Chapter 24: Metabolic Effects of Tumours

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Welcome to this special deep dive.

If you are joining us today, you are likely a college student staring down the barrel of your clinical biochemistry exams, specifically looking at Chapter 24.

Right.

Metabolic effects of tumors.

Exactly.

And we know this material can feel incredibly dense, but that is exactly why we are here.

Think of this as your personalized one -on -one tutoring session.

We are going to walk through this together at a pace that respects your time, but ensures you truly understand the mechanisms at play.

Our mission today is to tackle this chapter from top to bottom, mapping it all out so it actually sticks in your memory.

To frame this chapter in your mind right now, think of the body as a highly regulated manufacturing plant.

Sometimes, neoplastic or malignant cells go rogue.

They start acting like rogue factories.

Synthesizing things they shouldn't be.

Precisely.

Pumping out substances that you would never normally expect from that specific tissue.

These substances fall into two main categories, which beautifully divides our tutoring session today.

First, we have the biologically active compounds, right?

Yes, the active ones.

These actively alter metabolism and produce massive clinical effects, creating what we call hormonal syndromes.

And then the second category.

Second, we have biologically inactive compounds.

They don't make the patient sick directly, but they leave a trail we can detect in body fluids.

That makes them invaluable as tumor markers.

We're going to move chronologically through your material, linking the normal biochemistry directly to the pathophysiology, then to the lab tests, and finally to patient management.

It is the best way to make the concepts lock in.

Okay, let's unpack this starting with the first major section, which is the diffuse endocrine system.

There's a term introduced right away, apiodomas.

Ah, yes, apiodomas.

Essentially, these are tumors of cells that originally come from the embryonic ectoblast.

They share this unique ability for, well,

a manned precursor uptake and decarboxylation.

It is quite a mouthful.

It really is, but it's where the acronym APUD comes from.

Ultimately, these cells produce active amines.

The sympathetic nervous system, which includes the adrenal medulla and the sympathetic ganglia, comes from that embryonic neural crest.

So this is the region we're looking at when we talk about catecholamine producing cells.

If you're going to understand how these rogue factories cause havoc, you have to understand the normal assembly line first.

Let's slow down and look at the normal biochemical pathway for catecholamine synthesis.

If you're taking notes, write the sequence down.

The raw material starting the line is the amino acid tyrosine.

Tyrosine, got it.

An enzyme acts on tyrosine to convert it into dopa, that is dihydroxyphenylenine.

The next step on the line converts dopa into dopamine.

And from dopamine, we get noradrenaline.

Exactly, also known as norepinephrine.

And this step mostly happens right at the sympathetic nerve endings.

Finally, noradrenaline is converted into adrenaline or epinephrine.

Which happens in the adrenal medulla.

That final step is almost exclusively the job of the adrenal medulla, yes.

And obviously,

what the body builds, it eventually has to break down.

Understanding that breakdown or the exhaust from this factory seems crucial for the lab tests we use.

It is the entire basis of the diagnostics.

So adrenaline and noradrenaline get metabolized into these inactive intermediates called metadrenaline and normitadrenaline.

Which we collectively just call metanephrines.

From there, it's a multi -step process to finally break them down into HMA, which stands for 4 -hydroxy -3 -methoxymandelic acid.

Some older resources you read might call it VMA, but your material specifically wants you to remember HMA.

So let's look at the path of physiology.

What happens when a tumor forces that assembly line into overdrive?

Excess noradrenaline causes intense generalized vasoconstriction.

It just clamps the blood vessels down.

Clinically, you are going to see a patient with severe hypertension and extreme power or paleness.

But adrenaline is a different story.

Adrenaline is very different.

It actually dilates the blood vessels in the muscle, so its effect on blood pressure and pulse rate can fluctuate wildly.

It also messes with blood sugar, doesn't it?

Crucially, yes.

Adrenaline has an anti -insulin effect.

It tells the body to start breaking down glycogen into glucose.

That process is called glycogenolysis.

Right, which leaves the patient with hyperglycemia.

That perfectly sets up a really interesting clinical vignette from your notes.

Let's look at case one.

You have a 34 -year -old man who gets sent to a specialized clinic because his blood pressure is just wildly out of control, despite being on a bunch of standard medications.

What is exactly going on with him biochemically?

If we look at the lab values for that specific case, it paints a very clear picture.

In the clinic, his blood pressure was 186 over 104.

Which is dangerously high.

Especially for a young guy already on three different antihypertensives,

vendroflumethiazide, enoliprol, and philidipine.

His basic metabolic panel, the plasma sodium, potassium, urea, and creatinine were all perfectly normal.

But the 24 -hour urine test was the diagnostic smoking gun.

Yes.

