Inactivation of the transforming growth factor beta type II receptor in human small cell lung cancer cell lines

Transforming growth factor beta (TGF-beta) exerts a growth inhibitory effec t on many cell types through binding to two types of receptors, the type I and II receptors. Resistance to TGF-beta due to lack of type II receptor (R II) has been described in some cancer types including small cell lung cance r (SCLC). The purpose of this study was to examine the cause of absent RII expression in SCLC cell lines. Northern blot analysis showed that RII RNA e xpression was very weak in 16 of 21 cell lines. To investigate if the absen ce of RII transcript was due to mutations, we screened the poly-A tract for mutations, but no mutations were detected. Additional screening for mutati ons of the RII gene revealed a GG to TT base substitution in one cell line, which did not express RII. This mutation generates a stop codon resulting in predicted synthesis of a truncated RII of 219 amino acids. The nature of the mutation, which has not previously been observed in RII, has been link ed to exposure to benzo[a]-pyrene, a component of cigarette smoke. Since RI I has been mapped to chromosome 3p22 and nearby loci are often hypermethyla ted in SCLC, it was examined whether the lack of RII expression was due to hypermethylation. Southern blot analysis of the RII promoter did not show a ltered methylation patterns. The restriction endonuclease pattern of the RI I gene was altered in two SCLC cell lines when digested with Sma1. However, treatment with 5-aza-2'-deoxycytidine did not induce expression of RII mRN A, Our results indicate that in SCLC lack of RII mRNA is not commonly due t o mutations and inactivation of RII transcription was not due to hypermethy lation of the RII promoter or gene. Thus, these data show that in most case s of the SCLC cell lines, the RII gene and promoter is intact in spite of a bsent RII expression. However, the nature of the mutation found could sugge st that it was caused by cigarette smoking.