Prostaglandin endoperoxide synthase (cyclooxygenase) mRNA and protein production in mouse myoblasts and a differentiation-defective variant

Growth factor responsiveness: role of MyoD and myogenin

A differentiation-defective variant (DD-1) of the MM14 myoblasts acquired the ability to synthesize DNA in response to treatment with epidermal growth factor (EGF) (R. W. Lim and S. D. Hauschka, 1984, Dev. Biol. 105, 48) and no longer expressed myogenic determinant genes (i.e., MyoD and myogenin) (P.R. Mueller, and B. Wold, 1989, Science 246, 780). To determine the effect of expression of MyoD on EGF responsiveness, DD-1 cells were cotransfected with a MyoD expression vector and with pRSVneo. A clone, MyoDD-1 cells, which was G418 resistant, formed multinuclear syncitia, and also expressed MyoD and myogenin, was further characterized. EGF responsiveness, as assessed by DNA synthesis, was decreased 5- to 10-fold in the MyoDD-1 cells from that in G418-resistant control DD-1 cells, despite similar EGF receptor numbers and binding affinities of the receptors. Responsiveness of MyoDD-1 cells to fibroblast growth factor (FGF) was also diminished although to a lesser extent. To determine the effects of decreased myogenic determinant gene expression on mitogen responsiveness, MM14 myoblasts were grown in medium supplemented with 5 microM 5-bromo-2'-deoxyuridine (BUdR-MM14). BUdR-MM14 cells had decreased expression of MyoD and myogenin, did not fuse, and had an altered morphology, from round to flat. The BUdR effect on fusion and cell shape was reversed by growth in control medium. BUdR-MM14 cells were responsive to EGF and had enhanced responsiveness to FGF. The combined studies support the view that expression of MyoD and/or myogenin contributes to negative regulation of mitogen responsiveness.

Expression of growth-regulated genes in normal and SV40 transformed hamster fibroblasts

Transformation by the oncogenic virus SV40 has been shown to alter the expression of cellular genes at the level of RNA abundance. Many of these genes have yet to be identified. We have determined, by Northern blot analysis, the abundance levels of several growth-regulated genes in SV40-transformed cell lines to determine if their expression is altered and correlates with the ability of SV40 transformed cells to grow in low serum containing media. The mRNA abundance levels of the G1-specific genes 2A9/calcyclin, 2F1/translocase, and 4F1/vimentin were determined in the parental hamster fibroblast cell line, tk-ts13, and in two SV40 transformants, HR5 and HR8 cells, grown in medium containing 10% calf serum (normal medium) and in HR5 and HR8 cells adapted to passage in medium containing low serum. A spontaneous transformant of the parental line capable of growth in low serum in the absence of SV40 transformation (tk-ts13/1%), was also included in these studies. The low serum adapted SV40-transformed cells and the spontaneous tk-ts13 transformed cells grew more vigorously than their nonadapted counterparts in medium containing low serum. The low serum adapted cells also grew to higher saturation densities in low serum and to densities comparable to those in high serum, whereas the nonadapted cells grew to low saturation densities in low serum, but not as low as the untransformed parental.(ABSTRACT TRUNCATED AT 250 WORDS)