Cellular proliferation is a highly regulated process. During embryonic development, rapid cell growth is required to form the tissues of the body. In contrast, cellular proliferation in adults is slow, primarily serving to replace senescent cells. Adults, however, retain a limited capacity for rapid growth—for example, during wound healing. Regulation of this proliferative capacity is critically important. Errors in growth control can result in a variety of diseases, including cancer.
The local environment of cells acts as an important factor in controlling cellular proliferation. Changes in the local environment have profound effects on cellular physiology. Such changes include alterations in the physical-chemical properties of the environment, for example, temperature, osmotic stress, oxygen concentration, pH, and ionizing radiation. In addition, cells can respond to extracellular, biologically active molecules, including cytokines, growth factors, hormones, and the matrix to which the cells are attached.
Our research group is studying the molecular mechanisms that are employed by cells to respond to changes in the environment. A specific focus of our studies is the response of cells to inflammatory cytokines (for example, tumor necrosis factor and interleukin-1) and perturbations in the physical-chemical properties of the environment. Together, these cellular actions can be considered to be genetically programmed responses of cells to stress. Our studies have implications for the understanding of the stress response that occurs under physiological conditions (for example, during an immune response or during embryonic development) and under pathophysiological conditions (for example, stroke, heart disease, and tumorigenesis). In addition, the stress response is relevant to the treatments that are employed for many diseases, including radiation therapy and chemotherapy of patients with cancer. Thus, an understanding of the stress response will assist the rational design of novel therapeutic approaches.
A principal question we must answer to understand the mechanism of signal transduction is how a stimulus that is detected at the cell surface can be transmitted to the nucleus to initiate a program of gene expression that results in the creation of an appropriate physiological response to changes in the cellular environment. Protein kinases are one class of regulatory molecules that could account for this process. We are focusing our research on one subclass of these enzymes that causes the phosphorylation of proline-rich target sequences in substrate proteins. The mitogen-activated protein (MAP) kinases are one example of these enzymes. These protein kinases exist in multiple forms as part of an extended family of enzymes that are regulated by environmental stimulation.
The MAP kinases are activated by the sequential actions of other protein kinases that are arranged to form a signaling cascade where one protein kinase phosphorylates and activates the next protein kinase in sequence. A minimal signaling module consists of a MAP kinase, a MAP kinase kinase, and a MAP kinase kinase kinase. Different groups of MAP kinases are activated by different signaling modules that are composed of distinct protein kinases. Three major groups of MAP kinases have been identified by molecular cloning: the extracellular signal–regulated kinases, the p38 MAP kinases, and the c-Jun amino-terminal kinases (JNKs). A major focus of our studies is the JNK group of MAP kinases.
JNKs are activated by the exposure of cells to many forms of extracellular stress by a cascade that is formed by 1 of 14 MAP kinase kinase kinases and 1 of 2 different MAP kinase kinases. This complexity most likely exists because this signaling pathway is activated by many different stresses. The signaling cascade can be created through the interaction of the protein kinases within the cell. However, a JNK signaling module can also be assembled by the interaction of these protein kinases with other proteins that function as "molecular scaffolds." The JNK-interacting proteins JIP1, JIP2, JIP3, and JIP4 represent one family of mammalian scaffold proteins. These JIP proteins bind to a selective group of signaling proteins, including the mixed-lineage protein kinase group of MAP kinase kinase kinases, the MKK7 MAP kinase kinase, and the MAP kinase JNK. The scaffolds insulate the JNK signaling pathway from activation by inappropriate stimuli, enhance the activation of JNK by specific stimuli, and localize the activation of JNK to specific regions of the cell. In neurons, the JIP proteins accumulate in the neural projections that allow interneuronal communication. Studies using targeted gene disruption in mice demonstrate that JIP proteins are required for JNK activation in response to anoxia in neurons (e.g., stroke models) and also for JNK activation in response to metabolic stress in muscle and fat (e.g., in type II diabetes).
When the JNK signaling pathway is activated, the JNK protein kinases phosphorylate a group of protein targets of this signaling pathway. This group of proteins includes c-Jun, a component of the AP-1 transcription factor that is important for cellular responses to environmental stimulation. Studies of the physiological function of the JNK protein kinases have been performed using mice with targeted disruptions of genes that encode components of the JNK signaling pathway. These studies have demonstrated that the JNK signaling pathway contributes to stress-induced apoptosis (genetically programmed cell death) and also to the insulin resistance observed during metabolic stress (e.g., type II diabetes). Further studies designed to define the molecular mechanism and physiological significance of the JNK signal transduction pathway are in progress.
Our overall goal is to establish the molecular details of a signal transduction pathway that is initiated by environmental stress that leads to the regulation of gene expression. The major focus of our studies is to understand the role of the JNK stress signaling pathway in cancer and diabetes. Our current experiments are focused on identifying the genes that form stress-response pathways and determining the molecular mechanisms that account for the control of cell function by these signaling pathways.