By David N. Leff
When in 1872 Hungarian dermatologist Moritz Kaposi described the disease that bears his name, Kaposi's sarcoma was an extremely rare and lethal cancer.
He found the malignant purplish spots on the legs of elderly Central European Jewish men. Later, these lesions turned up on male Italians and on Central African Bantu tribespeople, young and old.
Today, Kaposi's sarcoma (KS) is endemic in those parts of Africa and the leading form of cancer that strikes Americans with AIDS. But unlike those afflicted with the pre-HIV "classic" KS, explained virologist Ornella Flore, "AIDS patients do not usually die because of KS. Most of them develop this cancer at the terminal stage of their disease. Thirty percent or more have the tumor, but they die most often from other diseases."
The multiple cancerous skin lesions that KS victims have on their legs, arms and often in their kidneys and lungs all harbor the same herpesvirus, now known as KSHV — Kaposi's sarcoma-associated herpesvirus. What's more, it also lurks in the tumors of classic KS sufferers.
This circumstantial evidence raised perplexity: How can an obviously infective herpesvirus cause cancer?
There was, to be sure, one precedent: Epstein-Barr virus, a herpes variant, is accountable for the deadly African malignancy, Burkitt's lymphoma. (See BioWorld Today, Jan. 27, 1997, p. 1.)
But before KSHV could be unequivocally indicted for cancer on the strength that it loitered in KS lesions, some key questions demanded answers:
* Could it actually infect the endothelial blood-vessel cells in the skin?
* How could that malignant infection be transmitted to healthy cells?
* Was KSHV actually oncogenic?
* If so, how did it transform its target cells to proliferate immortally?
A task force of virologists at Cornell University Medical College, in New York, took on this challenge. The results of their research appear in today's issue of Nature, dated Aug. 6, 1998, under the title "Transformation of primary human endothelial cells by Kaposi's sarcoma-associated herpesvirus." The paper's first author is Flore.
"We proved for the first time," she told BioWorld Today, "that the purified virus is able to transform the human cells. Nobody before was able to infect them."
To begin with, Flore and her co-authors purified the KSHV virus from a body-cavity lymphoma patient's cell line and used this virus to infect healthy bone-marrow endothelial cells.
"After five days," she recounted, "these cells showed a clear cytopathic effect, as I expected. Most of the cells were dead, but some had a spindle shape, which is typical of KS lesions. Fifteen days later," she went on, "these cells started to proliferate."
Experiments Buttress Circumstantial Evidence
"Normal healthy endothelial cells," Flore pointed out, "need special growth factors, especially vascular endothelial growth factor (VEGF) in order to multiply. I wanted to verify if these cells were able to grow without this supplement."
To her surprise, she found that by decreasing the concentration of VEGF, these cells, which were proliferating and which were infected by KSHV, started to produce the virus in its lytic phase. That is, it expressed the genes encoding the full structure of the virion, capsid and all, rather than the latent phase, in which KSHV lies low, awaiting its lytic cell-infecting action.
"I purified the lytic virus from these cells," Flore continued, "and infected other cells to prove that the virus was able to be transmitted and the cells had actually been transformed [made malignant]."
To demonstrate this transformation, the Cornell team conducted three experiments.
"Only transformed cells can grow in soft agar culture," Flore pointed out. "Our control cells did not grow in agar, but these cells proliferated. Another proof was their possessing telomerase activity, a feature of all transformed cells.
"But the most important finding," she went on, "was a paracrine effect, [like a locally acting factor].
"Only roughly five percent of the proliferating cells are infected," she pointed out. "The others are proliferating anyway, we think, because of the paracrine effect. The infected cells are probably producing some factors — which ones we don't know yet. These factors are up-regulating the VEGF receptor as well as a kinase domain receptor, KDR.
She explained, "Usually, in normal endothelial cells, after 10 or so passages, the KDR starts to be down-regulated. It's not expressed any more, and the cells no longer respond to the VEGF.
Bystander Immortalizing Effect Of Oncogenic Virus
"But these infected cells," Flore reported, "after one year and a half, still have the KDR up-regulated, and they still respond to the VEGF. They are proliferating, even though they are not infected. Probably, this virus is making the infected cells produce some factors. We don't know if they are cellular cytokines or viral cytokines."
Flore foresees clinical potential against KS, once the Cornell group has completed its ongoing follow-up research.
"Now we are proving that the virus is transformant," she said, "and attempting to prove, in mice, that it is oncogenic. We are also continuing to study this paracrine effect and KDR's up-regulation of the cells. If we had an antibody against that KDR receptor, we could try to block these cells and their proliferation by using it against the KDR. In this way, that receptor could be inhibited and the cells wouldn't respond to VEGF stimulation any more. So we can avoid their oncogenic proliferation."
But she cited unanswered questions in the way of this therapeutic outlook. "We don't know what factors are doing the up-regulating. Maybe it's not only the KDR up-regulation that can cause this proliferation.
"Another approach," she added, "will be to see what genes, if there are some genes, are being overexpressed in these transformed cells or if there are some that are down-regulated.
"That's why our group at Cornell has to continue this study," Flore concluded.
Meanwhile, the co-authors point out, "Kaposi's sarcoma, the commonest malignancy for HIV-infected people, is a growing public health problem as AIDS sufferers are surviving longer and are more vulnerable to cancer during the later stages of their disease." *