His normatonephrine level was 23 .1 millimoles per 24 hours.

A normal value is less than 2 .0.

His metanephrine was 17 .3, where normal is less than 1 .5.

So in plain English,

his body's exhaust pipe is just billowing out evidence of a massive catecholamine overload.

Precisely.

Those staggering elevations point to a single diagnosis.

A pheochromocytoma.

A catecholamine -secreting tumor of the chromophin tissue.

It is the absolute classic presentation of a young adult with refractory or treatment -resistant hypertension.

For your exams, you must memorize the classic clinical triad that accompanies this hypertension.

Sweating, headaches, and tachycardia.

That is the triad.

As a clinician, you also need to remember the 10 % rules for pheochromocytomas.

Broadly, about 10 % are extraadrenal, meaning they are found outside the adrenal medulla.

10 % are bilateral, affecting both sides.

And 10 % are malignant.

You also have to screen for familial syndromes, because these tumors frequently run in families alongside multiple endocrineoplasia, neurofibromatosis, and von Hippel -Lindau disease.

I do have a question about the labs, though.

Why are we meticulously measuring these breakdown products, the metanephrins, instead of just checking the patient's blood for the actual catecholamines?

What's fascinating here is that the breakdown products offer a much wider, more reliable diagnostic window.

Metanephrins are vastly more sensitive than measuring raw catecholamines or even HMMA.

Because the tumor might secrete them sporadically.

Exactly.

You can have a patient with a dangerous pheochromocytoma where their urinary catecholamines look perfectly normal on the day of the test, but their metanephrine levels are skyrocketing.

You also have to factor in that some extraadrenal tumors might only secrete dopamine, which alters the breakdown profile entirely.

This is huge.

But for your clinical practice, you have to be incredibly careful about lab interferences.

You cannot just order a 24 -hour urine collection blindly.

Dietary components and certain drugs will completely ruin the assay.

You have to watch out for the beta blocker libetolol, simple paracetamol, alpha -methyldopa, and cinnamet.

Cinnamet is the levodopa -carbidopa combo used for Parkinson's, right?

Correct.

And if collecting a giant jug of urine for 24 hours is unfeasible for a patient, we can use plasma metanephrins.

But you have to remember they can be falsely elevated if the patient has renal failure.

Let's say the tests are still ambiguous.

The material mentions a clonidine or pentalinium test.

How does that help?

It tests the autonomy of the factory, and a healthy person giving a suppressing agent like clonidine will force catecholamine levels to drop.

But a pheochromocytoma is a rogue factory.

It doesn't listen to the body's management anymore.

It just keeps secreting regardless of the suppression.

Yes.

Once you confirm the biochemistry, you use midging abdominal CT, MRI, or an MIBG scan, which uses a special radioisotope soaked up specifically by pheochromocytoma tissue.

There is a terrifying but amazing visual in figure 24 .2 of your text.

It graphs a patient's systolic blood pressure during the surgical removal of one of these tumors.

Just physically touching and manipulating the tumor during surgery causes it to dump massive amounts of adrenaline.

The blood pressure surges up to nearly 300, requiring a continuous nitroproside infusion just to keep the patient from stroking out on the table.

Before we leave the sympathetic nervous system, there is a brief mention of neuroblastomas.

From what I gather, these are sympathetic nervous tissue tumors, but they mostly happen in kids.

And unlike the adult tumors we just discussed, about 60 % of these are extra adrenal.

They can pump out just as many catecholamines, but some specifically produce dopamine and its breakdown product, homo -vanillic acid.

HVA, yes, which is what we would look for in the pediatric urine test.

Here's where it gets really interesting.

We are shifting focus from the adrenal glands down to the gut,

specifically to a group called the argentaffin cells.

These cells originate in the embryonic gut and hang out mostly in the eye lamin and appendix, but also spot the pancreas, stomach, and rectum.

They get their name because they reduce and stain with silver salts in a lab.

And when these cells turn into a tumor, they cause something called the carcinoid syndrome.

The biochemical assembly line here is different.

These argentaffin cells synthesize a completely different active immeridine 5 -hydroxy tryptamine, which you probably know better as serotonin or 5 -HT.

The pathway begins with a different amino acid, tryptophan.

An enzyme called tryptophan 5 -hydroxylase converts it into 5 -HTP.

Next, aromatic amino acid decarboxylase turns that 5 -HTP into serotonin.

Finally, monamine oxidases come in to break the serotonin down into an inactive exhaust product.

Called 5 -HIAA or 5 -hydroxyindole acetic acid, this 5 -HIAA is what gets excreted in the urine, and it's our primary diagnostic target.

But there is a really curious physiological quirk here, the liver metastasis rule.

The material points out that a tumor in the gut can be pumping out massive amounts of serotonin.

