Gene therapy of murine teratocarcinoma: separate functions for insulin-like growth factors I and II in immunogenicity and differentiation

Teratocarcinoma is a germ-line carcinoma giving rise to an embryoid tumor with structures derived from the three embryonic layers: mesoderm, endoderm, and ectoderm. Teratocarcinoma is widely used as an in vitro model system to study regulation of cell determination and differentiation during mammalian embryogenesis. Murine embryonic carcinoma (EC) PCC3 cells express insulin-like growth factor I(IGF-I) and its receptor, while all derivative tumor structures express IGF-I and IGF-II and their receptors. Therefore the system lends itself to dissect the role of these two growth factors during EC differentiation. With an episomal antisense strategy, we define a role for IGF-I in tumorigenicity and evasion of immune surveillance. Antisense IGF-I EC transfectants are shown to elicit a curative anti-tumor immune response with tumor regression at distal sites. In contrast, IGF-II is shown to drive determination and differentiation in EC cells. Since IGF-I and IGF-II bind to type I receptor and antisense sequence used for IGF-II cannot form duplex with endogenous IGF-I transcripts, it follows that this receptor is not involved in determination and differentiation.

Loss of tumorigenicity of rat glioblastoma directed by episome-based antisense cDNA transcription of insulin-like growth factor I

Malignant glioma is the most common brain tumor. The molecular basis of glioma tumorigenicity has not been defined. Cultured glioma cells accumulate high levels of insulin-like growth factor I (IGF-I) transcripts. We asked whether IGF-I expression is coupled to tumorigenicity, using a combined in vivo/in vitro system employing antisense RNA for IGF-I. An antisense IGF-I expression construct in an expression vector that incorporates Epstein-Barr virus replicative signals and the ZnSO4-inducible metallothionein I transcriptional promoter was assembled. Stable glioma transfectants were derived from C6 glioma cells, which constitutively express IGF-I. B-104 neuroblastoma cells, derived originally from the same tumor but not expressing IGF-I, were also transfected as controls. In the absence of ZnSO4, the C6 transfectants expressed high levels of IGF-I mRNA and protein as detected by in situ hybridization and immunocytochemistry, respectively. Addition of ZnSO4 in the culture medium resulted in high levels of antisense transcript accumulation and dramatically decreased levels of endogenous IGF-I mRNA and IGF-I protein. Subcutaneous injection of either nontransfected C6 parental cells or C6 cells transfected with vector without IGF-I sequences into rats resulted in large tumors after 2 weeks, as did transfected and nontransfected B-104 cells. However, the rats injected with transfected C6 cells yielded no tumors after 40 weeks of observation. Two weeks after injection of the transfected C6 cells a small cyst was apparent in six rats. Histologic sections revealed a few glioma cells infiltrated by a large number of mononuclear cells. No infiltration of mononuclear cells was apparent in the glioma tumors resulting from injection of parental (nontransfected) cells, suggesting that the parental cells, but not the antisense IGF-I transfectants, escape the host immune response.

Effects of actinomycin D and cycloheximide on transcript levels of IGF-I, actin, and albumin in hepatocyte primary cultures treated with growth hormone and insulin

The stability of several RNA transcripts in cultured hepatocytes is known to increase when serum is omitted from the culture medium. In order to investigate possible mechanisms for this phenomenon, we examined the effects of actinomycin D and cycloheximide on the levels of actin, albumin, and insulin-like growth factor I transcripts in primary cultures incubated in serum-free medium. The levels of IGF-I and albumin transcripts per culture increased for the first 4 hours following addition of actinomycin D and then declined. The levels of actin transcripts and total RNA per culture declined immediately following actinomycin D addition in a manner consistent with exponential decay. IGF-I and albumin transcript levels were relatively unaffected by cycloheximide, while actin transcript levels increased 7-fold over 7 hours. The half-lives of actin transcripts and total RNA were calculated to be 4.6 to 7.7 hours and 11 to 19 hours, respectively, with no statistically significant correlation with hormone treatment. The data suggest that the stability of albumin and IGF-I transcripts, but not actin transcripts, is controlled in part by an actinomycin D-sensitive process.

