How do pancreatic stellate cells become active and what is the relation with pancreatic cancer?

Hello everybody, this is not a usual topic, right?

Nowadays, I’m so into my school work. This is good because I wasn’t able to go to university because of the Covid situation so I was so out of my career plans and I felt lost. Now, I feel more connected to my plans and finally, I have an aim for my life. That’s why I am holding on to this goal that is about being a perfect scientist.

Also, I gave you a promise that I will write to you every week. I’m thinking to do this job more professionally nowadays so probably I will write more frequently. Sometimes I feel so overwhelmed so I couldn’t write anything but now I know I don’t have time for being tired. So, what I’m doing lately, I’ve just started my university and my work and trying to learn laboratory works. It is hard to get used to things that you never worked for yourself before, taking responsibility etc. On the other hand, it is so fun and I believe that will overcome everything with study hard. It’s good to be part of something, do something for your life.

As you know, as a part of being a researcher, we should read lots of papers. However, I was confused about what should I read, where should I start. That’s why I read papers sometimes quite easy, sometimes quite hard but I felt like I learned nothing. So I heard that in the beginning, I should read quite an amount of review papers. So here I am reading and sharing what I learned!

Here is the paper that I read. We are gonna talk about mechanisms and signal pathways that can activate stellate cells.

Why I’m studying about pancreatic cancer?

Pancreatic cancer is accepted as one of the most fatal cancer types. This type has a below %5 patient survival rate. Although there are classic treatment ways, why survival rate is so low? This is because pancreatic cancer cells have an important interaction with their stroma and stromal cells. We are calling this microenvironment. So if we need to improve the effective treatment we shouldn’t target only cancer cells, but we should target the cancer microenvironment too.

What is pancreatic stellate cells and why we target these cells for effective pancreatic cancer treatment?

Pancreatic stellate cells most abundant cells in the pancreatic cancer microenvironment. In normal situations, these cells are seen the passive form and regulate extracellular matrix (ECM) production. These cells become active form during tumorigenesis when pancreatic cancer cells or other stromal cells secrete some stimulating factors. When these cells become active, they lost their retinoids and express alfa-smooth muscle actin. Studies show that the active form of PS cells alter four things to stimulate pancreatic cancer progression;

  1. extra fibrosis
  2. promote tumor metastasis
  3. being resistant t chemotherapy and radiotherapy
  4. immune system modülasyonu

There are plenty of studies that can confirm pancreatic stellate cells’ these effects on cancer progression. For example, they confirmed that active PS cells supernatant use the PDGF also SDF-1/ CXCR signal pathways to increase cancer migration and proliferation. In addition, if PS cells and pancreatic cancer cells were co-cultured, they saw that metastatic and growth rates increased dramatically. Also, they showed that PS cells can induce paracrine signals that promote tumour formation.

So that’s why it’ so important to learn how these cells become active and what is the interaction between pancreatic cancer cells. Today, we will learn about ways that activate PS cells.

Simulators of PS cells

We can divide stimulate factors into five groups:

  1. Transcription factors and cytokines
  2. non-coding RNAs
  3. oxidative stress related factors
  4. hyperglycemia
  5. calcium signaling and ion channels

Transcription Factors and Cytokines

TGF-β is the most effective regulatory cytokine for fibrosis. TGF-β can activate the MAPKs signal pathway and increase the expression of JNK1 and ERK1’s mRNA levels. This cause of changing myofibroblastic form of PS cells and secrete a vast amount of type 1 collagen and fibronectin that are a variety of ECM. Interestingly, some studies show that after the initial external signal, PS cells can secrete their own TGF-β1 themselves like an autocrine signal and continuously be active form. This is important because studies show that PS cells are responsible for type 1 collagen for fibrosis in the first place. Fibrous tissue is an indicator of chronic pancreatitis and it is composed of type 1 and 3 collagens. And studies show that another type of TGF- β family member activin A can work with TGF-β1 and increase each other’s secretion and mRNA expression in PSCs.

PDGF (platelet-derived factor) on the other hand, main simulator for the active form of PSCs. Active PSCs express PDGFR-α and PDFGFR-β. Phosphorylation of PDFGFR-β activates PI3K and ERK pathways. These end up with increased proliferation and migration of PSCs. Studies show that blocking the ERK pathway, completely suppress the proliferation of PSCs.

Also, it’s well known that hypoxia-inducible factor-1α (HIF-1) is responsible for developing pancreatic diseases. Studies showed that hypoxia not only exists in cancer cells also in PSCs too. In the hypoxia state, HIF-1 accumulate and after that, it translocates into the nucleus and joins in the transcription of VEGF and other pro-fibrotic in PSCs.

