
M E
ME
Ewen
B J
BJ
Rollins
D002289:Carcinoma, Non-Small-Cell Lung / Q000235:genetics* / Q000473:pathology; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D002453:Cell Cycle; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D019941:Cyclin-Dependent Kinase Inhibitor p16; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D004273:DNA, Neoplasm / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic; D016147:Genes, Tumor Suppressor; D006801:Humans; D008175:Lung Neoplasms / Q000235:genetics* / Q000473:pathology; D008745:Methylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger / Q000235:genetics; D012334:RNA, Neoplasm / Q000235:genetics; D016160:Retinoblastoma Protein / Q000378:metabolism; D017384:Sequence Deletion; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004273:DNA, Neoplasm; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D012334:RNA, Neoplasm; D016160:Retinoblastoma Protein; D017346:Protein Serine-Threonine Kinases; C495900:CDK4 protein, human; C495911:CDK6 protein, human; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases