{"id":4038,"date":"2016-11-09T11:05:48","date_gmt":"2016-11-09T16:05:48","guid":{"rendered":"http:\/\/blogs.shu.edu\/cancer\/?p=4038"},"modified":"2021-07-02T08:51:44","modified_gmt":"2021-07-02T12:51:44","slug":"how-pd-1-abrogates-the-anti-tumor-immune-response","status":"publish","type":"post","link":"http:\/\/blogs.shu.edu\/cancer\/2016\/11\/09\/how-pd-1-abrogates-the-anti-tumor-immune-response\/","title":{"rendered":"How PD-1 abrogates the anti-tumor immune response"},"content":{"rendered":"

PD-1 inhibition (Figure 1) has quickly become a front-line therapy for non-small cell lung cancer<\/a> and melanoma<\/a>. Moreover, PD-1 and PD-L1 inhibitors are being tested in combination with other checkpoint inhibitors, targeted therapies, cancer vaccines, monoclonal antibodies, and other modalities. But, how does PD-1 blunt the anti-tumor immune response?<\/p>\n

\"Figure<\/a>

Figure 1. Cells have a protein on their surface called PD-1 (in orange above). When PD-1 binds to PD-L1 (yellow) on another cell, the T cell becomes deactivated. Most cancer cells have PD-L1 on their surface and escape being killed by turning off the T cell in this way. Anti-PD-1 antibodies (dark green) or anti-PD-L1 antibodies (light green) can prevent the tumor cell from binding PD-1 and thus allow T cells to remain active. http:\/\/www.curetoday.com\/articles\/fda-approves-frontline-opdivo-for-braf-mutant-melanoma<\/a><\/p><\/div>\n

Researchers at Beth Israel Deaconess Medical Center, Harvard Medical School examined the molecular pathways<\/a> through which PD-1 acts:<\/p>\n

PD-1 blocks cell cycle progression in the G1<\/sub>\u00a0phase. PD-1 suppressed the transcription of\u00a0SKP2<\/em><\/strong>, the substrate recognition component of the SCFSkp2<\/sup>\u00a0ubiquitin ligase that leads p27kip1<\/sup>\u00a0to degradation and resulted in accumulation of p27kip1<\/sup><\/strong> (Figure 2). Subsequently, T cells receiving PD-1 signals displayed impaired Cdk2 activation and failed to phosphorylate two critical Cdk2 substrates, the retinoblastoma gene product (Rb) and the TGF\u03b2-specific transcription factor Smad3<\/strong>, leading to suppression of E2F target genes but enhanced Smad3 transactivation (Figure 3). These events resulted in upregulation of the Cdk4\/6 inhibitor p15INK4B<\/sup><\/strong>\u00a0and repression of the Cdk-activating phosphatase Cdc25A. The suppressive effect of PD-1 on Skp2 expression was mediated by inhibition of both PI3K\/Akt and Ras\/MEK\/Erk pathways and was only partially reversed by IL-2, which restored activation of MEK\/Erk but not Akt. Thus, PD-1 targets Ras and PI3K\/Akt signaling to inhibit transcription of Skp2 and to activate Smad3 as an integral component of a pathway that regulates blockade of cell cycle progression in T lymphocytes.<\/p>\n

What are the effects of p27kip1<\/sup> and p15INK4<\/sup>?<\/em><\/strong><\/p>\n

p27 is a cyclin dependent kinase inhibitor that blocks the activity of Cyclin E-CDK2, which \u00a0phosphorylates pRb, thereby ushering the cell from G1 into S phase through the Restriction point (Figure 2). It also blocks Cyclin A-CDK2 from further phosphorylating pRb to maintain S phase. Accumulation of p27 in the nucleus, therefore, blocks cell cycle progression of T-lymphocytes that are being induced to act against cancer antigens. PD-1 expression by cancer cells blocks the proliferation of T-cells.<\/p>\n

P15INK4<\/sup> is a cyclin dependent kinase inhibitor that blocks the activity of Cyclind-CDK4,6, inhibiting it from hypophosphorylating Rb, thereby, rendering the cell cycle unresponsive to external proliferation signals.<\/p>\n

