Natural Compounds as Target Biomolecules in Cellular Adhesion and Migration: From Biomolecular Stimulation to Label-Free Discovery and Bioactivity-Based Isolation

Abstract

Plants and fungi can be used for medical applications because of their accumulation of special bioactive metabolites. These substances might be beneficial to human health, exerting also anti-inflammatory and anticancer (antiproliferative) effects. We propose that they are mediated by influencing cellular adhesion and migration via various signaling pathways and by directly inactivating key cell adhesion surface receptor sites. The evidence for this proposition is reviewed (by summarizing the natural metabolites and their effects influencing cellular adhesion and migration), along with the classical measuring techniques used to gain such evidence. We systematize existing knowledge concerning the mechanisms of how natural metabolites affect adhesion and movement, and their role in gene expression as well. We conclude by highlighting the possibilities to screen natural compounds faster and more easily by applying new label-free methods, which also enable a far greater degree of quantification than the conventional methods used hitherto. We have systematically classified recent studies regarding the effects of natural compounds on cellular adhesion and movement, characterizing the active substances according to their organismal origin (plants, animals or fungi). Finally, we also summarize the results of recent studies and experiments on SARS-CoV-2 treatments by natural extracts affecting mainly the adhesion and entry of the virus.

Keywords: CAM; SARS-CoV-2; biosensors; cell adhesion; integrin; isolation; movement; natural compound; preparation; viability.

Figure 1

Integrin family members and their ligands. Integrins are transmembrane heterodimer molecules containing α and β subunits. The figure illustrates the association of these subunits occurring in mammalian cells. As shown, 18 α and 8 β subunits form 24 different, distinct integrins. These can be grouped in subfamilies based on evolutionary relationships (different colors of α subunits), ligand specificity and, in the case of β2 and β7 integrins, restricted expression on white blood cells. Integrins can be grouped into two larger classes that bind to cell surface cell adhesion molecules (CAMs) and ECM ligands. They can be further classified as collagen-binding integrins (α₁β₁, α₂β₁, α₁₀β₁, and α₁₁β₁), RGD-recognizing integrins (α₅β₁, α_Vβ₁, α_Vβ₃, α_Vβ₅, α_Vβ₆, α_Vβ₈, and α_IIbβ₃), laminin-binding integrins (α₃β₁, α₆β₁, α₇β₁, and α₆β₄), and leukocyte integrins (α_Lβ₂, α_Mβ₂, α_Xβ₂, and α_Dβ₂). The β₂ integrin subunit (CD18) can pair with one of the four α subunits (α_L-CD11a, α_M-CD11b, α_X-CD11c, and α_D-CD11d) [16]. The α4β1 and α9β1 integrins recognize fibronectin and VCAM-1. The β2 and β7 integrins are restrictedly expressed by leukocytes). Asterisks show the alternatively spliced cytoplasmic domains. This figure is based on the study of Hynes [10] and Yue et al. [17]. (RGD, Arg-Gly-Asp sequence; VCAM-1, a vascular cellular adhesion molecule-11).

Figure 2

Adhesion molecule stimulation and mechanisms in endothelial cells. Inhibitory effect of a natural compound on cellular adhesion, typically of the human umbilical vein endothelial cell line (HUVEC), which are usually first treated with certain cytokines to stimulate the expression of CAMs.

Figure 3

Stimulation mechanism for cytokine modulation in lymphocytes. In vivo, lipopolysaccharide (LPS, from Gram-negative bacteria) stimulates the immune response by interacting with its leukocyte membrane receptor, CD14 (with TLR4-MD2), to induce the generation of cytokines such as TNF- α, IL-1, IL-1 α, IL-6.

Figure 4

The NF-κB pathway in endothelial cells. The compounds shown to have anti-inflammatory effects in vivo completely annulled LPS- and TNF-α-triggered nuclear translocation of NF-κB and NF-κB DNA-binding activity in endothelial cells, thus the production of cytokine receptors and ICAM, VCAM is inhibited.

Figure 5

The interaction between the ECM, MMPs and natural compounds in breast carcinoma. Baicalein blocked the expression of MMP-2/9, thus the degradation of the extracellular matrix is also inhibited.

Figure 6

Schematic illustration of the measurement steps of flow cytometry (A) and tetrazolium-based colorimetric MTT viability tests (B). (A) Propidium iodide is a popular red-fluorescent counterstain. Living cells are not permeable to it, but dead cells are (hence acquire dark coloring) and can be detected in the population after exposure treatment of the cells. (B) The basis of the MTT assay is that the yellow, water-soluble tetrazole becomes purple, insoluble formazan by the action of mitochondrial dehydrogenase of living cells. Cytotoxic and cytostatic activities can be determined from the optical density of the control and treated cells (ICM: incomplete medium, CM: complete medium). The formazan can be conveniently extracted by DMSO for colorimetric measurement.

Figure 7

Multicomponent study measuring cancer cell adhesion on a natural compound-treated coating [50]. A 384-well biosensor plate was used in the experiment (left) comprising the manipulation steps in a typical well (right). Representative kinetic curves are also shown (bottom).

Figure 8

Bioactivity-guided isolation of natural compounds.

Figure 9

Schematic illustration of SARS-CoV-2 attachment and two possible examples of ways to inhibit this process. (A) Virus can attach to ACE2 receptor or integrins of the host cell. RGD-integrin interaction occurs in calcium-dependent manner [274]. As the result of the process, the virus penetrates into the cell and starts to copy itself. (B) Several compounds bind to the spike protein or even may alter it [275] and prevent virus-receptor attachment. (C) Lowering divalent ion concentrations in the lungs with pulmonary EDTA chelation therapy may inhibit virus-host interaction [274].