Regulation of rat myosin light-chain synthesis in heterokaryons between 5-bromodeoxyuridine-blocked rat myoblasts and differentiated chick myocytes

5-Bromo-2-deoxyuridine regulates invasiveness and expression of integrins and matrix-degrading proteinases in a differentiated hamster melanoma cell

Cell interactions with the extracellular matrix play a critical role in regulating complex processes such as terminal differentiation and tumor progression. In these studies we describe a melanoma cell system that should be useful in addressing the regulation of cell-matrix interactions and the roles they play in regulating differentiation and cell invasiveness. CS (suspension)-1 melanoma cells are relatively well differentiated: they are melanotic, responsive to melanocyte-stimulating hormone, and express TA99, a melanosome membrane differentiation marker. Their repertoire of integrin receptors for extracellular matrix ligands is limited; in particular, they lack receptors for vitronectin, accounting for the observation that they are nonadherent when cultured in the presence of serum. CS-1 cells are noninvasive as well, and express low levels of both metalloproteinases and activated plasminogen activators. Treatment of these cells with melanocyte-stimulating hormone causes them to increase melanin production and assume an arborized phenotype, suggesting that it promotes their further differentiation. In contrast, treatment of CS-1 with the thymidine analog 5-bromodeoxyuridine, converts them to a highly invasive cell population (termed BCS-1) that loses its differentiated properties and responsiveness to melanocyte-stimulating hormone, acquires a broad integrin repertoire (including vitronectin receptors), and expresses elevated levels of metalloproteinases and activated urokinase. From these observations and findings of others on BrdU treatment of other developmental lineages, we hypothesize that BrdU both suppresses differentiation and promotes invasiveness of CS-1 cells. The demonstrated manipulability of CS-1 cells should make them extremely useful for studying the regulation of both terminal differentiation and tumor progression in the melanocyte lineage.

Spontaneous and induced differentiation of human melanoma cells

Malignant melanoma cells can differentiate spontaneously in vivo and in vitro into cells with a finite lifespan. Analysis of differentiating cells from primary melanomas in culture revealed a flat, fibroblast-like morphology and expression of the fibroblast-associated marker leucine aminopeptidase (LAP). Differentiation was also observed in a minor sub-population of permanent cell lines derived from metastatic lesions. An experimental model of melanoma cell differentiation was then developed, using the pyrimidine analog bromodeoxyuridine (BUdR). BUdR-treated cells had a flat morphology, were contact-inhibited, had up to 20-fold increased surface area, expressed LAP, no longer proliferated anchorage-independently in soft agar, and 3 out of 4 cell lines were non-tumorigenic in athymic nude mice. Our results show that models of differentiation of melanoma cells can be established that help to define pathways of differentiation.

Gene-specific DNA repair in terminally differentiating rat myoblasts

Preferential DNA repair in expressed genes has been well documented in proliferating mammalian cells following ultraviolet irradiation. It was of interest to learn whether excision repair is similarly selective in terminally differentiating cells. We have measured the removal of ultraviolet-induced cyclobutane pyrimidine dimers (detected as T4 endonuclease V-sensitive sites) from various genes in cultured L8 rat skeletal myoblasts. In these cells, the transcription of muscle-specific genes such as the embryonic myosin heavy chain (MHCemb) gene can be regulated by inducing cells to differentiate. L8 myoblasts are somewhat more sensitive than Chinese hamster ovary cells to ultraviolet radiation, and they exhibit relatively poor overall DNA-repair rates throughout differentiation. Irradiation severely reduces the rates of transcription and steady-state RNA levels for the genes studied. Although differences in kinetics are seen between the repair of active and inactive genes, repair rates are low relative to those previously measured in proliferating rodent cell lines. Repair efficiency in the MHCemb gene increases as it is activated during differentiation and, in fact, approaches 100% within 5 days, while that in the silent GAP43 gene is much lower. While repair efficiencies generally correlate with expression in the genes studied, the overall time course of repair appears to be prolonged in these cells compared to that in proliferating cells. These terminally differentiating cells seem to maintain a DNA damage surveillance and repair capacity for selected genes and/or genomic domains.

Uncoupling of muscle-specific protein expression in myocyte x myoblast heterokaryons

The inducibility of several rat skeletal muscle proteins was examined in heterokaryons formed by fusing differentiated chick myocytes to undifferentiated rat myoblasts. Chicken and rat proteins were distinguished using species-specific antibodies or by their different migrations in polyacrylamide or agarose gels. Both rat skeletal myosin light chain 1 and rat alpha-tropomyosin were induced in the heterokaryons. In contrast, neither rat acetylcholine receptors nor creatine kinase could be detected. These results suggest that chick myocytes may contain quantities of regulatory factors that are sufficient for the activation of some but not all of these rat muscle-specific proteins within the cellular context of the heterokaryon.

