2. Mechanism and obstacles of MPTT
2.1. Mechanism
PTT can be categorized into conventional PTT (≥45 ℃) and MPTT (<45 ℃). MPTT has been recognized as an emerging noninvasive approach for treating tumors that utilizes PTAs to convert light energy into heat energy, effectively destroying tumor cells when exposed to NIR light.[9, 26-28] This method offers several advantages, including minimal adverse reactions, high specificity, and repeatability. Under NIR irradiation, PTAs absorb photon energy and transition from their ground state to an excited state. Subsequently, the energy from electron excitation undergoes non-radiative decay via vibrational relaxation, returning the PTAs to their ground state through collisions with surrounding molecules. This increased kinetic energy results in the heating of the local microenvironment, inducing thermal effects.[29-32] When tissue temperature reaches approximately 41 ℃, the cell initiates a heat shock response, leading to the production of HSPs to mitigate initial heat damage. If the temperature continues to rise to 42 ℃, irreversible tissue damage occurs. Sustaining a temperature of 42-46 ℃ for 10 minutes will induce cell necrosis, while further temperature elevation to 46-52 ℃ will lead to rapid cell death due to microvascular thrombosis ischemia. Temperatures exceeding 60 ℃ cause instant cell death due to protein denaturation and membrane destruction. Conventional PTT operates at higher temperatures, resulting in greater tumor tissue killing efficacy but also causing more significant damage to normal tissues.[33-35] In contrast, MPTT, with its lower temperature range, causes less harm to normal tissues but has reduced tumor-killing efficacy. MPTT is often employed as a regulatory mechanism rather than a direct tumor tissue treatment. The success of both conventional PTT and MPTT heavily relies on the effectiveness of PTAs, which serve as critical intermediaries. The biological safety and photothermal conversion efficiency (PCE) of PTAs directly impact the therapeutic outcomes of PTT and MPTT.
2.2. Obstacles
2.2.1. HSPs
HSPs are a group of heat-responsive proteins that are widely distributed in both bacteria and mammals. When organisms are exposed to elevated temperatures, the production of HSPs is triggered to protect the organism from the damaging effects of heat stress. Many HSPs exhibit molecular chaperone activity, assisting in the proper folding and stabilization of other proteins.[36-38] HSPs can be categorized into five main types based on their molecular sizes: HSP110, HSP90, HSP70, HSP60, and small HSPs. HSPs play a crucial role in enhancing the stress resistance of cells, particularly heat resistance. They are known to confer a phenomenon known as heat tolerance. Heat tolerance refers to the ability of an organism to withstand a non-lethal thermal stimulus, which subsequently strengthens its resistance to a second, potentially lethal thermal stimulus. This adaptive response improves the organism’s chances of surviving under extreme heat conditions. The precise molecular mechanisms underlying heat tolerance are still not fully understood.[39, 40] However, numerous studies have consistently shown a positive correlation between the production of HSPs and the development of heat tolerance. In essence, when an organism is exposed to moderate heat stress, the upregulation of HSPs appears to be a key component of its defense mechanism, allowing it to better cope with subsequent, more severe heat stressors.
Currently, HSP70 is the most studied stress protein in the heat shock family, which has been widely found in prokaryotes and eukaryotes.[41-43] In biological cells, HSP70 is abundant and widely distributed. It is not only distributed in the cytoplasm, but also in the organelles. The HSP70 family mainly consists of four types of proteins: (1) Inducible HSP70 (also known as HSP72), which exists in the nucleus, is less expressed in normal cells, but increases rapidly after cell stress, and has a high affinity with adenosine triphosphate (ATP).[44, 45] (2) Structural HSC70 (Heat shock cognate 70, also known as HSP73) exists in the cytoplasm and is constitutionally expressed in all cells. It is a structural protein in mammalian cells, which only increases slightly after external stressor stimulation, and has a high affinity with ATP.[46, 47] (3) Glucose-regulated protein 78 (GRP78) (also known as HSPA5) is in the endoplasmic reticulum, slightly expressed under stress, and involved in a variety of cell life processes.[48-50] (4) Glucose-regulating protein 75 (HSP75) (also known as HSPA9) is mainly located in mitochondria, and its genes can be expressed in various tissues, and the expression level is related to the energy metabolism of cells themselves.[51, 52] The expression of protein function requires structural changes, and protein folding is the most common one. However, protein folding requires the involvement of several auxiliary elements, called chaperones, which are a class of constituent proteins that help the newly synthesized protein to fold correctly and assemble successfully without itself being the final assembly product. In biological studies, HSP70 has the functions of molecular chaperone, anti-apoptosis, anti-oxidation, innate immune response, and cellular immunity.[53-55] At the same time, in the study of medical tumors, HSP70 can be highly expressed in many malignant tumors, which is related to the inhibition of tumor cell apoptosis.
