1. Double-sided role of vibration and shaking
Removing vibration and shaking is a pivotal engineering task, which is showcased, for instance, in spacecraft attitude control systems that ensure a precise three-dimensional orientation. Vibration or shaking, such as precession caused by external torque, nutation stemming from off-axis angular momentum, and wobbling due to geometric misalignment, necessitate active and passive damping mechanisms. This principle extends beyond spacecraft to centrifugation techniques, pivotal not only in washing machines but also in a spectrum of biomedical apparatuses used for isolating cells and organelles and separating DNA and proteins. Consider an ultracentrifuge capable of surpassing speeds of 100 000 r∙min
−1 and generating forces akin to thousands of times Earth gravity (∼10 m∙s
−2). Thus, curtailing vibration and shaking becomes a cardinal rule in engineering practice. Contrarily, certain devices are purposely designed to take advantage of vibration and shaking, for instance, in chemical mixing, particle diffusion, and heat energy dissipation. Intriguingly, mounting evidence suggests that natural systems, including our bodies, cells, and biomolecules, are responsive to vibrations and shaking [
1]. This realization prompts the possibility of using their responsiveness to combat serious medical problems, notably, advanced cancer.
2. Alternative medicine to cell engineering
In medical applications, practices like massage and whole-body vibration have gained widespread acceptance. Massage therapy can alleviate diverse pathological states [
2]. Similarly, whole-body vibration has emerged as a popular method for easing muscle tension and strengthening the bones [
3]. An intriguing avenue is exploring their potential at a smaller level in cell engineering. Osteocytes, the predominant type of bone cells in the bone matrix, possess a remarkable ability to translate mechanical signals into biochemical cues, thereby contributing to bone formation triggered by loading [
4]. Furthermore, vibrations and shaking have been observed to stimulate the growth of various types of cells in part by enhancing nutrient and oxygen supplies [
5]. An interesting question is whether controlled vibrations and shaking could be employed to rectify pathological conditions by altering the fate of normal and cancellous cells.
3. Cell fate alterations in iPS cells and iTS cells
In cell engineering, induced pluripotent stem (iPS) cells and induced tumor-suppressing (iTS) cells are two examples of dramatically altering cell fates. In iPS cells, a quartet of factors—cMyc, Klf4, Oct4, and Sox2—exhibits the extraordinary ability to reprogram adult cells into a pluripotent state, resembling undifferentiated cells [
6]. This breakthrough holds immense promise in regenerative medicine and has sparked hope for rejuvenating aging or damaged cells. The process involved in generating iTS cells presents a counterintuitive approach. Traditional strategies focus on inhibiting tumor-promoting genes and signaling pathways to eradicate tumor cells. However, the generation of iTS cells diverges from this norm by involving the activation of oncogenic signaling (
Fig. 1) [
7]. Remarkably, the activation of specific factors like cMyc or Oct4 can facilitate the transformation into iTS cells. The unexpected revelation that an upsurge in cMyc or Oct4 can induce the formation of iTS cells marks yet another milestone. This discovery indicates that genes conventionally associated with promoting tumor growth might, under certain circumstances, prompt cells to produce and release proteins that suppress tumors. The creation of both iPS cells and iTS cells demonstrates the plasticity and adaptability of cellular behavior. To harness the full therapeutic potential of iTS cells while mitigating potential risks of manipulating oncogenic genes, a possibility for biomechanical and cell engineers is the potential of mechanical stimulation.
4. Generation of iTS cells by shaking
Various factors alter the growth and differentiation of both tumor cells and non-tumor cells. Efforts have focused on generating stimulus-triggered acquisition of pluripotency (STAP) cells akin to iPS cells through environmental stimuli, particularly mechanical stimulation. However, as of now, there is no substantiated evidence demonstrating the successful generation of STAP cells [
8]. In contrast, there have been promising developments in iTS cells. Biomedical engineers have gathered affirmative data indicating that certain tumor cells, like breast cancer cells, respond to low-intensity vibrations—such as those at 10 Hz under 1× Earth gravity for 0.5 to 1.0 hour—by suppressing migratory behavior [
9]. An attractive question has arisen: Can vibrations or shaking induce the transformation of non-tumor cells, such as mesenchymal stem cells (MSCs; the precursors to bone and muscle) and lymphocytes (a type of blood cell involved in immune responses), into iTS cells? Recent experimental findings involving both mouse and human cells suggest that this prospect holds promise [
10] (
Fig. 2).