5-bromo-2'-deoxyuridine blocks myogenesis by extinguishing expression of MyoD1

The pyrimidine analog 5-bromodeoxyuridine (BUdR) competes with thymidine for incorporation into DNA. Substitution of BUdR for thymidine does not significantly affect cell viability but does block cell differentiation in many different lineages. BUdR substitution in a mouse myoblast line blocked myogenic differentiation and extinguished the expression of the myogenic determination gene MyoD1. Forced expression of MyoD1 from a transfected expression vector in a BUdR-substituted myoblast overcame the block to differentiation imposed by BUdR. Activation of BUdR-substituted muscle structural genes and apparently normal differentiation were observed in transfected myoblasts. This shows that BUdR blocks myogenesis at the level of a myogenic regulatory gene, possibly MyoD1, not by directly inhibiting the activation of muscle structural genes. It is consistent with the idea that BUdR selectively blocks a class of regulatory genes, each member of which is important for the development of a different cell lineage.

Approximately 1 kilobase of sequence 5' to the two myosin light-chain 1f/3f gene cap sites is sufficient for differentiation-dependent expression

Approximately 1 kilobase of genomic DNA from the chicken fast myosin light-chain 1f/3f gene 5' to the transcriptional start sites for each light-chain mRNA was sufficient for differentiation-dependent, tissue-restricted expression. This was determined in primary chick myoblast cultures transfected with the chloramphenicol acetyltransferase (CAT) expression vector p8CAT containing these 5'-flanking sequences. The expression of CAT activity from both light-chain promoters was 10- to 20-fold higher in differentiated myotubes than in fibroblasts or myoblasts grown in bromodeoxyuridine. In contrast, the beta-actin and Rous sarcoma virus promoters joined to the CAT gene were expressed equally in all cell backgrounds tested. Even though the relative timing of light-chain 1f and 3f expression was altered, tissue-restricted, differentiation-dependent expression of the light-chain mRNAs was maintained with these 5' cis-acting sequence elements.

Approximately 1 kilobase of sequence 5' to the two myosin light-chain 1f/3f gene cap sites is sufficient for differentiation-dependent expression

Approximately 1 kilobase of genomic DNA from the chicken fast myosin light-chain 1f/3f gene 5' to the transcriptional start sites for each light-chain mRNA was sufficient for differentiation-dependent, tissue-restricted expression. This was determined in primary chick myoblast cultures transfected with the chloramphenicol acetyltransferase (CAT) expression vector p8CAT containing these 5'-flanking sequences. The expression of CAT activity from both light-chain promoters was 10- to 20-fold higher in differentiated myotubes than in fibroblasts or myoblasts grown in bromodeoxyuridine. In contrast, the beta-actin and Rous sarcoma virus promoters joined to the CAT gene were expressed equally in all cell backgrounds tested. Even though the relative timing of light-chain 1f and 3f expression was altered, tissue-restricted, differentiation-dependent expression of the light-chain mRNAs was maintained with these 5' cis-acting sequence elements.

Regulation of the growth of nonmuscle heart cells in culture

Primary cultures of neonatal rat hearts contain both striated muscle (myocytes) as well as nonmuscle heart cells (NMHC). Although myocytes do not divide in culture, NMHC do increase in number. The growth of NMHC is dependent on the concentration of serum in the media over a range of 1 to 10%. When compared to growth in 10%, cells in 1% serum have a prolonged doubling time and reach a maximum density that is 70% less. Thus, 1% serum which supports normal myocyte development is a useful culture media to also maintain muscle heart cell homogeneity by its failure to support optimum NMHC division.

The amplified expression of factors regulating myogenesis in L6 myoblasts

A strategy for increasing the expression of the factors regulating myogenesis was developed based upon the observation that increased amounts of regulatory factors could overcome the inhibition of differentiation produced by 5-bromodeoxyuridine (BUdR). L6 rat myoblasts were subjected to multiple cycles of cloning in progressively increasing concentrations of BUdR. The first clones to differentiate were picked and replated for the next cycle of selection. After 28 cycles in BUdR, cells were isolated that could differentiate in the presence of 8 microM BUdR. Cell hybrids between myoblasts subjected to 21 cycles of selection (BU21 cells) and differentiation-defective myoblasts exhibited a high probability of differentiation, consistent with the hypothesis that BU21 cells were overproducing factor(s) involved in the decision to differentiate. The selection of cells able to differentiate in the presence of BUdR may provide a general approach for increasing the expression of the regulatory molecules controlling terminal differentiation.

