Molecular Pathogenesis and Diagnostic Imaging of Metastatic Jaw Tumors.

Metastasis is the spread of malignant cells from a primary tumor to distant sites through lymphatics or blood vessels. Malignant lesions metastasizing to the oral and perioral region are a rarity indeed. Malignant lesions could metastasize to both soft tissue of oral cavity and the hard tissues of the jaws and recent meta-analysis showed that metastasis is more common in the jaws than oral soft tissues because of rich vascular supply. The incidence is very low when compared to the incidence of primary oral cancers; nevertheless, one has to include in the diagnostic workup, metastatic malignant lesions, when an irregular ill-defined radiolucency or radiodensity with ragged edges in noted. It could be a challenging task for a diagnostician, in cases with the presence and location of the primary tumor is unknown. Advanced oral imaging technologies and biochemical markers play a vital role in diagnosing such lesions.

INTRODUCTION

Malignant tumors involving the jaw bones are most often due to direct extension of the disease either from the oral cavity or from the surrounding tissues. Metastatic tumors of jaw bones constitutes about 1% of all the malignancies occurring in jaw and mostly affect the mandible region than the maxilla. The younger patient is affected more commonly with almost equal gender distribution. These metastatic tumors on jaws can deposit from any primary tumors arising from lungs, kidney, prostate, thyroid, and breast. In about 30% of cases, the oral lesions are the first sign of this disease and are very tough to diagnosis in histopathology because it takes the clones of primary cancer.[123]

The microenvironment of the bone matrix is a vast storehouse of growth factors such as platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), osteoprotegerin (OPG), bone morphogenetic protein (BMP), transforming growth factor (TGF)-β, and vascular endothelial growth factor (VEGF) and these factors get released during bone remodeling.[4] The release of these factors may promote cell homing and appears to promote colonization by stimulating vicious cycle and leads to tumor cell proliferation and progression of osteolytic and osteoblastic bone lesions.[56] Metastatic tumor cause changes in bone architecture, which predisposes the patient to a variety of skeletal complications.

PHYSIOLOGY OF BONE FORMATION AND RESORPTION

Bone is formed primarily of type I collagen and later gets mineralized with hydroxyapatite crystals. Continuous undergoing of osteoclast-mediated bone resorption and osteoblast-mediated bone formation are the coupled and sequential process of dynamic remodeling happening within the bone throughout the life. Osteoblasts, osteoclasts, and other cells involve a very close crosstalk within the bone microenvironment with the help of numerous growth factors, hormones, cellular molecules, and proteins, during this formation and resorption process.[78] Mesenchymal stem cells in the bone marrow stroma proliferate when it is stimulated by bone morphogenic proteins and other growth factors, and form pre-osteoblasts that differentiate into osteoblasts. Osteoblasts produce collagen matrix (osteoid) which is the precursor of bone and regulate bone mineralization. Bone regulating molecules such as parathyroid hormone receptors, prostaglandins, estrogen, Vitamin D3, and various cytokines will be expressed by osteoblast. They are also involved directly with the control of osteoclast differentiation through expression of receptor activator of nuclear factor κB ligand (RANKL).[79101112131415] Osteoclasts are derived from precursor cells in the monocyte-macrophage lineage. Osteoclast production is stimulated by prostaglandins, interleukin 6 (IL-6), IL-1, and macrophage colony-stimulating factors. Activation of osteoclast is based on the balance between osteoblast secreted RANKL and OPG levels. Activated osteoclasts degrade bone by binding to bone matrix through integrin proteins and secreting acid and lysosomal enzymes.[16171819202122232425] Wingless int (Wnt) pathway is also found to be a key regulator of osteoblast function and bone formation and activation of this Wnt/B-catenin signaling pathway leads to increased bone deposition [Figure 1].[726]

Figure 1

Physiology of bone

MECHANISMS OF METASTASIS

Metastasis is the spread of malignant cells from a primary tumor to distant sites. This occurs in a series of individual steps, which is called as “metastatic cascade.”