But the patient might not show any of the classic syndrome symptoms unless the tumor has metastasized to the liver.

Why does the location matter so much?

Think of the liver as the ultimate biological water filter.

All the blood draining from the intestines goes straight into the portal circulation, which flows directly into the liver.

So if a tumor in the ileum secretes a bunch of serotonin, the liver catches it.

And neutralizes it before it can reach the rest of the body.

But if that tumor spreads and implants itself on the liver itself, or if the primary tumor is located in the bronchus of the lung, those rogue secretions bypass the portal filter entirely.

They are dumped straight into the systemic circulation.

And when unfiltered serotonin hits the systemic circulation, the patient suffers.

We're talking severe diarrhea, sometimes so bad it causes profound malabsorption.

They get intense flushing of the skin.

They suffer from bronchospasm, so they can't breathe.

It even causes right -sided fibrotic heart lesions, like tricuspid incompetence and pulmonary stenosis.

Though weirdly, the heart damage doesn't seem to happen if the primary tumor is up in the bronchus.

You also have to consider the nutritional fallout.

The tumor is so greedy, demanding so much tryptophan to manufacture its serotonin, that it steals it away from the body's normal operations.

Normally, tryptophan is used to synthesize nicotinamide.

Without it, the patient develops a severe niacin deficiency, presenting as a pellagra -type syndrome.

To visualize this clinically, look at case 2 in your notes.

A 56 -year -old man presents to gastroenterology.

He has profuse diarrhea.

He is breathless from bronchospasm.

And his face is deeply flushed.

The doctors run a 24 -hour urinary 5 -HIAA test.

And his results are off the charts, right?

A normal 5 -HIAA result is less than 25 micromoles a day.

But this guy's result came back at an unbelievable 544 micromoles.

Exactly that high.

It definitively confirms the diagnosis of a metastatic ileal tumor.

For your exam, lock in these thresholds.

A daily 5 -HIAA excretion of more than 25 micromoles is elevated.

But anything greater than 100 micromoles, firmly indicates clinical carcinoid syndrome.

Getting a clean lab result for this seems incredibly tedious for the patient, though.

I'm looking at the dietary restrictions for the 24 hours leading up to the 5 -HIAA test.

You basically have to ruin their fruit salad.

You really do.

You have to tell them no plums, pineapples, kiwi fruit, avocados, tomatoes, walnuts, or bananas.

Because those foods are naturally packed with hydroxyendols that will mimic a tumor on the lab assay.

Exactly.

Once diagnosed, we can manage the symptoms with octreotide, a somatostatin analog that blocks the effects of all these chaotic mediators.

Now let's pivot to a situation where multiple rogue factories open up at the same time the pluriglandular syndromes.

Specifically,

multiple endocrine neoplasia or MEN.

This is classic flashcard material where two or more endocrine glands just start over -secreting hormones.

Is there a logical way to lock this into our memory?

Let's break it down genetically and anatomically.

These are usually familial autosomal dominant traits.

Start with MEN1.

This one is caused by a mutation in a tumor suppressor gene, appropriately named the MENEN gene.

To remember the glands involved, memorize the three P's.

First, the parathyroid glands, which develop hyperplasia or adenomas.

Second, the pancreas, specifically the islet cells, which can form gastronomas, insulinomas, vipomas, and so on.

And the third P is the anterior pituitary gland.

So three P's for MEN1, now for MEN2.

What drives MEN2?

MEN2 is driven by a different genetic error, a mutation in a proto -oncogene known as RET.

The triad here is medullary carcinoma of the thyroid gland,

fuchromocytoma, and parathyroid adenomas.

It's also worth noting there's a subvariant, MEN2B.

Yes, which features those endocrine tumors, but adds a morphonoid body habitus and mucosal neuromas.

The critical takeaway for MEN2 is the medullary thyroid carcinoma.

The thyroid has these paraphilicular C cells that normally secrete calcitonin.

When they turn malignant, plasma calcitonin spikes.

This makes calcitonin a remarkably specific tumor marker.

You can even use it to screen asymptomatic family members to see if they inherited the disease.

That brings us to one of the most mind -bending concepts in this entire chapter, ectopic hormone production.

This is where a tumor in one organ starts manufacturing a hormone that belongs to a completely different organ system.

How is that biologically possible?

If we connect this to the bigger picture, remember that every single cell in your body contains the entire genetic blueprint.

Your lung cells contain the code to make insulin.

They just keep that specific chapter of the DNA firmly locked or repressed.

When cells become malignant, their internal regulation shatters.

They can partly revert to a very early embryonic pluripotential state.

They de -repress parts of their genome.

Subtly, a rogue lung cell unlocks the genetic code for a hormone.

