Harini Krishnan

Plant Lectin Can Target Receptors Containing Sialic Acid, Exemplified by Podoplanin, to Inhibit Transformed Cell Growth and Migration

Cancer is a leading cause of death of men and women worldwide. Tumor cell motility contributes to metastatic invasion that causes the vast majority of cancer deaths. Extracellular receptors modified by α2,3-sialic acids that promote this motility can serve as ideal chemotherapeutic targets. For example, the extracellular domain of the mucin receptor podoplanin (PDPN) is highly O-glycosylated with α2,3-sialic acid linked to galactose. PDPN is activated by endogenous ligands to induce tumor cell motility and metastasis. Dietary lectins that target proteins containing α2,3-sialic acid inhibit tumor cell growth. However, anti-cancer lectins that have been examined thus far target receptors that have not been identified. We report here that a lectin from the seeds of Maackia amurensis (MASL) with affinity for O-linked carbohydrate chains containing sialic acid targets PDPN to inhibit transformed cell growth and motility at nanomolar concentrations. Interestingly, the biological activity of this lectin survives gastrointestinal proteolysis and enters the cardiovascular system to inhibit melanoma cell growth, migration, and tumorigenesis. These studies demonstrate how lectins may be used to help develop dietary agents that target specific receptors to combat malignant cell growth.

Serines in the Intracellular Tail of Podoplanin (PDPN) Regulate Cell Motility

Background: PDPN is a transmembrane receptor that promotes cell migration, but modifications that regulate its effects are not known. Results: PKA can phosphorylate PDPN, nonphosphorylatable PDPN promotes cell migration, and phosphomimetic PDPN fails to promote cell migration. Conclusion: PKA can phosphorylate PDPN to decrease cell migration. Significance: PDPN effects on cell motility are important for processes including embryonic development and cancer progression. Podoplanin (PDPN) is a transmembrane receptor that affects the activities of Rho, ezrin, and other proteins to promote tumor cell motility, invasion, and metastasis. PDPN is found in many types of cancer and may serve as a tumor biomarker and chemotherapeutic target. The intracellular region of PDPN contains only two serines, and these are conserved in mammals including mice and humans. We generated cells from the embryos of homozygous null Pdpn knock-out mice to investigate the relevance of these serines to cell growth and migration on a clear (PDPN-free) background. We report here that one or both of these serines can be phosphorylated by PKA (protein kinase A). We also report that conversion of these serines to nonphosphorylatable alanine residues enhances cell migration, whereas their conversion to phosphomimetic aspartate residues decreases cell migration. These results indicate that PKA can phosphorylate PDPN to decrease cell migration. In addition, we report that PDPN expression in fibroblasts causes them to facilitate the motility and viability of neighboring melanoma cells in coculture. These findings shed new light on how PDPN promotes cell motility, its role in tumorigenesis, and its utility as a functionally relevant biomarker and chemotherapeutic target.

Mechanisms of environmental chemicals that enable the cancer hallmark of evasion of growth suppression

As part of the Halifax Project, this review brings attention to the potential effects of environmental chemicals on important molecular and cellular regulators of the cancer hallmark of evading growth suppression. Specifically, we review the mechanisms by which cancer cells escape the growth-inhibitory signals of p53, retinoblastoma protein, transforming growth factor-beta, gap junctions and contact inhibition. We discuss the effects of selected environmental chemicals on these mechanisms of growth inhibition and cross-reference the effects of these chemicals in other classical cancer hallmarks.

Podoplanin: a marker for reactive gliosis in gliomas and brain injury.

Abstract Reactive astrogliosis is associated with many pathologic processes in the central nervous system, including gliomas. The glycoprotein podoplanin (PDPN) is upregulated in malignant gliomas. Using a syngeneic intracranial glioma mouse model, we show that PDPN is highly expressed in a subset of glial fibrillary acidic protein–positive astrocytes within and adjacent to gliomas. The expression of PDPN in tumor-associated reactive astrocytes was confirmed by its colocalization with the astrocytic marker S100&bgr; and with connexin43, a major astrocytic gap junction protein. To determine whether the increase in PDPN is a general feature of gliosis, we used 2 mouse models in which astrogliosis was induced either by a needle injury or ischemia and observed similar upregulation of PDPN in reactive astrocytes in both models. Astrocytic PDPN was also found to be coexpressed with nestin, an intermediate filament marker for neural stem/progenitor cells. Our findings confirm that expression of PDPN is part of the normal host response to brain injury and gliomas, and suggest that it may be a novel cell surface marker for a specific population of reactive astrocytes in the vicinity of gliomas and nonneoplastic brain lesions. The findings also highlight the heterogeneity of glial fibrillary acidic protein–positive astrocytes in reactive gliosis.