Non-coding RNAs

Lately, researchers thought that some miRNA expression alterations can be linked with PSCs activation. Studies show that pancreatic cancer-derived exosomes can increase the activity of PSCs and induce their fibrosis-related gene expressions via miRNAs. In addition, they showed that PSCs derived exosomes’ components connective tissue growth factor (CCN2) and miR-21 upregulated during the active state of PSCs. Eventually, this cause increased collagen production in the cells. Also, it’s better to note that there are miRNAs that show opposite effects too.

Oxidative stress related factors

Before that many studies confirm that, oxidative stress has a great impact on PSCs activation and chronic pancreatitis. One of the ROS inducers, exogenous H2O2 can promote alfa-smooth muscle actin production and migration of PSCs. Applying anti-oxidant chemical substances showed that alfa-smooth muscle actin and type 1 collagen expression decreased and inflammation in mice reduced.

In addition to this, it is known that PSCs cells and pancreatic acinar cells can metabolize ethanol to create oxidative stress factors that can promote PSCs activation. That’s why ethanol and smoking are important risk factors for chronic pancreatitis. In another study, they used a p38 blocker named SB203580 and showed that this inhibited the spontaneous activation of PSCs. With this information, they suggested that ethanol and its metabolites upregulated the MAPK signal pathway and expression of α-SMA in PSCs.

Lastly, studies showed that pressure can promote PSC activation and increase ROS levels in active PSCs. Intracellular pressure increase ERK phosphorylation and p38 MAPK signalling also induce secretion of TGF-β1 and collagens. Further studies showed that antagonists of p38 MAPK and extracellular signal-regulated kinase (MEK) inhibited pressure-induced α-SMA and cell proliferation. Therefore, increasing pressure may accelerate the progress of chronic pancreatitis in pancreatic tissue by activating PSCs.

Hyperglycemia

Studies showed that high glucose levels increase α-SMA and ECM protein expressions. In addition, high glucose can activate PSCs via the p38 MAPK signal and this result in ECM production. Some studies also showed that increasing glucose levels promote PSCs and pancreatic cancer cells interactions too.

Calcium signalling and ion channels

There are increasing studies that can suggest calcium signalling and ion channels can promote PSCs activations. They found that transient receptor potential (TRP) family’s members can contribute to PSCs activation. Researches showed that in the pressure-induced activation of PSCs, TRPC1-mediated calcium influx decreased. When they knockout TRPC1, saw that reduced cytokine production and decreased calcium influx in PSCs. There are other studies with other TRP members that can confirm calcium role in PSCs activation but still more efforts are needed.

Signalling pathways for PSCs activation

MAPKs

MAPKs signalling pathway takes part in many cellular events such as proliferation, apoptosis, survival and expression of inflammatory cytokines in the pancreas. ERK, JNK and p38 MAPKs are the main members of MAPKs.Studies showed that these main members increase chronic pancreatitis and PSCs source of producing MAPKs.

ERK, stimulate from RTKs( receptor tyrosine kinases) and activated from Raf and RasGTP enzymes. After that, it translocates into the nucleus and regulates transcription factors like activator protein-1 (AP-1). AP-1 is a transcription factor and it can be phosphorylated via MAPKs family members. Another clue for ERK signalling, researchers added ERK inhibitors to PSCs and they showed that CX3CR1 and SMA expression decreased significantly and they suggested that ERK may participate in process of fibrosis via regulating pancreatitis related cytokines. So we can say that ERK has a role in the migration, activation and matrix synthesis of PSCs. Also, studies suggest that ERK and JNK directly respond to the TGF-β1 and PDGF that are considered the most important factors of PSCs proliferation and ECM production.

Ras homolog gene family / rho-associated protein kinase (Rho/ROCK)

Rho is a GTP binding protein and ROCK is its downstream effector. Studies showed that they have a role in the actin cytoskeleton, stress fibres and cell morphology. Interestingly some reports showed that before the whole cells are filled with stress fibres, stress fibres that are found on activated PSCs increase. Some specific inhibitors for ROCK ( Y-27632 and HA-1077) prevent spontaneous PSCs activation via decomposing stress fibres. So we can indicate that PSCs activation ay related to Rho/ROCK pathway too. We can add that Y-27632 can decrease α-SMA expression and prevent proliferation also chemotaxis of PSCs mediated by PDGF so it can block the production of collagen. Result of this information, we can say that Rho/ROCK also promotes PSCs proliferation and chemotaxis.

