Endocytosis and cancer

Abstract

Endocytosis entails selective packaging of cell-surface proteins, such as receptors for cytokines and adhesion components, in cytoplasmic vesicles (endosomes). The series of sorting events that determines the fate of internalized proteins, either degradation in lysosomes or recycling back to the plasma membrane, relies on intrinsic sequence motifs, posttranslational modifications (e.g., phosphorylation and ubiquitination), and transient assemblies of both Rab GTPases and phosphoinositide-binding proteins. This multicomponent process is enhanced and skewed in cancer cells; we review mechanisms enabling both major drivers of cancer, p53 and Ras, to bias recycling of integrins and receptor tyrosine kinases (RTKs). Likewise, cadherins and other junctional proteins of cancer cells are constantly removed from the cell surface, thereby disrupting tissue polarity and instigating motile phenotypes. Mutant forms of RTKs able to evade Cbl-mediated ubiquitination, along with overexpression of the wild-type forms and a variety of defective feedback regulatory loops, are frequently detected in tumors. Finally, we describe pharmacological attempts to harness the peculiar endocytic system of cancer, in favor of effective patient treatment.

Figure 1.

Cell-surface structures regulated in tumors by vesicular trafficking. A schematic view of an epithelial cell, which is in close contact with both a highly polarized neighboring cell, via adherens junctions, and the underlying extracellular matrix (via focal adhesions). Several actin-filled projections of the plasma membrane are presented (e.g., a lamellipodium and several filopodia). In addition, ventrally located invadopodia are shown as actin-filled fingers that perforate the underlying extracellular matrix. Turnover of all presented surface structures is regulated by vesicular trafficking, which is outlined as a route starting at the clathrin-coated pit (CCP), leading to EEs, late endosomes, or multivesicular bodies (MVBs), and eventually reaches lysosomes. Both adhesion molecules, such as integrins and signaling receptors (e.g., EGFR), are transported to lysosomes through this pathway, but an alternative route recycles receptors back to the cell surface. In the case of integrins, this latter route involves the perinuclear recycling compartment (PNRC). Note that Figures 2–5 highlight portions of the general view shown in this scheme.

Figure 2.

Involvement of mutant p53 in lamellipodium dynamics and cell migration. To safeguard sustained forward movement of the leading edge, integrin and RTKs constantly internalize and directionally recycle by means of vesicular trafficking. A critical player is the RCP, a Rab11 effector, that physically binds with both RTKs and integrins such as the fibronectin receptor α5β1. This enables cotrafficking of adhesion and signaling molecules to the forefront of the leading edge. Importantly, the endosomal protein RCP, like its partner, Rab25, is overexpressed in some tumors and indirectly down-regulated by p63. The latter transcription factor is a member of a tumor suppressor family that includes also p73 and the wild-type form of p53. Notably, oncogenic mutant forms of p53, such as R175H and R273H, transcriptionally repress p63. Thus, in cancer cells expressing p53 mutants, both RTKs and certain integrins evade degradation in lysosomes, thereby enhancing cell migration and downstream signaling, primarily to the Akt pathway.

Figure 3.

Protein complexes regulating endosomal sorting of RTKs. Following ligand-induced phosphorylation of RTKs, such as the EGF-receptor (EGFR), RTKs are internalized via clathrin-dependent and -independent pathways in a manner dependent on several accessory proteins, as listed. These pathways merge at the EE and feed multivesicular bodies (MVBs). Receptor sorting into internal vesicles of the MVB likely precedes delivery to lysosomes for degradation. Alternatively, receptors that remain confined to the limiting membrane of MVBs often recycle back to the plasma membrane, thereby engaging in repeated cycles of activation. Rab proteins, phosphatidylinositol modifying enzymes, and posttranslational modifications of both RTKs and endocytic adaptors, for example, phosphorylation (denoted as P), ubiquitination (Ub), and neddylation (conjugation of Nedd8), have critical roles in endosomal sorting. Specific players of each trafficking branch are indicated. Note that aberrant expression of some players, in a way that skews recycling, has been associated with human cancer (see list in Table 1).

Figure 4.

Endosomal sorting regulates integrin-based focal adhesions. Focal adhesions containing active conformers of integrin β1 establish strong, yet dynamic, contacts between the ventral aspect of migrating cells and the underlying extracellular matrix. Both actin filaments and microtubules projecting from the microtubule-organizing center (MTOC) regulate cell adhesion. The latter polymers dissolve focal adhesions once they approach the plasma membrane. This requires protein kinases, such as FAK, and recruitment of both dynamin 2 and disabled 2 (Dab2), a clathrin adaptor, which directly binds with the cytoplasmic tails of integrin β-subunits. Once internalized by activated dynamin, the formed clathrin-coated vesicles mature to integrin-loaded EEs. These organelles are transported backward along microtubules to reach the PNRC and the recycling pathway, or these EEs deliver their cargo to lysosomes for degradation. These alternative itineraries are regulated by specific Rab proteins. Note that the flux of integrins through the endocytic pathway, rather than their surface levels, dictates migration speed. Accordingly, the levels of both Dab2 and dynamin display broad variation in several types of carcinomas (see Table 1).

Figure 5.

Endocytosis-mediated control of cell-to-cell junctions. Intact tight junctions and adherens junctions are essential for the integrity and polarity of epithelial sheets; their turnover is regulated by means of vesicular trafficking. Shown are two major components of tight junctions, occludin and claudin, and the epithelial cadherin, E-cadherin, the major component of adherens junctions. E-cadherin maintains calcium-dependent adhesion by means of homophilic extracellular interactions as well as association of the cytoplasmic tails with the actin cytoskeleton. This involves several types of catenins, including p120-catenin, an inhibitor of E-cadherin endocytosis. Tyrosine kinases such as Src have major roles in the disruption of adherens junctions; one mechanism involves inactivation of p120-catenin. Alternatively, tyrosine phosphorylation (denoted as P) of E-cadherin enables recognition by a Cbl-like ubiquitin ligase called Hakai that ubiquitinates (denoted as Ub) and sorts E-cadherin molecules to lysosomal degradation. Low expression of E-cadherin, attributable to genetic and other reasons, characterizes a broad spectrum of advanced tumors.