POSTTRANSCRIPTIONAL CONTROL REGULATES TRANSFORMING GROWTH-FACTOR-ALPHA IN THE HUMAN CARCINOMA KB CELL-LINE

Expression of epidermal growth factor receptor (EGF-R) antisense RNA r esults in a drastic reduction of EGF-R levels in the human carcinoma K B cell line and induces a reversion of their transformed phenotype (Mo roni, M. C., Willingham, M. C., and Beguinot, L. (1992) J. Biol. Chem. 267, 2714-2722), We used parental and EGF-R antisense KB clones as a genetic system to study, in the same cell line, the role of transformi ng growth factor alpha (TGF-alpha) in the establishment and maintenanc e of the transformed phenotype, KB cells produce TGF-alpha mRNA, and t heir conditioned medium is able to sustain growth of antisense cells, mimicking the effect of exogenous EGF or TGF-alpha. In antisense cells there is a marked reduction of TGF-alpha mRNA steady-state levels. In addition, the decrease in TGF-alpha parallels the levels of residual EGF-R in the various antisense clones, indicating a direct correlation between receptors and growth factor levels, The addition of exogenous TGF-alpha (10 ng/ml) to antisense clones induces TGF-alpha levels. Th e half-life of TGF-alpha mRNA is 40-60 min in antisense cells and more than 8 h in parental KB cells, as determined by actinomycin D decay c urves. This result indicates a predominant regulation of TGF-alpha mRN A at the post-transcriptional level. Nuclear Min-on experiments show t hat there is only a marginal effect at the transcriptional level, We c onclude that the autocrine loop responsible for the transformed phenot ype of the human carcinoma KB cell line is dependent on both elevated levels of EGF-R and the presence of TGF-alpha, In addition, TGF-alpha is able to induce its own mRNA via a signal due to activation of the E GF-R acting predominantly at the post-transcriptional level.