Expression of differentiated functions in heterokaryons between skeletal myocytes, adrenal cells, fibroblasts and glial cells

The regulation of both muscle and adrenal functions was examined in heterokaryons formed by fusing differentiated chick skeletal myocytes to Y1 mouse adrenal cells. Mouse fast skeletal myosin light chain one (LC1) synthesis was induced and acetylcholine receptor expression was maintained at muscle control levels. Steroid secretion, although reduced compared with Y1 X Y1 adrenal homokaryon control fusions, was nonetheless maintained at relatively high levels. Steroid secretion in the myocyte X adrenal heterokaryons was constitutively expressed and was not increased by exposure to either adrenocorticotrophic hormone or db-cAMP. The population of heterokaryons was thus simultaneously expressing both muscle and adrenal functions. The steroid secretion in these heterokaryons was compared to that in heterokaryons formed by fusing Y1 adrenal cells to either chick skin fibroblasts or rat C6 glial cells. Both of these sets of heterokaryons exhibited low baseline levels of steroid secretion that were inducible to control values by ACTH. These results extend previous observations showing that heterokaryons are functionally very different than cell hybrids, and exhibit a variety of phenotypic interactions. Although fibroblasts suppress muscle functions in heterokaryons, they are permissive for adrenal functions. C6 glial cells are permissive for both adrenal and muscle functions, and along with several other neurectodermal derivatives contain an inducible skeletal myosin light chain gene. Finally, myocytes and Y1 adrenal cells are mutually permissive for their differentiated functions, and Y1 adrenal cells contain an inducible myosin light chain gene.

Extinction of muscle-specific properties in somatic cell heterokaryons

In studies of gene regulation using somatic cell fusion techniques, the analysis of heterokaryons circumvents several problematic aspects of the more traditional approach utilizing proliferating hybrid cells. We have analyzed the expression of muscle specific properties in heterokaryons between muscle and nonmuscle cells in order to investigate whether differentiating cells contain regulatory factors that repress the expression of alternative developmental pathways. Heterokaryons and cybrids were derived from polyethylene glycol-mediated fusion of differentiated mononucleate chicken myocytes with mouse melanoma cells, mouse melanoma cytoplasts, chicken fibroblasts, or other chicken myocytes. Our results demonstrate that fusion of a myocyte with a nonmyogenic cell generally results in extinction of muscle-specific properties in the immediate fusion product. Myocyte X melanoma heterokaryons ceased to express the skeletal muscle forms of myosin, desmin and creatine kinase, reinitiated DNA synthesis, and showed a loss of spontaneous fusion competence within 96 hr after their formation. Although chicken myocyte X mouse melanoma heterokaryons showed extinction of muscle specific properties, they continued to synthesize protein and to incorporate [3H]hypoxanthine, presumably due to the continued production of constitutive chicken HPRT. That presence of the melanoma nucleus was required for extinction to be observed was demonstrated by the continued expression of muscle proteins in cybrids between chicken myocytes and melanoma cytoplasts. Significantly, heterokaryons between chicken myocytes and chicken fibroblasts also exhibited extinction of muscle proteins, demonstrating for the first time that extinction is not restricted to fusions in which at least one parental cell type was derived from an established cell line. Our results strongly support the notion that extinction reflects cell-type specific gene regulatory mechanisms operative during development.

Induction of muscle genes in neural cells

The regulation of skeletal muscle genes was examined in heterokaryons formed by fusing differentiated chick skeletal myocytes to four different rat neural cell lines. Highly enriched populations of heterokaryons isolated using irreversible biochemical inhibitors were labeled with [35S]methionine and analyzed on two-dimensional gels. Rat skeletal myosin light chains were induced in three of the four cell combinations. The one exception, the S-20 cholinergic cell line, not only failed to synthesize rat muscle proteins but also suppressed chick myogenic functions. Experiments with heterokaryons between chick myocytes and cells from whole embryonic rat brain cultures demonstrated that rat skeletal myosin light chains are inducible in normal diploid neural cells as well as in established neural cell lines. In contrast, dividing cell hybrids between rat myoblasts and rat glial cells were nonmyogenic. These results demonstrate that although neural cells may contain factors that prevent the decision to differentiate along myogenic lines in cell hybrids, most neural cell lines do not dominantly suppress the expression of muscle structural genes in heterokaryons. Furthermore, the skeletal myosin light chain genes in most neural cell lines are regulated by a mechanism that permits them to respond to putative chick skeletal myocyte-inducing factors. The "open" state of these myogenic genes may explain many of the reports of apparent "transdifferentiation" to muscle in neural cultures and neural tumors.