NF-κB

In mammalian cells, the synthesis of NF-κB is stimulated by cytokines, mitogens, ultraviolet, tumour necrosis factor- α (TNF-α) and IL-1. TNF- α and IL-1 activate PSCs and express IL-6, IL-8 and monocyte chemoattractant protein-1 ( MCP-1) via AP-1 that are target genes for the NF-κB pathway. This shows that NF-κB can promote the activation of pancreatic fibrosis via PSCs.

An additional study showed that one of the specific blockers of NF-κB (Bay11-7082) blocks the NF-κB pathway with clear up IL-8 and MCP-1 and nitric oxide synthase that are also induced by toll-like receptor (TLR) and galactin-1. This information also showed that TLR and galactin-1 may promote PSCs activation and fibrosis through the NF-κB pathway.

Peroxisome proliferator-activated receptor-γ (PPAR-γ)

PPAR-γ is a nuclear hormone receptor and is involved in adipose differentiation, proliferation, immune response, insulin secretion in adipose tissue and immune system. Studies indicated that PPAR-γ has a negative effect on PSCs proliferation. The ligand of PPAR-γ (Troglitazone) can reduce α-SMA expression and PSCs proliferation by PPAR-γ signalling. This transforms PSCs’ from an active state to a stationary state.

PI3K-Serine/threonine kinase (AKT)

PI3K family is a kinase family that can phosphorylate PI3-hydroxyl groups and produce inositol ester material that acts as a second messenger. During this phosphorylation process, PI3K attracts Akt ( protein kinase B) to the cell membrane. PI3K/Akt pathway has an impact on cell proliferation, anti-apoptosis, migration, transmembrane translocation and cell carcinogenesis. It’s well known that IL-1β, TNF-α and IFN-γ regulate the activity of PSCs and induce the expression of IL-32α that is the trigger for fibrosis. If there is an inhibitor, PI3K downregulates IL-32α expression. Also, studies have shown that PDGF mediated migration of PSCs occurred by activating PI3K/Akt pathway.

A janus-activated kinase (JAK)/signal transducer and activator of transcription (STAT)

JAK is a receptor-deficient tyrosine kinase and they are activated by phosphorylation of transcription factor STAT. STAT is found in the cytoplasm as a precursor molecule. It’s phosphorylated in tyrosine residue after that translocate into the nucleus to bind DNA. Studies showed that PDGF promotes the proliferation of PSCs via the JAK/STAT pathway. They also showed that PDGF-dependent activation of STAT1 and STAT3 were blocked by inhibitors of Src and JAK2 and this resulted in a reduction in fibrosis.

Smads

Smads proteins are small molecules that regulate intracellular signals. Reported that Smad proteins are functionally active in PSCs. First, they showed that TGF-β promotes Smad2,3 and 4 to form a kind of polymer to enter the nucleus. After that, it can regulate the transcription of the target gene. TGF-β promotes fibrosis via Smad2 and Smad3 pathway but inhibits the expression of Smad7. After members of Smads protein that we mentioned enter the nucleus they activate target genes after that they cause ECM production and fibrosis.

Hedgehog

Indian hedgehog (Ihh) is one of the main members of the Hedgehog family. The Ihh increase the migration of PSCs and expression of MMP1 which can degrade cell basement membrane to promote cell movement. Most of the hedgehog intercellular signal transductions are regulated by the transcription factor Gli1. And Ihh induces the accumulation of Gli1 in the nucleus of PSCs, suggesting that Ihh may activate Gli1-dependent signalling pathways. According to this information, hedgehog signalling is an indispensable pathway in the activation of PSCs and the production of ECM during fibrosis in chronic pancreatitis and pancreatic cancer.

CONCLUSION

This is my first review of an article and I must say that I struggle a lot. This is mostly because I have so many things to do, but I have no time for doing any of them. However, I don’t mind because I know when time pass, I will make an order. Anyway, I have to be quick though.

Also, I wanna say that this is a hard topic and there is literally so many things. Signal pathways affect other ways also they connected with each other. It is impossible that I tell you everything you need. I just try to write a small summary for myself doing repeat to understand better the main things. If you gonna need further information you should search more deeply. The thing is that I wanted to learn from this article which molecules and signal pathways activate PSCs and after that what is happening.

So here we saw that there is a strong belief about activated PSCs affect chronic pancreatitis and pancreatic cancer. The passive form of PSCs can maintain pancreatşc tissue architecture and ECM turnover. When PSCs become active, they cause unstable ECM turnover and contribute inflammatory microenvironment. We have some basic information about the activated state of PSCs but still, there are so many unclear things. If we understand the molecular mechanism of these pathways, we can find new therapeutic strategies.

Have a nice day, I hope you’ll learn something too and like it!

See you next time!

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