"Examining how cells protect themselves from various enviromental agents to either facilitate or protect from apoptosis is very interesting for basic academic research and for important applications for human health," says Michael Karin of the University of California, San Diego

The 20 microns between the cell membrane and the genetic material in the cell nucleus is the playground of the molecules of signal transduction, of the intricate and multifaceted redundancy of the pathways that take signals from the membrane and convert them into the exquisitely selective control of our genes. Within these pathways, the regulation of gene transcription is carried out by a multitude of hormones and growth factors, which in turn are affected by environmental stresses and a host of other phenomena. Over the past two decades, the study of signal transduction has grown into one of the hottest areas of research. With single-minded intensity, laboratories in academia and the biotech industry have taken to unraveling these pathways and the factors at work within them.

   In that hyper-competitive environment, few researchers have had the remarkable impact or maintained the extraordinary long-term productivity of molecular biologist Michael Karin of the University of California, San Diego. Karin, who ranked #12 in this publication’s recent list of the hottest biomedical researchers of the 1990s (see Science Watch, 9[3]:1-2, May-June 1998), has repeatedly broken new ground in elucidating how gene transcription is regulated by steroid hormones, growth factors, cytokines, and adverse environmental conditions. And Karin’s not just hot now; he’s always been. Since 1981, he’s had more than 70 of his papers cited at least 100 times each—and five papers cited more than 1, 000 times apiece. His reports cover a spectrum of molecules involved in signal transduction.

   Karin, now 47, graduated magna cum laude from Tel Aviv University in 1975 and went on to get his Ph.D. from UCLA in molecular biology, working in the laboratory of Harvey Herschman. Over the next six years, he led a peripatetic existence before settling in 1986 at UCSD, where he is now a professor of pharmacology. In 1993, Karin cofounded Signal Pharmaceuticals in San Diego with several other local scientists. From his office at UCSD, Karin spoke to Science Watch correspondent Gary Taubes.

            Considering the breadth of your work in signal transduction pathways, would you say you had a specific plan or simply followed where the pathways took you?

   Karin: The plan was always to understand signaling. That was my goal since the mid-1980s, to use these experimental systems to understand how signals are transmitted from cell surface to nucleus. We started in the nucleus, identifying the genetic elements that control gene transcription in response to signals generated at the cell surface. The next logical step was to identify the proteins that bind to these genetic elements and that somehow respond to these signals. Once the proteins were identified, we looked at mechanisms that regulate their activity. Once it became clear that it is regulated through protein phosphorylation—although I had a strong suspicion that that would be the case—we had to go after the protein kinases. Phosphorylation is the process whereby these kinases put phosphates on the proteins in the pathways and by doing so make them more, or less, active.

            How did you start off on this research path?

   Karin: With my Ph.D. thesis. It was on the regulation of metallothionein gene expression. Metallothioneins are small, low-molecular-weight, heavy-metal binding proteins whose expression is dramatically induced upon exposure of organisms or cells to heavy-metal ions such as cadmium or zinc. Their job is to protect us from a heavy-metal toxicity. So when you expose cells to heavy-metal ions you induce the transcription of the metallothionein genes. It’s a cellular protection mechanism, considered to be a stress response. So I’ve been involved in studying stress responses since way back. In addition to providing protection against heavy metal ions, these are the major zinc-binding proteins available to us. You really regulate the expression of these genes in response to many different signals, not only heavy metal ions, and thereby maintain zinc homeostasis. That's why the metallothionein promoter was such a good choice for a model in which one can study control of gene transcription in response to environmental and hormonal signals.

            Was it serendipitous?

   Karin: Not really. Part of it was by plan. My Ph.D. advisor, Harvey Herschman, thought about this as a system. It seemed like a good one for studying gene regulation. I started this in the B.C. days—before cloning. What happened A.C. is that my second post-doctoral project in San Francisco was to look at growth hormone gene regulation, and I realized that the metallothionein genes offer a better system to study what I wanted, which was regulation of transcription by glucocorticoid hormones—for example, cortisol or hydrocortisone. So I went back and, in collaboration with another fellow in the lab, isolated cDNAs for the human metallothionein genes, and consequently the genes themselves. Using the promoter of one of these genes, I conducted one of the earliest of what are nowadays called "promoter-bashing experiments." Back then it was pretty much the cutting edge of molecular biology. It’s where you take a promoter of a gene and subject it to all kinds of mutations—deletion mutations and so on—and find which elements in the promoter control its basal activity and which mediate its response to various signals. Starting with the human metallothionein IIA promoter, we outlined which genetic elements dictate basal promoter activity and which control induction of gene transcription in response to glucocorticoid hormones, phorbol ester tumor promoters, and heavy metal ions. This was one of the first, and still the best, examples of combinatorial control of gene transcription—demonstrating that the promoter has different elements that respond to different signals, and that overall promoter activity is the sum of the action of all of these different genetic elements. All genes work like this. It’s pretty fundamental. That work was done between 1980 and 1983, and that set me on the way of being interested in how signals—either hormones, or environmental signals such as metal ions and radiation—affect gene activity. Everything I have done ever since is related to that.

            Give us a rundown on the major discoveries along the way.

   Karin: One of the elements in the metallothionein promoter was a binding site for the glucocorticoid receptor. So, we started working on gene regulation by glucocorticoid hormones, and that's when I first became interested in the induction of cell death, because glucocorticoids are known to kill premature T cells. That was in the mid-1980s, even before the term "apoptosis" was popular.

   Another element in the metallothionein promoter, which turned out to be a gold mine in terms of binding sites for interesting DNA binding proteins, was a binding site for a transcription factor called AP-1. In collaboration with Bob Tjian's lab, we first described this transcription factor. And then my group and others went on to show that AP-1 is essentially a collection of protein dimers composed of Jun and Fos subunits. And the reason that this was very interesting at the time was that c-jun and c-fos were known as proto-oncogenes. But it was not clear until then what the gene products were doing. So that was a sign that products of some proto-oncogenes (those that are expressed in the nucleus) are actually turning on and off gene transcription, and the particular site to which they bind has turned out to be the element responsible for gene induction in response to phorbol ester tumor promoters and growth factors. This was pretty much the first connection between tumor promoters and proto-oncogenes. That was done in 1986-87 and resulted in our most highly cited paper. [See table, paper #1.]

   We dabbled with that for some time. Much of the output of the lab was really on AP-1, Jun and Fos, and using that system we have gotten into how the jun gene itself is regulated, and we have identified the protein kinase cascade that results in the phosphorylation of the c-jun protein itself and other transcription factors leading to induction of c-jun transcription. This, in turn, resulted in the identification of Jun kinases, the JNKs. We worked quite a lot on regulation of these protein kinases, and that took us out of the nucleus.

            So now you're heading toward the cell membrane?

   Karin: We pretty much follow our path in the kind of topics we work on, but the results are directing us. While the plan in the beginning was to follow a path, we just didn't know which path to follow.

   We first identified the protein kinases biochemically in 1993, but before that, in 1991, we showed that the Jun and Fos proteins are regulated by phosphorylation, which was quite a breakthrough. It told us how AP-1 activity is regulated. Then we biochemically identified the Jun kinase in 1993 and, in collaboration with Roger Davis, cloned it several months later.
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