Enhancement of 5-iododeoxyuridine-induced endogenous C-type virus activation by polycyclic hydrocarbons: apparent lack of parallelism between enhancement and carcinogenicity

Identification and expression analysis of two steamer-like retrotransposons in the Chilean blue mussel (Mytilus chilensis)

BACKGROUND

Disseminated neoplasia (DN) is a proliferative cell disorder of the circulatory system of bivalve mollusks. The disease is transmitted between individuals and can also be induced by external chemical agents such as bromodeoxyuridine. In Mya arenaria, we have cloned and characterized an LTR-retrotransposon named Steamer. Steamer mRNA levels and gene copy number correlates with DN and can be used as a marker of the disease. So far, the only mollusk where a retrotransposon expression relates to DN is Mya arenaria. On the other hand, it has been reported that the Chilean blue mussel Mytilus chilensis can also suffers DN. Our aim was to identify retrotransposons in Mytilus chilensis and to study their expression levels in the context of disseminated neoplasia.

RESULTS

Here we show that 7.1% of individuals collected in August 2018, from two farming areas, presents morphological characteristics described in DN. Using Steamer sequence to interrogate the transcriptome of M. chilensis we found two putative retrotransposons, named Steamer-like elements (MchSLEs). MchSLEs are present in the genome of M. chilensis and MchSLE1 is indeed an LTR-retrotransposon. Neither expression, nor copy number of the reported MchSLEs correlate with DN status but both are expressed at different levels among individual animals. We also report that in cultured M. chilensis haemocytes MchSLEs1 expression can be induced by bromodeoxyuridine.

CONCLUSIONS

We conclude that SLEs present in Mytilus chilensis are differentially expressed among individuals and do not correlate with disseminated neoplasia. Treatment of haemocytes with a stressor like bromodeoxyuridine induces expression of MchSLE1 suggesting that in Mytilus chilensis environmental stressors can induce activation of LTR-retrotransposon.

Activation of transcription and retrotransposition of a novel retroelement, Steamer, in neoplastic hemocytes of the mollusk Mya arenaria

Bivalve mollusks of the North Atlantic, most prominently the soft shell clam Mya arenaria, are afflicted with an epidemic transmissible disease of the circulatory system closely resembling leukemia. The disease is characterized by a dramatic expansion of blast-like cells in the hemolymph with high mitotic index. Examination of hemolymph of diseased clams revealed high levels of reverse transcriptase activity, the hallmark of retroviruses and retroelements. By deep sequencing of RNAs from hemolymph, we identified transcripts of a novel retroelement, here named Steamer. The DNA of the element is marked by long terminal repeats and encodes a single large protein with similarity to mammalian retroviral Gag-Pol proteins. Steamer mRNA levels were specifically elevated in diseased hemocytes, and high expression was correlated with disease status. DNA copy number per genome was present at enormously high levels in diseased hemocytes, indicative of extensive reverse transcription and retrotransposition. Steamer activation in M. arenaria is an example of a catastrophic induction of genetic instability that may initiate or advance the course of leukemia.

Virus-like particles in cultured C3H/St mouse cells treated with a carcinogenic polycyclic hydrocarbon

Virus-like particles with cylindrical form were found in cultured alveolar macrophages and lung fibroblasts of C3H/St mice, after treating these cells with a carcinogenic polycyclic hydrocarbon, 1, 2, 5, 6,-Dibenzanthracene (DBA). Their morphology looked identical to those observed in vivo in the reactive cells which engulfed DBA crystals implanted into the brain and muscle of the same mouse strain. These particles were not found in either the untreated cells of C3 H/St mice or the treated cells of BALB/c mice. In the alveolar macrophages, these particles appeared first at 3 days after DBA treatment and reached the maximum number around the 30th day. They still kept their morphology in the degenerating cells which had lost the cytoplasmic organelles. These findings suggest the possibility that DBA induced the expression of the viral genome endogeneous to C3H/St cell.

A new tumor promoter, dihydroteleocidin B, enhances cell growth and the production of murine leukemia virus by fibroblasts

The effects of tumor promoters on the expression of murine leukemia virus (MuLV) were studied in tissue culture. Dihydroteleocidin B, an indole alkaloid, recently found to be a tumor promoter, enhanced not only the production of Moloney MuLV (M-MuLV) by a mouse fibroblast cell line, C3H2K, persistently infected with M-MuLV, but also growth on the C3H2K cells. The production of infectious M-MuLV by M-MuLV-infected C3H2K cells that had been treated with dihydroteleocidin B for 1-7 days was four or five times higher than that of control cells. C3H2K cells grew faster and became stationary at higher cell densities in the presence of dihydroteleocidin B than in its absence. The tumor-promoting phorbol ester phorbol-12, 13-didecanoate and 12-O-tetradecanoylphorbol-13-acetate also enhanced the production of M-MuLV, but their effects were not so strong as that of dihydroteleocidin B. These tumor promoters, however, did not induce production of endogenous MuLV in C3H2K or K-BALB cells.