Src Points the Way to Biomarkers and Chemotherapeutic Targets

The role of Src in tumorigenesis has been extensively studied since the work of Peyton Rous over a hundred years ago. Src is a non-receptor tyrosine kinase that plays key roles in signaling pathways controlling tumor cell growth and migration. Src regulates the activities of numerous molecules to induce cell transformation. However, transformed cells do not always migrate and realize their tumorigenic potential. They can be normalized by surrounding nontransformed cells by a process called contact normalization. Tumor cells need to override contact normalization to become malignant or metastatic. In this review, we discuss the role of Src in cell migration and contact normalization, with emphasis on Cas and Abl pathways. This paradigm illuminates several chemotherapeutic targets and may lead to the identification of new biomarkers and the development of effective anticancer treatments.

Contact Normalization or Escape from the Matrix

Cancer is a complex process that involves interactions between numerous cell types. In many cases, tumor cell expansion is prevented by other cells in the microenvironment. The growth and morphology of genetically transformed cells can be normalized by junctional communication with surrounding nontransformed cells. Tumor cells need to overcome this process, called “contact normalization”, before they can realize their malignant and metastatic potential. Here, we describe some fundamental aspects that underlie contact normalization, and how this information can be used to develop innovative ways to detect and treat many forms of cancer.

Podoplanin: An emerging cancer biomarker and therapeutic target

Podoplanin (PDPN) is a transmembrane receptor glycoprotein that is upregulated on transformed cells, cancer associated fibroblasts and inflammatory macrophages that contribute to cancer progression. In particular, PDPN increases tumor cell clonal capacity, epithelial mesenchymal transition, migration, invasion, metastasis and inflammation. Antibodies, CAR‐T cells, biologics and synthetic compounds that target PDPN can inhibit cancer progression and septic inflammation in preclinical models. This review describes recent advances in how PDPN may be used as a biomarker and therapeutic target for many types of cancer, including glioma, squamous cell carcinoma, mesothelioma and melanoma.

PKA and CDK5 can phosphorylate specific serines on the intracellular domain of podoplanin (PDPN) to inhibit cell motility.

Podoplanin (PDPN) is a transmembrane glycoprotein that promotes tumor cell migration, invasion, and cancer metastasis. In fact, PDPN expression is induced in many types of cancer. Thus, PDPN has emerged as a functionally relevant cancer biomarker and chemotherapeutic target. PDPN contains 2 intracellular serine residues that are conserved between species ranging from mouse to humans. Recent studies indicate that protein kinase A (PKA) can phosphorylate PDPN in order to inhibit cell migration. However, the number and identification of specific residues phosphorylated by PKA have not been defined. In addition, roles of other kinases that may phosphorylate PDPN to control cell migration have not been investigated. We report here that cyclin dependent kinase 5 (CDK5) can phosphorylate PDPN in addition to PKA. Moreover, results from this study indicate that PKA and CDK5 cooperate to phosphorylate PDPN on both intracellular serine residues to decrease cell motility. These results provide new insight into PDPN phosphorylation dynamics and the role of PDPN in cell motility. Understanding novel mechanisms of PDPN intracellular signaling could assist with designing novel targeted chemotherapeutic agents and procedures.

Chapter 6:Contact Normalization: Mechanisms and Pathways to Biomarkers and Chemotherapeutic Targets

Identifying cancer biomarkers and chemotherapeutic targets has become an essential task for the successful treatment of cancer. In this chapter we describe how elucidating mechanisms that underlie contact normalization – the ability of non-transformed cells to normalize the growth and morphology of neighboring tumor cells – can be used to help identify these targets and elucidate their roles in transformed cell growth and migration. Contact normalization is mediated by junctional communication between transformed and non-transformed cells. Here, we describe the potential roles of cadherens junctions and gap junctions in this process. In addition, we describe recent findings of novel tumor suppressors and tumor promoters, including Fhl1, Sdpr, Pdpn and specific miRNA species, that are affected by contact normalization. The study of contact normalization can guide us toward more specific ways to diagnose and treat cancer in an efficient manner, as well as understand molecular mechanisms behind its pathophysiology.

Src and podoplanin forge a path to destruction

Cancer and arthritis present an enormous challenge to society. They share pathogenic pathways that involve extracellular matrix degradation, tissue invasion, and inflammation. Most cancer and arthritis treatments affect normal cell function to cause significant adverse effects in patients. Specific pathways that promote cancer and arthritis progression must be elucidated to design more targeted and effective therapeutics. The Src kinase and podoplanin (PDPN) receptor are upregulated in cancer cells, fibroblasts, synoviocytes, and immune cells that increase tissue invasion and inflammation to promote both cancer and arthritis. In this review, we discuss how Src and PDPN forge a path to tissue destruction, and how they can serve as targets for therapeutics to combat cancer and arthritis.