Sieving through the cancer secretome

Abstract

Cancer is among the most prevalent and serious health problems worldwide. Therefore, there is an urgent need for novel cancer biomarkers with high sensitivity and specificity for early detection and management of the disease. The cancer secretome, encompassing all the proteins that are secreted by cancer cells, is a promising source of biomarkers as the secreted proteins are most likely to enter the blood circulation. Moreover, since secreted proteins are responsible for signaling and communication with the tumor microenvironment, studying the cancer secretome would further the understanding of cancer biology. Latest developments in proteomics technologies have significantly advanced the study of the cancer secretome. In this review, we will present an overview of the secretome sample preparation process and summarize the data from recent secretome studies of six common cancers with high mortality (breast, colorectal, gastric, liver, lung and prostate cancers). In particular, we will focus on the various platforms that were employed and discuss the clinical applicability of the key findings in these studies. This article is part of a Special Issue entitled: An Updated Secretome.

Introduction

With the advances in health care, cancer has emerged as among the top diseases in many parts of the world, with global estimates of 12.7 million new cases and 7.6 million cancer deaths each year [1], [2]. Cancers of the breast, colorectum, and lung are some of the most frequently diagnosed cancers that occur anywhere regardless of human development. Unfortunately, most of the clinically useful cancer biomarkers available currently, such as carcinoembryonic antigen (CEA) and alpha-fetoprotein (AFP), are still lacking in specificity and sensitivity [3]. There is thus an urgent need for more novel and useful biomarkers, not only for early diagnosis and prognosis but as therapeutic targets as well.

Serum- or plasma-based assays remain the most popular choice for clinical screening and diagnosis. The blood circulation extends throughout the entire human body and is thus a repository of signals released from any tissue within the body. Furthermore, obtaining blood samples is simple and non-invasive. Ironically, biomarker discovery using serum or plasma samples is anything but simple. These studies are often confounded by the broad dynamic range of serum and plasma protein concentrations. The presence of high abundance proteins such as albumin, haptoglobin, transferrins and immunoglobulins hinders the detection of tumor-specific biomarkers, which are usually at very low concentration in the ranges of nanograms per milliliter [4]. As a result, biomarker discovery studies have been slowly shifting towards proteomics analyses of other proximal biological fluids and in particular, the “secretome”: proteins secreted from cancer tissue specimens and cell lines.

The term “secretome” was originally used by Tjalsma and colleagues to describe the total proteins that are released by a cell, tissue or organism [5]. These secreted proteins constitute about 10–15% of the proteins encoded in the human genome and are involved in important physiological processes such as immune defense, blood coagulation, matrix remodeling and cell signaling [6]. Proteins can be released into the extracellular space through two mechanisms: the classical secretory pathway and the non-classical secretory pathways. In the classical secretory pathway, proteins targeted for extracellular release are synthesized as protein precursors which usually contain signal peptides located at the N-terminus. These signal peptides direct the proteins to the rough endoplasmic reticulum (ER) and subsequently to the Golgi apparatus, following which the proteins would then be released into the extracellular environment in Golgi-derived secretory vesicles. Alternatively, proteins can be exported through ER/Golgi-independent mechanisms which are also known as the non-classical secretory pathways. In these non-classical secretory pathways, proteins may be exported by targeting endosomes recycling back to the plasma membrane, directly translocating across the plasma membrane, or through exosomal secretion [7]. Exosomes are intralumenal vesicles (ILVs) which are formed by inward budding from the limiting membrane of multivesicular bodies (MVBs). During the formation process, some cytosolic proteins may be incorporated into the invaginating membrane and become engulfed in the ILVs. As a result, when a MVB eventually fuses with the plasma membrane, the ILVs within are released extracellularly together with their cargo of cytosolic proteins [8].

The neoplasm is by no means a stand-alone entity. Tumor cells constantly interact with their extracellular environment to create favorable conditions for tumor progression, such as inducing angiogenesis or degrading the extracellular matrix to facilitate metastasis. These interactions are mediated by a variety of proteins secreted by the tumor cells, including growth factors, chemokines, cytokines, adhesion molecules, proteases and shed receptors. In the same way, surrounding stromal cells are also recruited by tumor cells to actively release proteins which further the progression of the neoplasia [6]. Thus, the cancer secretome can be described as constituting of proteins released from cancer-associated stromal cells, as well as all proteins that are secreted by cancer cells through classical or non-classical secretory pathways [6], or shed from the cell surface [9]. These secreted proteins present a promising source of serological biomarkers, because they are most likely to enter the blood circulation.