HSP90 (also known as oncoprotein) is another well studied stress protein, which is one of the most widely tested targets for cancer therapy. HSP90 is a class of energy-dependent chaperone molecules that maintain cell differentiation, growth, and survival by regulating the conformational stability, folding and function of its ”customer service protein” with the help of co-chaperone (Hop, Stil).[56-58] In vivo, HSP90 has more than 400 customer proteins, including transmembrane tyrosine kinases (HER/neu, EGFR, MET, IGF21R), metastable signaling proteins (Akt, Raf21, IKK), mature signaling proteins (p53, kit, Flt3, v2Src), Chimeric signaling proteins (NPM2ALK, Bcr2Abl), steroid hormone receptors, and cell cycle regulatory factors (cdk4, cdk6), which are closely related to the occurrence and evolution of tumors, and are overexpressed or continuously activated in malignant tumors.[59-61] HSP90 is like a ”switch” in the human body, which is upstream of many cell signaling channels. These customer service proteins are its ”daughter switches”. By changing their conformation, HSP90 can affect the survival of cancer cells. When the existing therapies reduce the dependence of tumor cells on the original drug inhibition pathway through the branch signaling pathway due to the tumor gene changes, a large number of HSP90 client proteins are often directly or indirectly involved in the regulation of these new pathways.[62-64] Therefore, inhibition of the function of HSP90 can lead to the instability of its client proteins and eventually degradation through the proteasome pathway. In addition, the dependence of tumor on branching signals was cut off, and the drug resistance of downstream targets of HSP90 was solved.[65, 66] Relevant studies have proved that some drug-resistant tumors also show significant sensitivity to HSP90 inhibitors, suggesting that HSP90 inhibitors can be used as a therapeutic strategy against drug resistance.
2.2.2. Autophagy
Autophagy is a cellular process that involves the degradation of dysfunctional cellular components within cells through lysosomes. This essential process allows cells to break down and digest damaged or denatured organelles, proteins, nucleic acids, and other biological macromolecules. Autophagy serves as a mechanism for providing raw materials necessary for cell regeneration and repair, effectively recycling cellular materials.[67-71] It is a dynamic process that can be regulated in response to various stimuli, such as inadequate nutrition or hypoxia, leading to increased autophagy levels. Autophagy is a widespread phenomenon in eukaryotic cells and can be categorized into three main types: macroautophagy, microautophagy, and chaperone-mediated autophagy. This process is tightly regulated and is a routine part of cell growth, development, and homeostasis. Autophagy helps maintain a balance between the synthesis, degradation, and recycling of cellular products. Recent research has shown that autophagy is activated to varying degrees during the differentiation process of many cell types. It plays a role in various cellular processes, including angiogenesis, osteogenic differentiation, lipogenesis, neurogenesis, and more. Autophagy also plays a protective role in cancer treatment. MPTT can enhance the autophagy levels in tumor cells. Inhibition of autophagy has been found to sensitize tumor cells to MPTT, effectively suppressing tumor growth under low-temperature conditions and significantly improving the efficiency of photothermal therapy.[72-74] Furthermore, the autophagy-related protein Beclin 1 and peptide molecules derived from it can further enhance the autophagy levels in tumor cells, building upon the autophagy induction achieved through MPTT. Therefore, autophagy is a crucial cellular process involved in the degradation and recycling of cellular components. It plays a multifaceted role in various cellular processes and has emerged as a protective mechanism in cancer treatment, particularly in the context of MPTT.[75-77] Enhancing autophagy levels, either through MPTT or Beclin1-induced autophagy, may reduce the therapeutic outcomes in cancer treatment.