Cytoplasmic modification of nuclear gene expression

Epigenetics and memigenetics

The field of epigenetics is expanding rapidly, yet there is persistent uncertainty in the definition of the term. The word was coined in the mid-twentieth century as a descriptor of how intrinsic, yet largely unknown, forces act with genes to channel progenitor cells along pathways of differentiation. Near the end of the twentieth century, epigenetics was defined more specifically as the study of changes in gene activity states. In some definitions, only those activity states that are inherited across cell division were considered. Other definitions were broader, also including activity states that are transient, or occurring in non-dividing cells. The greatest point of disagreement in these current definitions, is if the term should concern only inherited activity states. To alleviate this disparity, an alternative term, 'memigenetics', could be used in place of epigenetics to describe inherited chromatin activity states. The advantage of this term is that it is self-defining, and would serve to emphasize the important concept of cell memory. It would also free the term epigenetics to be used in a broader sense in accord with the meaning of the prefix 'epi', that is, as a descriptor of what is 'over' DNA at any point in time.

Maternally inherited susceptibility to cancer

Tumor microenvironment promotes mtDNA mutations. A number of these mutations will affect cell metabolism and increase cell survival. These mutations are positively selected and contribute to other tumor features, such as extracellular matrix remodeling and angiogenic processes, thus favoring metastases. Like somatic mutations, although with less marked effects, some mtDNA population polymorphisms will affect OXPHOS function, cell metabolism, and homeostasis. Thus, they could behave as inherited susceptibility factors for cancer. However, in addition to epidemiological evidence, other more direct clues are required. The cybrid approach can help to clarify this issue.

Active tissue-specific DNA demethylation conferred by somatic cell nuclei in stable heterokaryons

DNA methylation is among the most stable epigenetic marks, ensuring tissue-specific gene expression in a heritable manner throughout development. Here we report that differentiated mesodermal somatic cells can confer tissue-specific changes in DNA methylation on epidermal progenitor cells after fusion in stable multinucleate heterokaryons. Myogenic factors alter regulatory regions of genes in keratinocyte cell nuclei, demethylating and activating a muscle-specific gene and methylating and silencing a keratinocyte-specific gene. Because these changes occur in the absence of DNA replication or cell division, they are mediated by an active mechanism. Thus, the capacity to transfer epigenetic changes to other nuclei is not limited to embryonic stem cells and oocytes but is also a property of highly specialized mammalian somatic cells. These results suggest the possibility of directing the reprogramming of readily available postnatal human progenitor cells toward specific tissue cell types.

Subcellular localization of phosphatidylinositol synthesis

It is well-established that the endoplasmic reticulum is the major site of phosphatidylinositol (PtdIns) synthesis. The PtdIns synthetic ability of other organelles, such as plasma membrane and nucleus, remains controversial. In the present study, we re-examine this question by comparing PtdIns synthesis in isolated cytoplasts (enucleated cells) with that in corresponding karyoplasts (nuclei surrounded by plasma membrane but lacking most cytoplasmic components). We report that cytoplasts are competent to carry out both basal and stimulated PtdIns synthesis as well as polyphosphoinositide hydrolysis, while karyoplasts can neither synthesize PtdIns nor hydrolyze phosphoinositides in response to agonists. The karyoplasts are, however, capable of synthesizing phosphatidylcholine (PtdCho), as previously reported. From these data, we conclude that PtdIns synthesis is limited to cytoplasmic components, and cannot be sustained by either plasma membrane or nucleus under conditions that permit robust PtdCho synthesis.

Epigenetic Theories of Cancer Initiation

I argue that carcinogenic insults injure many cells rather than mutate a few. This results from evidence that such insults convert too many cells to a precancerous state and that too many of the converted cells then revert to plausibly involve mutation and its repair; from evidence that the delays between such insults and chemically demonstrable mutations are long enough to easily allow nonmutational mechanisms to work; from evidence that even ionizing radiation first acts on the cytoplasm and mainly affects cells unhit by it; from the fact that such insults induce proto-oncogene expression far too quickly to do so by mutation; and from the fact that fusions of various cells and cell parts show that the tumorous or nontumorous nature of the product depends on its cytoplasmic rather than its nuclear component. I further argue that reduced DNA methylation, modifications of the histone code, and tissue disorganization are the three main mechanisms of epigenetic cancer initiation. Hypomethylation would result from DNA excision repair. Moreover, a methyl-deficient diet is carcinogenic and demethylation is also known to be carcinogenic via the histone code. Finally, I strongly argue for tissue disorganization as a mechanism of cancer initiation. This results from evidence that skin carcinogens disrupt the dermal/epidermal connection and from the fact that tumorigens swiftly disrupt gap junctions, as well as from evidence that such disruption is tumorigenic.

