Is the product of the src gene a promoter?

Abstract

Addition of a potent promotor, 12-O-tetradecanoylphorbol 13-acetate (TPA), to primary avian tendon or chicken embryo fibroblast cells infected with a temperaturesensitive mutant of Rous sarcoma virus produced a complete transformed phenocopy at the non rmissive temperature by the criteria tested. While normal, uninfected cultures also shifted towards a transformed phenotype after TPA addition, they did not achieve the same degree of morphological and biochemical alteration seen in virus-infected, TPAtreated cells.It is proposed that viral carcinogenesis, despite its rapidity, may occur in two stages: an "initiation" step caused by expression of a part of viral genome other than src (or by integration) and a promotion step (itself a multistep process) caused by the activation of the src gene. The src gene product could be enhanced or replaced by other promoting agents.We have gained tremendous knowledge of the structure and the genetics of RNA tumor viruses in recent years (1-3), yet we know relatively little about the mechanism(s) of viral carcinogenesis. The early optimism that viral transformation could provide a simple model for studying malignancy and that, once we identified the "oncogene" (4), we would understand cancer, has given way to skepticism and to an abrupt shift of interest from tumor viruses to other environmental carcinogens.The rapidity and completeness by which RNA tumor viruses could transform cells in culture andthe simplicity of the assay systems were the initial arguments for the use of RNA tumor virus-transformed cells as models for studying malignancy. These same reasonings, however, now are used to The isolation of active ingredients of croton oil by Hecker (7) has opened a whole new area of research, that of mechanism of action of tumor-promoting agents 8-11. It is now apparent that many characteristics induced by Rous sarcoma viruses (RSVs) are also induced by tumor promoters (10, 11). The overlap, however, is incomplete. Indeed, most of the effects of tumor promoters on normal cultured cells are analogousqualitatively and quantitatively-to the effects of serum (12) or other growth factors and are less pronounced than those caused by viral transformation. In addition, some characteristics, such as transformed morphology (11) and growth in agar, are not induced by the promoters.In this paper we show that at 410C addition of 12-0-tetradecanoylphorbol 13-acetate (TPA), a potent chemical promoter, to chicken cells infected with LA24, a temperature-sensitive (ts) mutant of RSV with a defect in the sarc gene, produces a more exact phenocopy of transformed cells than does TPA treatment of normal cells. The product of the arc gene at the nonpermissive temperature, therefore, appears to be either activated or replaced by the promoter. We propose that the product of the src gene itself may be a promoter rather than the initiator of transformation and that viral carcinogenesis may occur in two stages similar to those operating in mouse skin (13). The predictions of the model and the a...

Tumor promoters alter gene expression and protein phosphorylation in avian cells in culture.

“…These TPA-induced alterations were enhanced in ts-RSV-infected cells kept at nonpermissive temperatures as reported (16) and were found to be sensitive to cycloheximide in both types of cells (Fig. 3).…”

“…However, in vitro assays do not always reflect the state of enzymatic activity in the intact cell (27). The (5), and that increased levels of phosphotyrosine are not found in all virally or chemically transformed cells (27) (16).…”

“…CEF were prepared as described (16,17). The wild-type (wt) and the temperature-sensitive (ts) mutant of the Prague (PrA) strain of RSV, LA-24 (18), were focus purified in our laboratory.…”

Diverse mitogenic agents induce the phosphorylation of two related 42,000-dalton proteins on tyrosine in quiescent chick cells.

“…Tumor promoters induce many of the changes which accompany malignant transformation (5,6,23,47,64). The phorbol ester TPA is mitogenic for several types of cells in culture (21,22,54).…”

Activation of the 9E3/cCAF Chemokine by Phorbol Esters Occurs via Multiple Signal Transduction Pathways That Converge to MEK1/ERK2 and Activate the Elk1 Transcription Factor

“…Wounding is also a tumor promoter because it can cause cancer to develop at the edges of wounds inflicted in areas that have been exposed previously to a carcinogen (10 -15). The v-src oncogene, which is the transforming protein of the Rous sarcoma virus (a retrovirus that causes tumors in chickens), also has been shown to be a tumor promoter (16). The 9E3 gene and its product, the chicken chemotactic and angiogenic factor (cCAF), 1 are stimulated to high levels by the v-src oncogene and also by phorbol esters and wounding/inflammation (17)(18)(19)(20)(21).…”