TGF beta 1 and TGF beta 2 are potential growth regulators for low-grade and malignant gliomas in vitro: evidence in support of an autocrine hypothesis

Low-grade astrocytomas, anaplastic astrocytomas and glioblastomas in vitro were found to ubiquitously produce the mRNA of transforming growth factor-beta (TGF beta). TGF beta 1 and TGF beta 2 mRNA were expressed to a lesser degree among the hyperdiploid malignant gliomas. By radioreceptor assay of conditioned medium, TGF beta was secreted predominantly in latent form, in both latent and active form, or only in active form within a panel of low-grade and malignant gliomas. The TGF beta receptor (types I, II, and III) was evident among the glioma lines. Many near-diploid gliomas were growth-inhibited by TGF beta 1 and TGF beta 2 in vitro. Most hyperdiploid glioblastomas showed a positive mitogenic response to exogenous TGF beta 1 and TGF beta 2. A synergistic or additive mitogenic interaction with epidermal growth factor and insulin was observed among some. Under serum-free conditions, anti-TGF beta antibody neutralized the expected growth-regulatory effect of endogenous TGF beta, thus establishing the specificity of the response in vitro. TGF beta 1 also enhanced the clonogenicity of certain gliomas which had been growth-stimulated in monolayer. Thus, basic elements in support of an autocrine hypothesis have been demonstrated: TGF beta mRNA was expressed among low-grade and malignant gliomas, TGF beta was secreted in latent and/or active form into conditioned media and appeared to serve as an endogenous regulator of glioma proliferation in vitro. The mitogenic response, either positive or negative, correlated with the degree of anaplasia and karyotypic divergence.