Four Human Ras Homologs Differ in Their Abilities to Activate Raf-1, Induce Transformation, and Stimulate Cell Motility

Abstract

Human cells contain four homologous Ras proteins, but it is unknown whether each of these Ras proteins participates in distinct signal transduction cascades or has different biological functions. To directly address these issues, we assessed the relative ability of constitutively active (G12V) versions of each of the four Ras homologs to activate the effector protein Raf-1 in vivo. In addition, we compared their relative abilities to induce transformed foci, enable anchorage-independent growth, and stimulate cell migration. We found a distinct hierarchy between the four Ras homologs in each of the parameters studied. The hierarchies were as follows: for Raf-1 activation, Ki-Ras 4B > Ki-Ras 4A >>> N-Ras > Ha-Ras; for focus formation, Ha-Ras >/= Ki-Ras 4A >>> N-Ras = Ki-Ras 4B; for anchorage-independent growth, Ki-Ras 4A >/= N-Ras >>> Ki-Ras 4B = Ha-Ras = no growth; and for cell migration, Ki-Ras 4B >>> Ha-Ras > N-Ras = Ki-Ras 4A = no migration. Our results indicate that the four Ras homologs significantly differ in their abilities to activate Raf-1 and induce distinctly different biological responses. These studies, in conjunction with our previous report that demonstrated that the Ras homologs can be differentially activated by upstream guanine nucleotide exchange factors (Jones, M. K., and Jackson, J. H. (1998) J. Biol. Chem. 273, 1782-1787), indicate that each of the four Ras proteins may qualitatively or quantitatively participate in distinct signaling cascades and have significantly different biological roles in vivo. Importantly, these studies also suggest for the first time that the distinct and likely cooperative biological functions of the Ki-ras-encoded Ki-Ras 4A and Ki-Ras 4B proteins may help explain why constitutively activating mutations of Ki-ras, but not N-ras or Ha-ras, are frequently detected in human carcinomas.

Ras Antagonist Farnesylthiosalicylic Acid (FTS) Reduces Glomerular Cellular Proliferation and Macrophage Number in Rat Thy-1 Nephritis

“…Similar results have been obtained using indirect antagonism of Ras with HMG-CoA Reductase inhibitors and prenylation inhibitors, which reduce the C terminal prenylation required for Ras function. Although the specific function of each of the Ras isoforms is not known, different Ras isoforms have been shown to differ in their ability to activate certain effectors (21,22), and it may be that the different isoforms of Ras play distinct roles in control of proliferation acting via different downstream cascades.…”

K-Ras4A splice variant is widely expressed in cancer and uses a hybrid membrane-targeting motif

“…formation of transformed foci and enable anchorage-independent growth is significantly greater than that of oncogenic K-Ras4B expressed at the same level (13). A recent study suggested that the presence of oncogenic K-Ras4A was necessary to initiate tumor formation and growth in a mouse model of induced lung carcinogenesis through KRAS mutation (14).…”

Farnesyltransferase and geranylgeranyltransferase I inhibitors and cancer therapy: Lessons from mechanism and bench-to-bedside translational studies

“…There are important di erences in the function of the di erent Ras proteins. H-Ras has been shown to activate preferentially the PI3-kinase/Akt survival pathway, whereas K-Ras prefers to activate the Raf/ MEK/Erk transformation pathway (Voice et al, 1999;Yan et al, 1998). Therefore, in human cancers that harbor wild type H-Ras, but that are dependent on the H-Ras/PI3-kinase/Akt pathway for survival, FTIs may be very e ective at inducing apoptosis.…”