Retinoblastoma protein in non-small cell lung carcinoma, cells arrested for growth by retinoic acid.
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The growth of non-small cell lung carcinoma (NSCLC) cells was inhibited by retinoic acid after 16 h of treatment. However, the growth of all cell lines except one became refractory to retinoic acid after 48 h of exposure. The expression of the hyperphosphorylated retinoblastoma protein species (RB p110) was observed to be increased fivefold to tenfold in NSCLC cells within 16 h of exposure to retinoic acid. In the H460a and H226b cell lines, p110 showed some conversion to the underphosphorylated p105 form after 24 h of retinoic acid treatment. After 48 h, p105 became the predominant form, along with a 60K M(r) species. After 72 h, expression of all RB protein species became almost undetectable in H460a and H226b cells, concomitant with increases in cell growth rates. A different pattern of RB expression was observed in the H322j and H358 cell lines. In these cells, both p110 and p105 were induced within 8 h of retinoic acid treatment. Furthermore, the elevated levels and the phosphorylation state of RB in retinoic acid-treated H322j and H358 cells remained essentially unchanged for up to 72 h and only low amounts of the 60K M(r) species were detected. I believe that a post-translational mechanism is responsible for the retinoic acid-mediated induction of RB, since levels of RB mRNA remained relatively unchanged during the time course of retinoic acid exposure. I conclude that retinoic acid inhibited the growth of NSCLC cells by inducing high levels of RB and that increases in RB levels occurred as a result of either an increase in stability of the protein or by downregulation of an RB-specific protease. The inhibition of growth by retinoic acid must involve other molecular events, since the H596b cell line, which lacks RB protein, exhibited growth properties in the presence of retinoic acid similar to those of cells in which RB expression was induced.