 | Mordechai Liscovitch, Ph.D.Professor |
Caveolin-1 is an essential protein constituent of plasma membrane caveolae - non-clathrin-coated, flask-shaped invaginations of the plasma membrane. Caveolin-1 is a principal component of the caveolar coat and a regulator of caveolae-dependent signaling and endocytosis. In addition, caveolin-1 exhibits an unusual ability to interact with and modulate multiple signaling pathways, suggesting that its expression is likely to profoundly affect cell function and cell fate. Caveolin-1-null mice exhibit tissue-specific hyperplasia and increased sensitivity to oncogene- and carcinogen-induced tumorigenesis, in the mammary gland and skin, respectively. These results led to the suggestion that caveolin-1 is a growth-inhibitory protein that may act as a tumor-suppressor. However, this idea is inconsistent with the fact that caveolin-1 is highly expressed in many cancer cell lines. This was initially demonstrated by us in human multidrug resistant cancer cells (Lavie et al., 1998) and by others in mouse metastatic prostate cancer cells. A large body of data that has accumulated in recent years reveals that in many forms of cancer caveolin-1 expression is up-regulated. Furthermore, the expression of caveolin-1 is often positively correlated with the tumor cell grade and its progression stage and, in some cases, the expression of caveolin-1 is an independent predictor of poor disease prognosis (Liscovitch et al., 2005). These data highlight an important question: Why is a putative tumor suppressor protein like caveolin-1 highly expressed in so many cancer cells? One possibility is that in such cancer cells caveolin-1 promotes cell survival. Indeed, the ability of caveolin-1 to effect both growth-inhibitory and survival-promoting actions may explain its divergent expression in early vs. advanced stage human cancers (Liscovitch et al., 2005). Therefore, the main focus of our current research is to elucidate the function(s) of caveolin-1 in human cancer cell lines and to examine the hypothesis that its expression in advanced stage, multidrug resistant and/or metastatic cancer is related to its pro-survival actions.
We are interested in the role of signal-activated phospholipases and lipid-derived messengers in control of cell growth, differentiation and function. Activation of phospholipase D by extracellular signals constitutes a putative signal transduction pathway and is involved in intracellular membrane traffic (see recent review; phospholipase D links). We are studying the cellular and molecular physiology of eukaryotic phospholipase D isozymes, including their localization, mechanisms of activation and possible functions (see a list of our recent publications). Currently, we are engaged in identification and cloning of a second yeast phospholipase D gene; we study the differential localization of mammalian phospholipase D isozymes in specific membrane microdomains; we investigate the possible role of phospholipase D2 in caveolae-mediated endocytosis and signaling; and we explore the action(s) and target(s) of phosphatidic acid as a mediator of specific cellular events.
We designed a novel, general and simple approach for generation of protein alleles that can be regulated by a small-molecule drug. The new procedure involves insertion into a given protein of a chemical-genetic Œswitch¹ which consists of a genetically-encoded peptide that binds a small-molecule ligand with high affinity. The insertion position is selected empirically to confer ligand-dependent modulation of the mutant protein¹s activity. We demonstrated the feasibility of the new method by its application to the TEM-1 beta-lactamase antibiotics resistance gene. We generated several ligand-sensitive mutants of TEM-1, two of which are inhibited by the ligand, whereas a third is stimulated by the ligand. Our preliminary results suggest that the method may be applied to any protein given an appropriate activity assay. ŒRegulatable¹ mutant alleles could be utilized to determine the effect of drug-dependent activation or inhibition of the protein on cancer-related cellular phenotypes and help establish its cellular function. This method will therefore be used to determine the role played by selected proteins, including caveolin-1, in mediating the phenotypic changes that are associated with cancer cell progression to a full-fledged malignant state.