Emerging evidence suggests that structural fibers, intricately linking the cell membrane, cytoskeleton, and nuclear envelope, may serve as conduits for transmitting mechanical vibrations to chromatin nestled within the nucleus. This transmission mechanism appears pivotal in triggering the intricate cascade leading to the production and subsequent secretion of tumor-suppressing proteins [
9]. Furthermore, recent advancements in mass spectrometry-based proteomics analyses have shed light on the consistent enrichment of key tumor-suppressing proteins in iTS cells. Notably, proteins like histone H4 have been identified across various methodologies utilized to generate iTS cells, underscoring their fundamental role in the anticancer properties exhibited by these specialized cells [
10].
5. Shaker therapy in combination with immunotherapy
Shaking may bring a new dimension to current cancer treatment. It has dual benefits through its anticancer effects directly on cancer cells and indirectly through non-cancer cells that can be harvested from a patient with cancer. One of the specific applications for shaking-treated iTS cells involves their combination with chimeric antigen receptor (CAR)-T cell immunotherapy [
11]. CAR-T therapy involves engineering T cells, collected from a patient with cancer, to develop a specific receptor known as CAR. This engineered CAR molecule exhibits a high affinity for cancer cells. While T cells function to eliminate cancer cells as part of the immune system, the inclusion of CAR enhances their anticancer capabilities by directing them toward cancerous cells.
In addition to MSCs, our research incorporated T lymphocytes, integral to CAR-T cell therapy, to explore their combined impact on tumor suppression. Employing a conventional tube shaker, we simulated an acceleration akin to 1× Earth gravity. The resulting proteins in conditioned medium derived from T cells exhibited notable efficacy in impeding breast cancer cell metabolism and migration. The significance of viscosity became apparent, with the viscosity of the regular medium measured at 0.8 cP (1 cP = 10
−3 Pa∙s
−1). To augment its antitumor potency, we increased viscosity to 10.0 cP using methylcellulose. A pressing question is whether mechanical stimulation can not only induce the conversion of T cells into iTS cells but also activate T cells. This potential avenue, possibly leveraging antibodies targeting T cell surface receptors, carries profound implications for immune regulation within the tumor microenvironment. If mechanical stimulation demonstrates efficacy in activating T cells, it has the potential to revolutionize immunotherapy approaches designed to target tumors. Encouraging evidence already exists, as illustrated by a study utilizing ultrasound, a form of mechanical stimulation typically operating above 20 kHz [
12].
Anticipation mounts for the collaboration between shaking-treated iTS cells and CAR-T cells, potentially yielding a more comprehensive anti-tumor effect. While CAR-T cells directly target cancer cells via antigen-specific recognition, iTS cells show promise in suppressing tumor growth through the secretion of tumor-suppressing proteins. Notably, while CAR-T cells require direct contact, iTS cells can act indirectly. We understand that CAR-T cell therapy faces challenges like antigen escape and immunosuppressive tumor microenvironments, where iTS cells offer complementary benefits by addressing broader aspects of tumor growth. Moreover, engineered iTS cells may target specific tumor microenvironment features, potentially enhancing overall therapeutic efficacy.
6. Future perspective
Presently, cancer treatment encompasses diverse approaches like surgery, chemotherapy, radiotherapy, targeted therapy, immunotherapy, and adjunctive methods such as massage and physical exercises targeting specific tissues or the whole body. A key inquiry emerges: Could introducing vibration and shaking of patient-derived cells introduce a novel dimension to conventional cancer treatments? While mechanical stimulation can alter the behaviors of cancer cells, temporarily shrinking them before returning to normal within hours, with varying impacts on growth inhibition across cancer types, our primary target of mechanical stimulation is not cancer cells but cancer-fighting cells such as MSCs and T cells. Besides vibration and shaking, other forms of biophysical stimuli, including electric excitation, ultrasound, and light irradiation, may trigger mechanotransduction and alter the progression of cancer cells and the anticancer action of non-cancer cells [
11], [
12]. To delve into this prospect, conducting thorough molecular signaling analyses becomes imperative. Understanding whether mechanotransduction is optimally effective at resonance frequencies and assessing how the viscous cell culture medium enhances responses in these cells are crucial inquiries. Addressing these questions holds promise in advancing mechanically driven iTS cell-mediated cancer treatments.
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