\"Figure<\/a>

Figure 2. Orderly progression through the cell cycle involves passage through sequential checkpoints. Full holoenzyme activity of the cyclin D1-Cdk4 complex is induced by mitogen recruitment of CAK. The cyclin D1-Cdk4 complex phosphorylates the pRB protein leading to sequential phosphorylation by cyclin E-Cdk2 and release of free E2F. The phosphorylation of pRB, and relief of transcriptional repression by pRB induces genes involved in the induction of S-phase entry. https:\/\/www.bioscience.org\/2000\/v5\/af\/A562\/fulltext.htm<\/a><\/p><\/div>\n

What does Smad3 do?<\/em><\/strong><\/p>\n

The SMAD proteins are a family of transcription factors<\/a> consisting of 8 members, SMAD1-8, which are further subdivided into 3 classes based on structural and functional properties.\u00a0Receptor-regulated SMADs (R-SMADs), SMAD1, 2, 3, 5, and 8, are the only SMADs directly phosphorylated and activated by the kinase domain of type I receptors. Upon phosphorylation, R-SMADs form a complex with the common SMAD, SMAD4, resulting in nuclear accumulation of activated complexes. In the nucleus, R-SMAD\u2013SMAD4 complexes cooperate with transcriptional coregulators that further define target gene recognition and transcriptional regulation.\u00a0The inhibitory SMADs, SMAD6 and SMAD7, constitute the third class, which function to inhibit TGF-\u03b2 signaling (Figure 3).<\/p>\n

\"Figure<\/a>

Figure 3. TGF-\u03b2 ligands bind type I and type II receptors at the cell surface. Subsequently, the type I receptor (ALK5) becomes phosphorylated by the type II receptor. This leads to phosphorylation of SMAD2 and SMAD3, which form a complex with SMAD4. Activated complexes accumulate in the nucleus where they cooperate with DNA-binding cofactors to regulate target gene transcription. SMAD2 and SMAD3 also bind to TIF1\u03b3. In embryonic stem cells, SMAD2\/3-TIF1\u03b3 recognizes specific chromatin marks, promoting access of SMAD2\/3-SMAD4 to otherwise repressed targets. TIF1-\u03b3\u2013SMAD2\/3 promotes erythroid differentiation whereas SMAD4-SMAD2\/SMAD3 complexes inhibit proliferation. In certain cell types, JNK and p38 are phosphorylated by TAK1 and constitute, together with the PI3K-AKT-FOXO axis, ERK, and PAR6, so-called noncanonical signaling responses to TGF-\u03b2. The dashed line indicates unclear molecular mechanism. http:\/\/www.bloodjournal.org\/content\/125\/23\/3542?sso-checked=true<\/a><\/p><\/div>\n

Since T-cells deficient in SMAD3<\/a> are resistant to growth inhibition by TGF-b (Figure 4). It follows that transactivation of SMAD3 induced by PD-1 blocks proliferation of T-cells, thereby abrogating the anti-cancer immune response.<\/p>\n

\"Figure<\/a>

Figure 4. Smad3\u2212\/\u2212 T cells are resistant to growth inhibition by TGF-\u03b21. Whole splenocytes (A) or thymocytes (B) from WT or Smad3\u2212\/\u2212 mice were stimulated with anti-CD3 plus anti-CD28 in the presence or absence of TGF-\u03b21 for 72 h, and [3H]TdR was added during the last 18 h of culture. The data are expressed as the mean [3H]TdR incorporation \u00b1 SE of three separate experiments for quadruplicate cultures. \u00b6, Different from vehicle (0 point) at p < 0.05. http:\/\/www.jimmunol.org\/content\/172\/7\/4275.full<\/a><\/p><\/div>What is SKP2 and how is it related to PI3K\/Akt and Ras\/MEK\/Erk?<\/em><\/strong><\/p>\n

