Human arteries engineered in vitro.

There is a pressing need to develop methods to engineer small-calibre arteries for bypass surgery. We hypothesized that the rate-limiting step that has thwarted previous attempts to engineer such vessels from non-neonatal tissues is the limited proliferative capacity of smooth muscle cells (SMCs), which are the main cellular component of these vessels. Ectopic expression of the human telomerase reverse transcriptase subunit (hTERT) has been shown recently to extend the lifespan of certain human cells. We therefore introduced hTERT into human SMCs and found that the resulting cells proliferated far beyond their normal lifespan but retained characteristics of normal control SMCs. Importantly, using these non-neonatal SMCs, we were able to engineer mechanically robust human vessels, a crucial step towards creating arteries of clinical value for bypass surgery.

Mutational analysis defines a minimum level of telomerase activity required for tumourigenic growth of human cells.

A hallmark of cancer cells is the ability to proliferate indefinitely. This acquisition of an immortal lifespan usually requires the activation of telomerase, the enzyme that elongates telomeres. Human telomerase is minimally composed of the reverse transcriptase subunit hTERT, and the RNA subunit hTR. While hTR is ubiquitously expressed in human cells, the hTERT subunit is generally transcriptionally repressed in most normal somatic cells, but is illegitimately activated to restore telomerase activity in cancer cells. Indeed, in the thousands of different human tumours assayed, 85% were scored positive for telomerase activity. However, the levels of telomerase activity detected in tumour samples can vary substantially and even some normal somatic cells have been found to have low levels of enzyme activity. As the functional significance of low levels of telomerase activity is unclear, we investigated whether there is a minimum level of telomerase activity required for tumourigenesis. Using mutants of hTERT that induce varying levels of telomerase activity, we show that there does indeed exist a threshold of activity required for the processes of immortalization, transformation and tumourigenesis. Thus, low levels of activity detected in certain somatic cells would not be expected to contribute to tumourigenesis, nor does the mere detection of telomerase in cancer cells necessarily signify an immortal lifespan.

Distinct requirements for Ras oncogenesis in human versus mouse cells.

The spectrum of tumors associated with oncogenic Ras in humans often differs from those in mice either treated with carcinogens or engineered to sporadically express oncogenic Ras, suggesting that the mechanism of Ras transformation may be different in humans. Ras stimulates primarily three main classes of effector proteins, Rafs, PI3-kinase, and RalGEFs, with Raf generally being the most potent at transforming murine cells. Using oncogenic Ras mutants that activate single effectors as well as constitutively active effectors, we find that the RalGEF, and not the Raf or PI3-kinase pathway, is sufficient for Ras transformation in human cells. Thus, oncogenic Ras may transform murine and human cells by distinct mechanisms, and the RalGEF pathway--previously deemed to play a secondary role in Ras transformation--could represent a new target for anti-cancer therapy.