Newly synthesized RNA: simultaneous measurement in intact cells of transcription rates and RNA stability of insulin-like growth factor I, actin, and albumin in growth hormone-stimulated hepatocytes

The levels of several RNA transcripts in cultured hepatocytes are regulated by transcriptional and post-transcriptional mechanisms and are affected by growth hormone and insulin. We assessed the effects of these hormones on transcription rates and the stability of insulin-like growth factor I, actin, and albumin transcripts in intact cells of primary cultures of rat hepatocytes by analyzing thiol-labeled, newly synthesized RNA isolated by mercurated agarose affinity chromatography. The application of this concept to the measurement of transcript stability is presented in detail. The data indicate that growth hormone stimulates the transcription rates of insulin-like growth factor I, actin, and albumin genes. The stability of all three transcripts, particularly albumin, appears to be lower in growth hormone-containing medium than it is in insulin-containing medium. The experiments indicate that the rates of transcription and/or degradation of albumin mRNA are influenced by hormonal treatment. However, the cells maintain roughly constant albumin transcript levels independent of hormone treatment by compensatory changes in the rates of transcription and degradation.

Hepatocytes express blood coagulation factor XII (Hageman factor)

The liver synthesizes blood coagulation factor XII (Hageman factor). The specific cell that expresses factor XII, however, has not been previously identified. We used primary rat hepatocytes cultured in serum-free medium to study the transcription, de novo synthesis, and secretion of factor XII. A 32P-labeled human factor XII complementary DNA probe was used for RNA blot hybridization. A single band of hybridization at 2.4 kilobases appeared in blots of polyadenylated RNA derived from 24-hour hepatocyte cultures. This corresponds to the known size of factor XII-processed primary transcript (messenger RNA). Cultured hepatocytes secreted labeled factor XII when tritiated leucine was added to the medium, indicating that the hepatocytes used 3H-leucine to synthesize factor XII de novo. In these hepatocyte cultures immunoreactive factor XII levels progressively increased in 24 hours and factor XII clotting activity increased in parallel. Cycloheximide inhibited the accumulation of both immunoreactive and coagulant factor XII. Secreted factor XII from the rat hepatocytes comigrated with authentic rat plasma factor XII at 80,000 molecular weight in a Western immunoblot. These data indicate that cultured hepatocytes transcribe, synthesize, and secrete authentic factor XII.

Expression of insulin-like growth factor I in cultured rat hepatocytes: effects of insulin and growth hormone

The effects of GH and insulin the accumulation of insulin-like growth factor I (IGF-I) RNA transcripts and the secretion of immunoreactive IGF-I protein were studied in rat liver hepatocytes cultured in serum-free medium. GH at concentrations of 10 ng/ml or greater stimulated the accumulation of IGF-I RNA transcripts relative to actin transcripts in poly(A)+ RNA isolated from cultured hepatocytes. The time course of IGF-I transcript accumulation in response to GH appeared to be biphasic. The transcript levels rose dramatically during the first 2 h after exposure of the cultures to GH, declined between 3 and 6 h, but reaccumulated by 24 h after exposure to GH. The presence of insulin did not influence the effect of GH on the accumulation of IGF-I RNA transcripts, although insulin did elevate IGF-I transcript levels in the absence of GH. Analysis of RNA pulse-labeled with thiouridine followed by purification of the thiol-labeled RNA using mercurated agarose indicated that GH probably acts by increasing IGF-I transcription. Insulin also affected the release of immunoreactive IGF-I into the culture medium. In the presence of insulin, immunoreactive IGF-I accumulated in the culture medium to approximately the same extent over a 24-h period regardless of whether GH was also present. In the absence of insulin, immunoreactive IGF-I accumulated in the medium only if GH was present. The results suggest that insulin may play an important role in both IGF-I transcript accumulation and the secretion of IGF-I from cultured hepatocytes.