SKP2 encodes a member of the F-box protein family which is characterized by an approximately 40 amino acid motif, the F-box. The F-box proteins constitute one of the four subunits of ubiquitin protein ligase complex called SCFs (SKP1-cullin-F-box)<\/a>, which function in phosphorylation-dependent ubiquitination. This protein is an essential element of the cyclin A-CDK2 S-phase kinase. It specifically recognizes phosphorylated cyclin-dependent kinase inhibitor 1B (CDKN1B, also referred to as p27 or KIP1) predominantly in S phase and interacts with S-phase kinase-associated protein 1 (SKP1 or p19). In addition, this gene is established as a protooncogene causally involved in the pathogenesis of lymphomas.<\/p>\n

PD-1 blocks the transcription of SKP2 leading to higher levels of p27; this blocks Cyclin E-CDK2 complexes from ushering-in the Restriction points, thereby arresting the cell cycle.<\/p>\n

PD-1 does so by blocking PI3K\/Akt and Ras\/MEK\/Erk, pathways that induce SKP2 (Figure 5). Akt controls Skp2 stability and the subcellular localization of Skp2<\/a>. First of all, the positive correlation between Skp2 expression and Akt activity was found in a panel of breast cancer cell lines. Moreover, inhibition of Akt1 activity in breast cancer cells caused down-regulation of Skp2 expression, indicating that elevated Akt activity could be one major cause of the observed up-regulation of Skp2 in breast cancer. Cooperation between ERK and Skp2<\/a> has also been found in human breast cancer whereby synergistic activity of the two oncogenes has been shown to increase p27 degradation.<\/p>\n

\"Figure<\/a>

Figure 5. Diagram of Skp2\u2019s cross-talks with other major signaling pathways in breast cancer. PI3K\/Akt, mTOR, PPAR\u03b3, ERK, FoxP3, and IGF regulate the expression of Skp2 in the breast cancer. Chemical compounds including Skp2 inhibitors and natural agents inhibit cell growth and induce apoptosis through down-regulation of Skp2 expression in breast cancer. Skp2, S-phase kinase associated protein 2; ERK, extracellular signal-regulated kinase; IGF-1, insulin-like growth factor-1; FoxP3, forkhead box P3; mTOR, mammalian target of rapamycin; PI3K, phosphatidylinositol 3-kinase; PPAR\u03b3, peroxisome proliferator-activated receptor-\u03b3. http:\/\/journal.frontiersin.org\/article\/10.3389\/fonc.2011.00057\/full<\/a><\/p><\/div>\n","protected":false},"excerpt":{"rendered":"

PD-1 inhibition (Figure 1) has quickly become a front-line therapy for non-small cell lung cancer and melanoma. Moreover, PD-1 and PD-L1 inhibitors are being tested in combination with other checkpoint inhibitors, targeted therapies, cancer vaccines, monoclonal antibodies, and other modalities. But, how does PD-1 blunt the anti-tumor immune response?<\/p>\n","protected":false},"author":2252,"featured_media":3979,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[711,28,6,1],"tags":[278,284,893,89,749,1803,1801,41,40,218,1805,251,1804,1802,1577],"class_list":["post-4038","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-active-immunotherapy","category-checkpoint-inhibitors","category-immunology-immunotherapy","category-uncategorized","tag-aktpkb","tag-erk","tag-keytruda","tag-nivolumab","tag-opdivo","tag-p15ink4","tag-p27kip1","tag-pd-1","tag-pd-l1","tag-pembrolizumab","tag-pi3kakt","tag-ras","tag-skp2","tag-smad3","tag-tgf-"],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/posts\/4038","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/users\/2252"}],"replies":[{"embeddable":true,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/comments?post=4038"}],"version-history":[{"count":4,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/posts\/4038\/revisions"}],"predecessor-version":[{"id":4889,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/posts\/4038\/revisions\/4889"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/media\/3979"}],"wp:attachment":[{"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/media?parent=4038"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/categories?post=4038"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/blogs.shu.edu\/cancer\/wp-json\/wp\/v2\/tags?post=4038"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}