Step 1: Epithelial-mesenchymal transition (EMT) and breach of the basement membrane barrier

Step 2: Dissociation of tumor cells from the bulk tumor

Step 3: Invasion into the neighboring tissue

Step 4: Intravasation into preexisting and newly formed blood and lymph vessels (angiogenesis)

Step 5: Transport of disseminated tumor cells through vessels

Step 6: Extravasation of tumor cells from vessels

Step 7: Establishment of disseminated cells (which can stay dormant for a prolonged period of time) at a secondary anatomical site and

Step 8: Outgrowth of micrometastasis and macrometastasis/secondary tumors.[2728293031]

Recent studies in this area have suggested yet another step, to be added at the beginning of the cascade and therefore designated as:

Step 0: The creation of target site before the first tumor cells arrive at this distant location, “premetastatic niche.”[5263233]

Each step creates one or more physiological barriers to the spread of malignant cells. Tumor cells have to overcome all of those barriers, for successful metastasis

Microenvironment within the bone contains two groups of cells which contribute to the metastatic bone niche formation. They are stromal cells and transient cells. Stromal cells arise from mesenchymal cells in the marrow (adipocytes, fibroblasts, and osteoblasts). They sustain the differentiation and proliferation of cancer cells through molecules such as vascular cell adhesion molecule, syndecan-1, and matrix metalloproteinase 2 (MMP-2). Transient cells (erythrocytes, T-cells, and platelets) all aid tumor growth and metastases through various pathways and molecules.[71634] EMT is likely to contain several intermediate steps such as downregulation of epithelial proteins, loss of cell polarity, loss of cell-cell adhesion, cell scattering, secretion of protein degrading enzymes, induction of mesenchymal proteins, and inhibition of apoptosis.[2135363738] Continuous cycle of actin polymerization and depolymerization results in extension of cell membrane protrusions which help cell migration.[2139]

Cell death induced by inappropriate or loss of cell adhesion is called Anoikis. When tumor cells enter unfamiliar environments, anoikis could obstruct metastasis by inducing apoptosis. Anoikis suppression, therefore, is likely to be an essential requirement for tumor cells to successfully metastasize to distant sites.[214041] Tumor cell invasion alone is not sufficient to produce distant metastases but also the transport of malignant cells through blood and/or lymph vessels. Passive diffusion of nutrients and oxygen becomes rate limiting for the tumor nodule and restrict avascular tumors to grow beyond 1 mm in diameter. Hence, it is then forced to enter a state called tumor dormancy (cells cease dividing but survive in a quiescent state while waiting for appropriate environmental conditions to begin proliferation again).[42] At this stage, angiogenic switch (which is a loss of balance between stimulation and inhibition of new blood vessel growth) gets activated and the tumor grows beyond its diffusion limit.[43] The presence of tumor cells in regional lymph nodes draining the primary tumor site can precede distant metastasis to visceral organs.[2144]

Earlier studies suggested that tumor-associated macrophages play a crucial role in guiding tumor cells to blood vessels and sites of intravasation (directional cell migration).[45] Extravasation of the transported tumor cell at predestined site is depended on integrins and ezrin, possibly suppressing anoikis.[46] Integrins have a vital position in the interaction of tumor cells not only with platelets and leukocytes but also with endothelial cells. The interaction of tumor cells with platelets also plays an important role in extravasation of tumor cells. Metastasis can be impaired by the presence of anticoagulation agents within the blood vessels.[21] “Soil and seed hypothesis” stating that certain tumor cells (seeds) will selectively colonize distant organs (soil) because of the presence of a favorable environment for their localization and growth. An emerging concept has recently challenged this existing model of metastasis by demonstrating that the potential to metastasize is encoded in the bulk of the tumor and is present early in tumor pathogenesis.[52147]

The basic principle for the formation of a primary tumor and metastatic tumors remains same. Metastasis suppressor genes when overexpressed impair metastasis without affecting primary tumor growth.[21] Cancer stem cells (CSCs) in particular that eventually establish the macro metastases. Till date, the link between CSCs and metastasis is the overexpression of stem cell-associated genes in metastatic tumors.[2148] Cancer-associated fibroblasts which are often referred as myofibroblast or activated fibroblast is a major component of tumor stroma. When these cells get in contact with the tumor cells, it stimulates tumor growth and angiogenesis.[214950]

