Growth control mechanisms in normal and neoplastic mammalian cells

Sammanfattning: The main theme of the studies presented in this thesis is, the growth control mechanisms whose loss in normal cells predispose to or cause cancer. The balance between growth inhibitory and stimulatory mechanisms is crucial for the development and maintenance of a normal animal.PDGF, a growth factor for cells of mesenchymal origin, is implicated in normal developmental processes as well as neoplasia. The alternative splicing of exon 6 in PDGF-A gene transcripts gives rise to two different proteins with different compartmentalization properties. The PDGF-A chain homodimers, PDGF-AAL, encoded PDGF A-splice variant remain associated with the cell membrane. Studies of a human fibrosarcoma cell line, U-2197, revealed a high expression level of the cell associated PDGF-AAL which upon release increased autophosphorylation of the endogenous PDGF receptors, suggesting an autocrine loop. PDGF-A gene and PDGFR-α gene found to be co-amplified in the U-2197, indicating an optimised system for growth in these cells, i.e. amplified growth factor receptor as well as a local autocrine supply of the mitogen.Members of TGFβ superfamily are potent regulators of the growth and differentiation of a wide range of cell types. Intracellular mediators of TGF-β signalling, SMADs, transduce signals from serine/threonine kinase receptors to the nucleus where they affect transcription of target genes. A new class of SMAD proteins has been identified whose members, the inhibitory SMADS, antagonise TGF-β signals by interfering with agonistic SMADs activity. Smad6 and Smad7 are two closely related TGF-β antagonists identified in mammalian cells. Overexpression of Smad7 inhibited the cellular response to TGF-β whereas expression of an anti-sense Smad7 construct showed an enhancing effect on this response. The inhibitory SMADs may act in a negative feedback loop, as their expression is induced by the same ligands whose action they antagonise.Density dependent growth inhibition is a growth control mechanism often lost in transformed and malignant cells. Cells in dense culture are refractory to the mitogen stimulation although, the mitogenic signals were shown to be processed to some extent. The expression of immediate-early genes in dense culture stimulated with mitogen was induced. The activity of cyclin dependent kinases (CDKs), the pivotal kinases in G1/S transition, showed to be density dependent and decreased by increasing cell density. pRb, a tumour suppressor and growth regulatory protein, remained unphosphorylated in mitogen treated dense culture. The cessation of CDKs kinase activity in dense cultures was shown to be accompanied with increasing expression of inhibitory proteins of these kinases, CKIs. The impaired expression of a positive regulator of CDKs, Cdc25A phosphatase, was another feature of dense cultures.