Effects of ageing on the growth and differentiated function of transfected human thyrocytes

Abstract

Senescence in primary cultures of mammalian cells is characterised by cessation of growth after a number of cell divisions; this may be associated with loss of some differentiated functions. Recent studies on bovine adrenocortical cells have suggested that expression of simian virus-40 (SV40) early region in these cells may prevent phenotypic losses due to senescence. We report here data on growth and differentiated function of two human thyrocyte cell lines (SGHTL-34 and -45) generated by the transfection of primary thyrocytes with the plasmid pSV3 neo which contains the SV40 early region. Growth was assessed by fluorometric DNA estimations and calculation of cell population doubling time; function was assessed by binding studies using 125I-bovine thyrotrophin (TSH) and measurement of cyclic adenosine 3',5'-monophosphate (cAMP) response to stimulation with TSH, forskolin and cholera toxin. After 3–12 months in stable culture there was a gradual increase in the doubling time of both cell lines over a 3-month period (SGHTL-34 cells, early 34.5 ± 4.5 h, late 301 ± 111.6 h; SGHTL-45 cells, early 53.4 ± 4.4 h, late 148.3 ± 26.3 h; mean ± SEM). Scatchard analysis demonstrated a loss of the high affinity TSH receptor over the same time period. The increase in cAMP in response to 1000 μU/ml TSH declined until the cells became unresponsive (SGHTL-34 early, cAMP 10.3 ± 0.7 pmol/well; late, cAMP −0.4 ± 0.3 pmol/well; SGHTL-45 early, cAMP 11.3 ± 1.1 pmol/well, late, cAMP 0.3 ± 0.1 pmol/well). The cAMP responses to forskolin and cholera toxin were unaffected. This loss of response to TSH in SGHTL-45 cells was unaffected by the removal of TSH from the culture medium. We conclude that pSV3 neo transfected human thyrocytes, whilst having a longevity of great use for biochemical, endocrinological and immunological studies, are not immortal and that they demonstrate parallel loss of growth and differentiated function during ageing.