Cancer Stem Cells-Origins and Biomarkers: Perspectives for Targeted Personalized Therapies

Abstract

The use of biomarkers in diagnosis, therapy and prognosis has gained increasing interest over the last decades. In particular, the analysis of biomarkers in cancer patients within the pre- and post-therapeutic period is required to identify several types of cells, which carry a risk for a disease progression and subsequent post-therapeutic relapse. Cancer stem cells (CSCs) are a subpopulation of tumor cells that can drive tumor initiation and can cause relapses. At the time point of tumor initiation, CSCs originate from either differentiated cells or adult tissue resident stem cells. Due to their importance, several biomarkers that characterize CSCs have been identified and correlated to diagnosis, therapy and prognosis. However, CSCs have been shown to display a high plasticity, which changes their phenotypic and functional appearance. Such changes are induced by chemo- and radiotherapeutics as well as senescent tumor cells, which cause alterations in the tumor microenvironment. Induction of senescence causes tumor shrinkage by modulating an anti-tumorigenic environment in which tumor cells undergo growth arrest and immune cells are attracted. Besides these positive effects after therapy, senescence can also have negative effects displayed post-therapeutically. These unfavorable effects can directly promote cancer stemness by increasing CSC plasticity phenotypes, by activating stemness pathways in non-CSCs, as well as by promoting senescence escape and subsequent activation of stemness pathways. At the end, all these effects can lead to tumor relapse and metastasis. This review provides an overview of the most frequently used CSC markers and their implementation as biomarkers by focussing on deadliest solid (lung, stomach, liver, breast and colorectal cancers) and hematological (acute myeloid leukemia, chronic myeloid leukemia) cancers. Furthermore, it gives examples on how the CSC markers might be influenced by therapeutics, such as chemo- and radiotherapy, and the tumor microenvironment. It points out, that it is crucial to identify and monitor residual CSCs, senescent tumor cells, and the pro-tumorigenic senescence-associated secretory phenotype in a therapy follow-up using specific biomarkers. As a future perspective, a targeted immune-mediated strategy using chimeric antigen receptor based approaches for the removal of remaining chemotherapy-resistant cells as well as CSCs in a personalized therapeutic approach are discussed.

Keywords: CAR cells; biomarkers; cancer stem cells; precision therapy; senescence; targeted therapy.

Figure 1

The origin of CSCs at tumor initiation: The two hypotheses of CSC generation. (A) The proliferation and differentiation of adult tissue resident stem cells is part of the physiological regeneration program that maintains tissue homeostasis. Adult tissue resident stem cells divide asymmetrically and generate transient amplifying cells, which possess a high proliferative capacity. These cells terminally differentiate; a process during which they will lose their proliferative capacity to finally support organ homeostasis. (B) Tumors can be generated by step-wise accumulation of several mutations (red lightening) that transform differentiated cells and cause a de-differentiation. Tissue resident stem cells as well as their progeny can accumulate mutations that lead to uncontrolled and niche independent growth. Heterogeneous tumors are generated. CSCs share phenotypic characteristics and several markers have been described in solid as well as in liquid cancers.

Figure 2

Kinetic of tumor development in pre-, early-, and late-therapeutic period upon application of chemo- and/or radiation therapy: current situation in the clinic. (A) In the pre-therapeutic situation, heterogeneous tumors are composed of several cell types, including CSC, tumor cells, TAMs, and CAFs; all characterized by biomarkers. (B) In the early post-therapeutic period, upon application of the first-line treatment (that currently uses mostly chemo- or radiotherapeutic regimens) several important changes occur in the tumor, in particular: tumor cells or CAFs die due to the therapy or become senescent, whereas CSCs mostly survive the treatment. Senescent cells (tumor cells and CAFs) attract immune cells toward the senescent site via SASP. The SASP therefore plays a positive role and attracts immune cells in this early post-therapeutic situation. Attracted immune cells promote the clearance of dead, of necrotic, and senescent tumor cells and CAFs. (C) In the late post-therapeutic situation uncleared senescent tumor cells and senescent CAFs and SASP thereof play a negative (pro-tumorigenic) role and support tumor development. SASP molecules provide stimulating factors for CSCs for further uncontrolled proliferation as well as their maintenance. Also, remaining senescent tumor cells acquire additional mutations that promote activation of a stemness phenotype and finally a tumor relapse.

Figure 3

Targeted personalized second-line therapy as a future perspective. (A) Analysis of post-therapeutic biopsy samples: follow-up studies need to be included into regular clinical post-therapeutic relapse analysis. After therapy, local biopsies of remaining tumor tissue and/or satellite tissue should be taken periodically (even after several years post-therapy) and a multivariant analysis for biomarkers has to be performed, including the analysis of CSC biomarkers, pro-inflammatory cytokines, senescent markers as well as markers for CAFs. (B) Targeted second-line therapy needs to be performed based on the analysis described in (A) and will include a specific targeted eradication of remaining cells that could promote tumor relapse and metastasis. Targeted therapies comprise CAR-based approaches targeting CSCs as well as senescent cells or CAFs and TAMs. They also include senolytic drugs to deplete senescent cells independent of CAR approaches.

Figure 4

Targeted personalized first-line therapy as a future perspective. (A) Pre-therapeutic period: local biopsies before the therapy would allow to determine the heterogenic composition of the tumor, consisting of several biomarkers to be analyzed (CSC, CAFs, and TAMs biomarkers, tumor cell antigens, as well as e.g., T-cell compositions). (B) First-line targeted personalized therapeutic approach—therapeutic regimens could combine several approaches: the chemotherapy and small molecules (both selected based on tumor genotype), combined with immunotherapies (antibodies and checkpoint inhibitors based on tumor and analysis of T-cell phenotype), as well as CAR cell-based therapies targeting CSCs, CAFs, and TAMs. Combination therapy will allow a precise and efficient targeting of the heterogenic tumor composition from the beginning on.