An abnormal stimulation of the cAMP pathway has been recognized as the primary event in various pathological situations that lead to goitrogenesis or thyroid tumors. Thyroid adenomas are monoclonal neoplasms that become independent of thyroid stimulating hormone (TSH) in their secretory function and growth. Mutated forms of the TSH receptor (TSHR) and the adenylyl cyclase-activating Gsa protein, which confer a constitutive activity on these proteins, have been observed in human adenomas. The FRTL-5 rat thyroid cell line is a permanent but untransformed line; the growth of which depends on the presence of TSH, and at least in part, on the stimulation of the cAMP pathway. In order to compare the oncogenic potential of the activated mutant Gsa protein and the constitutively activated TSHR, we have transfected FRTL-5 cells with an expression vector bearing either the cDNA of the Gsa gene carrying the A201S mutation or the cDNA of the TSH receptor carrying the M453T mutation recently identi®ed in a case of congenital hyperthyroidism. The expression of these two cDNAs was driven by the bovine thyroglobulin gene promoter. We show that, although the expression of both the Gsa or TSHR mutant proteins leads to TSHindependent proliferation and to constitutive cAMP accumulation in FRTL-5 cells, only the mutant TSHR is able to induce neoplastic transformation, as demonstrated by growth in semi-solid medium and tumorigenesis in nude mice.

The purpose of this study was to follow the evolution of normal and tested rat embryo lungs in organ culture and to classify the induced lesions in relation to the in vivo tumour histogenesis. Lungs from 15 day old Wistar rat embryos were maintained in organ culture in M 199 medium and 20% horse serum and treated with benzo[a]pyrene (B[a]P) or cigarette smoke condensate (CSC). Explants were studied 30 days of culture. The control explants display a prevalence of epithelial tissue along with a reduction in connective tissue, whereas treated explants show similar morphological alterations after both B[a]P and CSC. The percentage of the observed lesions depends on the strain of rats used. Alterations are classified into preneoplastic stages according to Shabad's pattern developed for rat lung tumours in vivo. This classification permits the assessment and quantitation of the carcinogenic effect of a substance. To prove that lung explants may undergo neoplastic transformation after treatment in vitro, normal and carcinogen-treated lung organ explants were dispersed and, after cell subculture, injected i.p. into isogenic rats. Tumours were obtained after only 5 subcultures with treated cells, whilst with the controls, tumours were obtained after 21 subcultures.