Signal-Transducing Proteins.

Several examples of oncoproteins that mimic the function of normal cytoplasmic signal-transducing proteins have been found. Most such proteins are strategically located on the inner leaflet of the plasma membrane, where they receive signals from outside the cell (e.g., by activation of growth factor receptors) and transmit them to the cell's nucleus. Biochemically, the signal-transducing proteins are heterogeneous. The best and most well studied example of a signal-transducing oncoprotein is the RAS family of guanine triphosphate (GTP)-binding proteins (G proteins).

The RAS Oncogene.

The RAS proteins were discovered as products of viral oncogenes. Point mutation of RAS family genes is the single most common abnormality of dominant oncogenes in human tumors.

Approximately 15% to 20% of all human tumors contain mutated versions of RAS proteins.[54] Several distinct mutations of RAS have been identified in cancer cells, all of which dramatically reduce the GTPase activity of the RAS proteins. The mutations generally involve codons 12, 59, or 61 of HRAS, KRAS, and NRAS. The frequency of such mutations varies with different tumors, but in some types it is very high. For example, 90% of pancreatic adenocarcinomas and cholangiocarcinomas contain a RAS point mutation, as do about 50% of colon, endometrial, and thyroid cancers and 30% of lung adenocarcinomas and myeloid leukemias.[55] [56] [57] In general, carcinomas (particularly from colon and pancreas) have mutations of KRAS, bladder tumors have HRAS mutations, and hematopoietic tumors bear NRAS mutations. RAS mutations are infrequent in certain other cancers, particularly those arising in the uterine cervix or breast.

Several studies indicate that RAS plays an important role in mitogenesis induced by growth factors. For example, blockade of RAS function by microinjection of specific antibodies blocks the proliferative response to EGF, PDGF, and CSF-1. Normal RAS proteins are tethered to the cytoplasmic aspect of the plasma membrane, and they flip back and forth between an activated, signal-transmitting form and an inactive, quiescent state. Recently it was found that these proteins may also be found in the endoplasmic reticulum and Golgi membranes, where they can be activated by growth factor binding to the plasma membrane, through a still-uncertain mechanism. [58] In the inactive state, RAS proteins bind guanosine diphosphate (GDP); when cells are stimulated by growth factors or other receptor-ligand interactions, RAS becomes activated by exchanging GDP for GTP ( Fig. 7-32 ). Activated RAS, in turn, acts on the MAP kinase pathway by recruiting the cytosolic protein RAF-1. The MAP kinases so activated target nuclear transcription factors and thus promote mitogenesis. In normal cells, the activated signal-transmitting stage of the RAS protein is transient because its intrinsic GTPase activity hydrolyzes GTP to GDP, thereby returning RAS to its quiescent ground state (described below).

The orderly cycling of the RAS protein depends on two reactions: (1) nucleotide exchange (GDP by GTP), which activates RAS protein, and (2) GTP hydrolysis, which converts the GTPbound,

active RAS to the GDP-bound, inactive form. Both these processes are enzymatically regulated. The removal of GDP and its replacement by GTP during RAS activation are catalyzed by a family of guanine nucleotide-releasing proteins that are recruited to the cytosolic domain of activated growth factor receptors by adapter proteins. More importantly, the GTPase activity intrinsic to normal RAS proteins is dramatically accelerated by GTPase-activating proteins (GAPs). These widely distributed proteins bind to the active RAS and augment its GTPase activity by more than 1000-fold, leading to rapid hydrolysis of GTP to GDP and termination of signal transduction. Thus, GAPs function as "brakes" that prevent uncontrolled RAS activity. The response to this braking action of GAPs seems to falter when mutations affect the RAS gene. Mutant RAS proteins bind GAP, but their GTPase activity fails to be augmented. Hence the mutant proteins are "trapped" in their excited GTP-bound form, causing, in turn, a pathologic activation of the mitogenic signaling pathway. The importance of GTPase activation in normal growth control is underscored by the fact that a disabling mutation of neurofibromin (NF-1), a GTPase-activating protein, is also associated with neoplasia (see discussion of tumor suppressor genes below).

In addition to RAS, other members of the RAS signaling cascade (RAS/RAF/MAP kinase) may also be altered in cancer cells. Thus, mutations in BRAF, one of the members of the RAF family, have been detected in more than 60% of melanomas and in more than 80% of benign nevi.[59] [60] This suggests that dysregulation of the RAS/RAF/MAP kinase pathway may be one of the initiating events in the development of melanomas, although it is not sufficient by itself to cause tumorigenesis.

Recent studies have revealed that, in addition to its role in transducing growth factor signals, RAS is also involved in regulation of the cell cycle. As described above, the passage of cells from G1 to the S phase is modulated by cyclins and CDKs. RAS proteins can indirectly regulate the levels of cyclins by activating the MAP kinase pathway and the AP-1 transcription factor.

Figure 7-32Model for action of RAS genes. When a normal cell is stimulated through a growth factor receptor, inactive (GDP-bound) RAS is activated to a GTP-bound state. Activated RAS recruits RAF and stimulates the MAP-kinase pathway to transmit growth-promoting signals to the nucleus. The mutant RAS protein is permanently activated because of inability to hydrolyze GTP, leading to continuous stimulation of cells without any external trigger. The anchoring of RAS to the cell membrane by the farnesyl moiety is essential for its action.

Figure 7-33The chromosomal translocation and associated oncogenes in Burkitt lymphoma and chronic myelogenous leukemia.

Figure 7-34Amplification of the N-MYC gene in human neuroblastomas. The N-MYC gene, normally present on chromosome 2p, becomes amplified and is seen either as extra chromosomal double minutes or as a chromosomally integrated, homogeneous staining region. The integration involves other autosomes, such as 4, 9, or 13. (Modified from Brodeur GM: Molecular correlates of cytogenetic abnormalities in human cancer cells: implications for oncogene activation. In Brown EB (ed): Progress in Hematology, Vol 14. Orlando, FL, Grune & Stratton, 1986, pp. 229–256.)

TABLE 7-9-- Selected Tumor Suppressor Genes Involved in Human Neoplasms

Date: 2016-04-22; view: 638