Transforming growth factor-β1 induces growth inhibition of a human medullary thyroid carcinoma cell line despite an increase in steady state c-myc messenger ribonucleic acid levels

Medullary thyroid cancer (MTC) is an endocrine tumor of the thyroid C-cells which provides an important experimental model for studies of tumor differentiation and progression. We investigated the effects of transforming growth factor-β 1 (TGF β 1) on the growth and functional characteristics of a human medullary thyroid carcinoma cell line (TT). Because the c-myc protooncogene may play an important role in the growth inhibition induced by TGF β 1, we also assessed steady state c-myc messenger RNA (mRNA) levels in these cells. A 6-day exposure of TT cells to TGF β 1 resulted in a dose-dependent inhibition of cell proliferation. In addition, TGF β 1 exposure led to a 3-fold increase in nonadherent floating TT cells in the culture supernatants. The floating cells exhibited ultrastructural features of dying or apoptotic cells, including chromatin condensation, cytoplasmic and nuclear vesicularization, and DNA degradation with evidence of internucleosomal DNA “laddering.” Despite inhibition of cell proliferation, steady state c-myc mRNA levels were 3.6 ± 0.6-fold higher in cells exposed to TGF β 1 compared to those in control cells (P < 0.001). Exposure of cells to a 15-base antisense c-myc oligonucleotide (10 μM) resulted in an attenuation of the TGF β 1-induced growth inhibition and induction of cell death. TGF β 1 also resulted in an approximately 3-fold decrease in steady state calcitonin and calcitonin gene-related peptide mRNA levels. Finally, using a sensitive bioassay for TGF β, TT cells were shown to produce and activate significant amounts of TGF β, particularly under conditions of serum deprivation. Our data thus indicate that TGF β 1 has multiple effects on TT cell growth and function. It induces growth inhibition in the presence of an increase in steady state mRNA levels of the c-myc protooncogene, which is usually associated with cell proliferation. In addition, TGF β 1 accelerates apoptosis in TT cells.