Mitochondrial genomes in intraspecies mammalian cell hybrids display codominant or dominant/recessive behavior

A unique type of nonstochastic mitochondrial DNA (mtDNA) segregation was found in mammalian cells. In human cell hybrids isolated from the fusion of HeLa cells with 23, GM639, A549, or 293 cells, HeLa mtDNA was always lost from the hybrids, whereas both parental mtDNAs were maintained in hybrids of HeLa X 143BTK-. Similar phenomena were observed in mouse cell hybrids isolated by the fusion of cells with different mtDNA types. Types 1, 2, and 3, can be distinguished from each other by restriction fragment-length polymorphisms. The mouse cell hybrids between cells with type 1 and type 2 mtDNA always lost type 2 mtDNA, whereas the hybrids between cells with type 2 and type 3 mtDNA retained both types stably. These observations suggest that either a codominant or a dominant/recessive relationship may be present in intraspecies mitochondrial genomes of human and mouse cells. When the mitochondrial genomes in cell hybrids are codominant, stochastic segregation occurs while nonstochastic segregation occurs when they are in the dominant/recessive relationship. These concepts may help elucidate organelle heredity in animals.

Cytoplasmic suppression of malignancy

Using both normal and transformed rat liver epithelial cells to prepare cytoplasmic hybrids (cybrids) we have found evidence to support the theory that the cytoplasm from a normal cell can suppress tumorigenicity. A unique aspect of this study is that all of the cells utilized, both normal and malignantly transformed, were derived from an original cloned cell. We found that fusing cytoplasts from normal cells to malignantly transformed whole cells resulted in cybrid clones which, when injected into newborn rat pups, isogenic with those from which the cell culture was initiated, yielded tumors in 51% of the animals injected compared to 92% of the animals injected with the tumorigenic parent. Those animals that did develop tumors from the cybrid cells survived longer than those injected with cells from the tumorigenic parent. Thus, the cybrid, formed of cytoplasm from both parents, was less tumorigenic than the malignantly transformed parent cell. When reconstituted cells were prepared by fusing cytoplasts from normal cells with karyoplasts from malignantly transformed cells, a situation in which essentially all of the cytoplasm of the reconstituted cell is derived from normal cells, the tumorigenic phenotype was extinguished.

Differentiation apparently repressed by the nucleus. Rapidly-induced pigmentation of enucleated melanoma cells

There is evidence for cytoplasmic control over gene expression in cell differentiation, but still very little is known of the intracellular mechanism, nuclear, cytoplasmic, or both, which actively initiates the differentiation of one cell type into another. Here the role of the cytoplasm was examined in the induction of differentiation of cultured mouse melanoma cells by melanocyte-stimulating hormone and alkaline medium. Intact cells were compared with cytoplasts, cells enucleated by centrifugation in the presence of cytochalasin D (CD). Surprisingly, early inductions of pigment (melanin) synthesis and of the principal melanin-synthesizing enzyme activity, tyrosinase, could be achieved in cytoplasts. Indeed these early changes were slower in nucleated cells and were accelerated by the inhibitor of protein synthesis, cycloheximide. Thus the initial activation of tyrosinase and melanin synthesis--although not necessarily any other or later aspects of melanoma cell differentiation--is apparently controlled through a labile, transcription- and translation-dependent repression. To our knowledge this is a novel mechanism for the initiation of differentiation; its generality remains to be tested.

A method for analyzing transcription using permeabilized cells

Brief exposure of Friend cells to a buffered hypotonic solution containing 1% Tween 80 caused permeabilization and allowed incorporation of [3H]UTP into RNA. The incorporation was inhibited 85-97% by 20 micrograms/ml actinomycin D and the reaction product was completely hydrolyzed by 0.1 M KOH. UMP incorporation was nearly linear for 60 min at 23 degrees C; however, at 37 degrees C it ceased after 15-20 min of rapid incorporation. The inhibition of UMP incorporation by 2 micrograms/ml alpha-amanitin was much greater at 23 degrees C than at 37 degrees C. The molecular weight of the RNA synthesized in permeabilized cells is broadly distributed with about 83% larger than 18 S. In vitro transcription of the mouse beta-major globin gene was studied by hybridizing 32P-labeled nascent RNA to filter-bound DNA sequences representing this gene and its flanking regions. After induction by hexamethylene-bisacetamide, Friend cells exhibited more than fivefold increases in the rate of transcription for the beta-major globin gene as compared to the uninduced control cells. Induction also caused an increase in the transcription rate of the 3'-flanking region located downstream from the poly(A) addition site. Thus, the primary transcription unit of beta-major globin gene is essentially the same in permeabilized cells as that previously reported for nuclei isolated from the same cell line. In addition, permeabilized cells actively initiate RNA synthesis as determined by the incorporation of a thiol group at the 5' initiating nucleotide, when synthesis was in the presence of [gamma-S]-labeled nucleoside triphosphates. Permeabilized cells are about 7-11 times more active than isolated nuclei in the synthesis of both in vitro-initiated and total RNA.