MOLECULAR EVENTS INVOLVED IN METASTATIC TUMORS OF JAWBONES

Large size primary cancer favors pre metastatic niche formation.[265152] Chemokine receptor type 4 (CXCR4) is expressed on orthotropic cancer cells. Bone microenvironment secretes stromal derived factor-1 and generates a chemoattractant signal through its receptor ligand, for which CXCR4 responds. Marrow rich metaphysis of jaw bone has abundant sinusoids and sluggish blood flow (especially in mandibular retromolar area which has more red bone marrow than other jaw sites) influences the interaction between endothelium and tumor cells. This attachment of metastatic cells gets extravasated latter and are colonized in the bone marrow.[52653545556] Parathyroid hormone-related peptide (PTHrP) upregulates RANKL expression and decreases OPG expression.[5758] Soluble RANKL stimulates osteoclastogenesis by binding directly to RANK.[59] IL and TNF-α increases osteoclastogenesis, enhances the effect of PTHrP, promotes osteoclast activation and survival.[6061] M-CSF upregulates RANKL expression on stromal cells. It has chemotactic role for attracting osteoclasts to resorptive sites and prolongs survival of the mature osteoclast by inhibiting apoptosis. to resorptive sites and prolongs survival of the mature osteoclast by inhibiting apoptosis.[62] TGF-β in the absence of RANKL directly stimulates osteoclast formation.[63] VEGF induces angiogenesis and promotes osteoclastogenesis.[64] MMPs help osteoclast-mediated bone resorption.[6566]

VEGF and MDA-BF-1 play a vital role in differentiation and activation of osteoblast.[6768] Proliferation of the osteoblast is regulated by BMP,[69] IGF's,[70] TGF-β,[71] uPA,[72] FGF's,[73] and ET-1.[7475] OPG inhibits osteoclastic activity by binding to RANKL.[76] PDGF-BB promotes angiogenesis [Table 1].[4777879]

Table 1

Osteolytic and osteoblastic metastasis

Biochemical markers

Biomarkers have been used to assess the response to therapy or for the detection of bone metastases. Osteoblastic activity is analyzed through the of levels of bone-specific alkaline phosphatase, osteocalcin, and type I procollagen C-propeptide in serum, whereas osteoclastic activity is identified through serum levels of C-terminal telopeptide of type I collagen and tartrate-resistant acid phosphatase and urinary levels of type I collagen cross-linked N-telopeptides. Urinary type I collagen crosslinked N-telopeptides and C-terminal telopeptide of type I collagen appear to be the most useful.[8]

Imaging in bone metastasis

Bone metastatic tumors primarily consist of four architectural patterns: osteolytic, osteoblastic, osteoporotic, and mixed. Bone metastases are mostly multiple during the time of diagnosis.[7] Eosinophilic granuloma, lymphoma, chronic osteomyelitis, Paget's disease, stress fractures, osteodystrophy, multiple myeloma, and secondary osteoarthritis should be considered as differential diagnosis during the imaging evaluation of jaw bones [Figure 2]. Advanced imaging aids such as radiography, computed tomography (CT), magnetic resonance imaging (MRI), bone scintigraphy, and positron emission tomography play an essential part in the detection, diagnosis, and determination of the extent of the disease, prognosis, treatment planning, and follow-up for monitoring these patients.[7808182838485868788]

Figure 2

Advantages and limitations of imaging techniques in diagnosing metastatic jaw tumors

CONCLUSION

Metastasis of jaw bones is one of the most debilitating problems in patients with malignancies. Prevention or limitation of bone metastasis would considerably improve the quality of lives of patients diagnosed with advanced malignancies. The detailed molecular mechanisms responsible for both osteolytic and osteoblastic metastases are just being unraveled with recent advancements in molecular biology. When bone metastases are diagnosed or suspected, further imaging-guided techniques may be required to confirm, to establish the extent of the disease, and to find the primary tumor. Radiograph, CT, MRI, bone scintigraphy, and PET are considered to be the most accurate imaging modality to date, but they still have limitations such as high cost, lower availability, lack of specificity, and the inability to detect micrometastases. Further improvement of imaging techniques is mandatory to improve their accuracy for the well-being and the quality of life in affected patients.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.