Protein kinase C as a therapeutic target.

The recognition of protein kinase C (PKC) as the long soughtafter receptor for the tumor-promoting phorbol esters established the potential role of PKC in carcinogenesis and as a potentially important target for cancer therapeutics (1, 2). PKC is a family of serine/threonine kinases that regulate a variety of cell functions including proliferation, gene expression, cell cycle, differentiation, cytoskeletal organization cell migration, and apoptosis (3). The PKC family was the first recognized receptor of diacylglycerol. Aberrant PKC activation can lead to diseases of cellular dysregulation such as cancer and diabetes. The existence of multiple isozymes of PKC raised the question of whether each PKC isozyme has a specific function. The PKC family includes isozymes [a, hI, hII, g, y, q, u, D, E/L (mouse/ human), and ~] which are involved in signal transduction from membrane receptors to the nucleus (Fig. 1; ref. 4). Differences in PKC isozyme protein structure and substrate preferences have allowed the family to be divided into three groups (2). First, the conventional PKC isozymes (a, hI, hII, and g) are calcium-dependent and are phospholipidand diacylglycerolactivated kinases. Second, the novel isozymes (y, q, A, and u) are calcium-insensitive, phospholipid-dependent, and diacylglycerol-dependent. Third, the atypical PKC isozymes (D and E/L) are both calciumand diacylglycerol-insensitive enzymes. The activation mechanism of the PKC isozyme family is clearly different among the three subgroups: conventional, novel, and atypical PKC, but whether or not each isozyme in a subgroup has a specific function or activation mechanism has not been clarified. The activation and degradation of the PKC isozymes is controlled spatially and temporally (5). PKC is a single polypeptide with four conserved regions and four variable regions having a COOH-terminal catalytic domain and a NH2terminal regulatory domain (2). The multiple functions of PKC in signal transduction are regulated by targeting PKC to specific intracellular compartments. Several PKC isozymes may be capable of phosphorylating the same substrate. The constitutive, lipid-dependent protein kinase activity of purified PKC delayed the realization that PKC phosphorylation plays a fundamental role in their catalytic activities. PKC phosphorylation in vivo is well documented and is important in the maturation of the enzyme to a fully functional form localized correctly in the cell. The identification of physiologically relevant substrates of the 11 known PKC isozymes is of obvious importance for a complete understanding of the mechanisms by which this family of serine/threonine protein kinases relays information from intracellular stimuli to biological responses. Many proteins have been identified as PKC substrates both in vitro and in vivo . Interestingly, however, knowledge of PKC substrates is not currently matched by the detailed understanding of PKC regulation (2–4, 6). PKC isoforms are widely distributed in tissues, although some isoforms are selectively expressed. PKCg is selectively expressed in the central nervous system and spinal cord, PKCu is expressed by skeletal muscle and hematopoietic cells, and PKCh is expressed in pancreatic islet cells, monocytes, brain, and retinal tissue (2). Many kinases display overlapping substrate specificities in vitro and can functionally compensate for each other in single-gene knockout experiments. Early investigations regarding the role of PKC isoforms in various intracellular signaling events were based on a linear paradigm (6). Recent studies show that signaling molecules aggregate to form multiprotein complexes, a phenomenon that seems to hold a number of various types of molecules in close proximity. The formation of these complexes may facilitate signal transduction and allow the modulation of biological functions within the cell. PKC isozymes do not function in isolation but exist in complexes with other signaling molecules. PKChI, PKCq, and PKCg are targets in phorbol ester– mediated tumor promotion/progression, whereas PKCy is tumor-suppressive (Fig. 1). Disulfide forms of thiols have oxidative regulatory effects on PKC isozymes and PKC Sthiolation by disulfiram induces differential regulatory effects on PKC isozymes that correlate with the cancer-preventive activity of disulfiram (7, 8). Overexpression of PKCy stimulates apoptosis in a wide variety of cell types through a mechanism that is incompletely understood (9). PKCy-deficient cells are impaired in their response to DNA damage–induced apoptosis, suggesting that PKCy is required for an apoptotic response to stress. Using adenoviral vectors, Santiago-Walker et al. (9) found a modest increase in PKCy activity but PKCa or PKCq activity was not able to selectively stimulate quiescent cells to initiate G(1) phase cell cycle progression. The PKCy-infected cells arrest in S phase and proceed to caspase-dependent apoptotic cell death. Further delineating the PKC isozymes, Gustafson et al. (10) showed that atypical PKCL is required for Bcr-Abl-mediated resistance of human K562 chronic myelogenous leukemia cells to paclitaxel-induced apoptosis. When BcrAbl was expressed in Bcr-Abl-negative HL-60 promyelocytic leukemia cells, the expression of PKChI, PKChII, and PKCL was induced whereas expression of PKCy was decreased. Thus, Bcr-Abl-mediated transformation involves transcriptional activation of the PKCL gene leading to Bcr-Abl-mediated chemoresistance. Understanding the mechanisms by which PKC contribute to malignancy remains a challenge (11). One model critically implicates conventional PKChII (diacylglycerol-responsive) and atypical PKCE/L (diacylglycerol-unresponsive) in cancer cell transformation and invasion. PKCE/L is markedly upregulated in human colon and lung tumors, in an in vivo model Review