Transforming Normal Human Cells Into Tumor Cells

Transforming Normal Human Cells Into Tumor Cells

by Jeremy Cherfas

WHAT'S HOT IN BIOLOGY...

Rank	Paper	Citations This Period Jul- Aug 01	Rank Last Period May- Jun 01
1	M.D. Adams, et al., *"The genome sequence of Drosophila melanogaster,"* Science, 287(5461):2185-95, 24 March 2000. [35 institutions worldwide] *296WE	62	1
2	H.M. Berman, et al., "The Protein Data Bank," Nucl. Acids Res., 28(1):235-42, 1 January 2000. [Rutgers U., Piscataway; NIST, Gaithersburg, MD; U. Calif., San Diego; Burnham Inst., La Jolla, CA] *276UT	58	2
3	J.C. Venter, et al., "The sequence of the human genome," Science, 291(5507):1304-51, 16 February 2001. [14 institutions worldwide] *402MX	58	3
4	A.A. Alizadeh, et al., "Distinct types of diffuse large B-cell lymphoma identified by gene expression profiling," Nature, 403(6769):503-11, 3 February 2000. [8 U.S. institutions] *282PM	39	8
5	B.D. Strahl, D. Allis, "The language of covalent histone modifications," Nature, 403(6765):41-5, 6 January 2000. [U. Virginia Hlth. Sci. Ctr., Charlottesville] *273BM	37	4
6	K. Palczewski, et al., "Crystal structure of rhodopsin: a G protein-coupled receptor," Science, 289(5480):739-45, 4 August 2000. [U. Washington, Seattle; RIKEN Harima Inst., Hyogo, Japan; Tokyo Inst. Technol., Yokohama, Japan] *341FZ	37	6
7	J.A. Romashkova, S.S. Makarov, "NF-kB is a target of AKT in anti-apoptotic PDGF signalling," Nature, 401(6748):86-90, 2 September 1999. [U. North Carolina, Chapel Hill] *232MK	32	†
8	A. Bateman, et al., "The Pfam protein families database," Nucl. Acids Res., 28(1):263-6, 1 January 2000. [Sanger Ctr., Cambridge., U.K.; Washington U. Sch. Med., St. Louis, MO; Karolinska Inst., Stockholm, Sweden] *276UT	32	†
9	A. Bairoch, R. Apweiler, "The SWISS-PROT protein sequence database and its supplement TrEMBL in 2000," Nucl. Acids. Res., 28(1):45-8, 1 January 2000. [Centre Med. U., Geneva, Switzerland; Europ. Bioinformatics Inst., Cambridge, U.K.] *276UT	32	†
10	W.C. Hahn, et al., "Creation of human tumour cells with defined genetic elements," Nature, 400(6743):464-8, 29 July 1999. [6 U.S. institutions] *221FZ 31	31	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

n 1983 researchers created the first mouse tumor cells by making changes to two genes. It took another 16 years to repeat the trick in human cells, not least because three genes are needed. The paper at #10 outlines how Robert A. Weinberg and his group at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, transformed human cells.

The crucial essential gene reflects a clear difference between humans and mice. Normal mouse cells express a gene for telomerase, an enzyme that is responsible for lengthening the telomeres, regions at the ends of chromosomes. Telomeres stop the ends of the chromosome fraying. Human cells, by contrast, do not normally make telomerase. The telomeres shorten after every cell division, which limits the number of times a cell can divide (see Science Watch, 11[3]:8, May/June 2000). The enzyme is, however, active in all known human tumor cells, and could contribute to their immortality.

Immortality, though, is not the same as transformation. Immortal cells keep on dividing, but they still need growth factors and must be anchored to a solid support. Transformed cells do not need growth factors and will form colonies in the absence of a solid support.

"We wondered what it was about the mouse cells that made them so easy to transform and hypothesized that it might have something to do with the presence of the enzyme telomerase," says Bill Hahn, a postdoctoral fellow in Weinberg's laboratory, and one of the lead authors on the Nature paper. The question is whether human cells "need" an active telomerase gene to be transformed into tumors, or whether that is just an accompanying change.

Hahn and Weinberg had previously shown that expression of hTERT, which codes for the telomerase subunit that actually lengthens the telomeres, confers immortality on some (but not all) types of human cell. To see whether hTERT makes cells tumorigenic—rather than merely immortal—Weinberg's group set about putting it into cells along with two other "cancer" genes.

One was ras, the first known oncogene, discovered by Weinberg in 1980. The other oncogene was the large-T antigen of simian virus 40. Cells expressing only ras and large-T quickly went into crisis and died out. Cells with hTERT and the two oncogenes became immortal. Could they, however, form tumors?

In soft agar, with no solid support, cells with all three genes rapidly formed colonies; those with only the two oncogenes did so poorly and very rarely, and those with one oncogene and hTERT never. Furthermore, injected into immune-deficient nude mice, the three-gene cells quickly turned into fast-growing solid tumors, a process that required no further changes to the cells. The others did not.

Weinberg's group has demonstrated that the transformation of human cells involves at least four different pathways. The maintenance of telomeres by hTERT confers immortality. Large-T is known to interact with two cellular control pathways involving the tumor-suppressor genes p53 and retinoblastoma (the first tumor-suppressor gene to be isolated, also by Weinberg, in 1983). And ras acts on the control of cell division to permit unchecked growth.

"It is the end of one line of work," Weinberg tells Science Watch, in that after more than 15 years "we are close to enumerating the total set of genetic and biochemical changes that is required to transform a normal human cell into a tumor cell." But it is also the beginning of several new types of work. The transformed cells are not capable of metastasis; Weinberg's lab is looking for the genes that enable transformed cells to invade other tissues and metastasize. "The genetic determinants of these latter steps in tumor progression are still poorly defined," Weinberg says.

Hahn adds another question: "How close are these models to real cancer cells?" In a preliminary effort his lab used the same three genes successfully to convert human mammary epithelial cells into tumor cells (see B. Elenbaas, et al., "Human breast cancer cells generated by oncogenic transformation of primary mammary epithelial cells," Genes & Development, 15[1]:50-65, 1 January 2001). "The human cell model behaved in many similar ways to cells derived from patients," Hahn says. Hahn's group now has its sights set on what he calls "a whole cadre of model systems." That will allow better understanding of just what happens when a cell is transformed into a tumor and, with this understanding, the hope of more effective therapy.

Dr. Jeremy Cherfas is Science Writer
at the International Plant Genetic Resources Institute, Rome, Italy.