Inactivation of TOR causes an inhibition of cellular growth, a reduction in cell size, and a suppression of cell cycle progression. In addition to well described changes in protein synthesis and ribosome biogenesis, recent studies have suggested that other cell processes are likely to contribute to these growth effects of TOR. The present study identifies endocytosis as one such process. Our results demonstrate that the clathrin-uncoating ATPase Hsc70-4 interacts genetically with TOR and Tsc1, and that bulk endocytosis is stimulated in cells with activated TOR signaling. Conversely, we find that TOR activity inhibits the endocytic degradation of nutrient transporters such as Slimfast. Together, these endocytic effects of TOR promote both the bulk and targeted uptake of nutrients and other biomolecules required for cell mass increase (). In addition to this direct role in cellular biosynthesis and growth, nutrients also act as potent regulators of TOR signaling. Indeed, Slimfast was previously identified as an upstream activator of TOR (Colombani et al., 2003
). Our findings that disruption of endocytosis effects cell size, rapamycin sensitivity, and TOR kinase activity are consistent with an additional role for endocytosis upstream of TOR.
Figure 7. Inverse regulation of bulk endocytosis and targeted endocytic degradation. (A) Under conditions favorable for growth, TOR promotes bulk endocytic uptake and inhibits the endocytic turnover of specific nutrient importers such as Slimfast. (B) In growth-inhibitory (more ...)
Mutations that disrupt endocytosis are likely to have both positive and negative effects on nutrient uptake and cell growth because they inhibit bulk endocytic uptake, as well as degradation of nutrient transporters and other signaling molecules. Thus, the overall effects of endocytic disruption on nutrient uptake, cell growth, and TOR signaling are difficult to predict a priori. Our results suggest that both the cellular context and the specific step at which endocytosis is blocked influence the growth response. Thus, in fat body cells, expression of ShiK44A resulted in an increase in cell size, whereas loss of Hsc70-4 function caused reduced cell size. We note that these changes mirror the effects of these mutations on Slimfast levels; whereas both ShiK44A expression and Hsc70-4 mutation decreased bulk endocytic uptake, only ShiK44A resulted in increased levels of Slimfast. In contrast, both ShiK44A and Hsc70-4 mutants led to the increased size of wing imaginal disc cells, suggesting that in these cells the growth-inhibitory effects of endocytic degradation of membrane proteins such as Slimfast predominate over the potential positive effects of increased bulk uptake. Similarly, our results indicate a complex effect of endocytosis on TOR signaling. Partial reduction in Hsc70-4 levels lead to an increase in TOR signaling, as was evident in the eyTOR interaction and rapamycin resistance. In contrast, larvae that are homozygous mutant for Hsc70-4 show a decrease in TOR kinase activity. These results suggest that modest inhibition of endocytosis may increase TOR signaling, whereas a complete block of endocytosis may reduce it.
A striking parallel to the inverse regulation of bulk and targeted endocytic processes by TOR can be observed in its effects on autophagy in yeast. Through autophagy, random portions of cytoplasm are nonselectively engulfed within double membrane–bound vesicles for delivery to the lysosome. Activation of TOR causes this nonselective form of autophagy to be suppressed, and, instead, the autophagic machinery engages in a selective type of autophagy known as the cytoplasm–vacuole targeting (CVT) pathway, which is responsible for lysosomal delivery of specific hydrolases (Klionsky and Emr, 2000
). Thus, TOR acts as a switch between selective and nonselective autophagy. TOR may also be involved in switching between clathrin-and caveolae/raft-mediated endocytosis in higher eukaryotes. A genome-wide survey of protein kinases found that RNAi-mediated inactivation of TOR in HeLa cells inhibited clathrin-dependent processes such as transferrin uptake and vesicular stomatitis virus infection, and stimulated cavelolae/raft-dependent events (Pelkmans et al., 2005
). Together, these findings suggest that TOR may control the specificity of membrane trafficking components. In addition, our results show that S6K, which is an important TOR substrate, acts downstream of TOR in promoting bulk endocytosis, but is not involved in the suppression of starvation-induced autophagy.
The identification of endocytosis as a TOR-controlled function adds to the growing list of cell processes regulated by TOR, including protein synthesis, ribosome biogenesis, autophagy, metabolic gene expression, and cytoskeletal organization. How these distinct functions interact to achieve a coordinated growth response is only beginning to be understood. One likely mechanism involves the common use of molecular components and cellular substrates by different cell functions, as in the case of selective and nonselective autophagy, bulk endocytosis, and endocytic degradation. Two or more distinct branches of TOR signaling may also act cooperatively to control the same target, as in the case of Slimfast regulation by both translation and endocytosis, or may act in opposition, as previously observed for the role of S6K in limiting autophagy. Finally, distinct TOR complexes may converge on the same targets with opposing effects, as in the regulation of Akt by TOR-raptor versus TOR–rictor complexes (Shah et al., 2004
; Sarbassov et al., 2005b
). The finding that TOR signaling regulates the levels of Slimfast, which was previously shown to function upstream of TOR, adds another layer of complexity to the TOR signaling network.