It has no business making.

And it starts synthesizing it in massive quantities.

And because this factory is entirely off the grid ectopic, it has zero feedback control.

The body can't tell it to stop.

So essentially, these cancer cells are unlocking a time capsule of their own DNA, reverting back to a fetal state just to survive.

That is wild.

Let's group some of the clinical examples from the text so they're easier to digest.

Let's start with the lungs, because small cell carcinoma of the bronchus seems to be a massive offender here.

It is a frequent culprit for two major ectopic syndromes.

First, it can secrete ectopic antidiuretic hormone, or ADH.

This causes the body to inappropriately retain water, diluting the blood and leading to severe hyponatramia.

Second, small cell lung cancer can secrete ectopic adrenocorticotophin hormone, or ACTH.

And ACTH normally tells the adrenal cortex to pump out hormones.

So what does this look like in the patient?

Ectopic ACTH overstimulates the adrenal cortex to secrete massive amounts of glucocorticoids.

Because glucocorticoids also have a secondary mineralocorticoid effect, they force the kidneys to dump potassium and hold on to salt.

This presents a severe hypokalemic alkalosis.

It clinically mimics a totally different disease, primary hyperaldosteronism.

Another lung cancer, squamous cell carcinoma of the bronchus, does something similar but with calcium.

It produces parathyroid hormone -related protein, or BTHRP.

It looks and acts just like true parathyroid hormone, but standard lab tests won't catch it.

It aggressively pulls calcium out of the bones, causing a hypercalcemia that comes on much more rapidly than standard parathyroid disease.

Okay, let's group the next two, erythropoietin and IGF2.

Erythropoietin stimulates red blood cell production, leading to polycythemia.

We see this ectopically produced by hepatocellular carcinomas.

IGF2, or insulin -like growth factor 2, is fascinating.

Large mesenchymal or liver tumors secrete it, and it acts like a massive overdose of insulin, causing severe hypoglycemia.

If you check the patient's labs, their actual native insulin and C -peptide levels will be appropriately rock bottom.

Because the healthy pancreas is desperately trying to stop the low blood sugar, but the IGF2 is driving the crash anyway.

Finally, we have tumors that mimic reproductive or thyroid hormones.

We see carcinomas of the lung or liver producing HCG,

causing gynecomastia in men or precocious puberty in kids.

Ectopic prolactin causes unexpected galacturia,

and testicular teratomas or choreocarcinomas can secrete a TSH -like substance that acts like thyroid -stimulating hormone, though actual hyperthyroidism is rare.

So what does this all mean?

We've covered the biologically active compounds, the rogue factories making things that actively harm the body.

Let's pivot to the biologically inactive compounds.

These are the tumor markers.

We need to manage expectations immediately.

The ideal tumor marker is a myth.

An ideal marker would be 100 % sensitive, meaning if you have the cancer, the test is always positive, and 100 % specific, meaning if you don't have the cancer, the test is never positive.

That doesn't exist.

Therefore, we almost never use these markers to screen the general public or make a primary diagnosis.

Their superpower is in mapping the timeline.

We use them to determine prognosis, monitor if chemotherapy is working, and spot a relapse months or years before it shows up on a scan.

Let's apply that to a real scenario.

There is a case in the chapter about a 68 -year -old non -smoking woman who had surgery for colon cancer 10 months ago.

Before her surgery, they checked her CEA carcinomebryonic antigen.

Normal for a non -smoker is under 2 .5, but hers was at 43.

Right after the tumor was removed, it plummeted to less than 4 .0.

Three months later, she was sitting at a perfect less than 2 .5.

But at her latest checkup, it spiked back to 19.

That case perfectly traces the true utility of a marker.

The surgery was an initial success, and the normalizing CEA proved it.

But that recent spike to 19, that is the biological alarm bell.

The tumor has recurred.

Your text illustrates this exact principle with a graph in figure 24 .4,

showing three hypothetical patient trajectories.

One patient's marker stays high and they pass away.

Another normalizes and stays in remission.

And a third mirrors our case, where the line drops beautifully, but then climbs again as the disease relapses.

That brings us to a marker everyone has heard of.

Prostate -specific antigen, or PSA.

It's a protein that normally helps liquidize semen.

But diagnostically, it seems fraught with gray areas, mostly because it heavily overlaps with benign prostatic hyperplasia, or BPH, which is just standard non -cancerous prostate enlargement.

The overlap is significant.

PSA naturally creeps up as men age and their prostates enlarge, so you always have to use age -adjusted reference ranges.

You also have to interrogate the patient's medication list.

A common BPH drug called finasteride will artificially slash their PSA levels by up to 50%.

To help clear the fog between BPH and cancer,

labs look at the ratio of free to total PSA in the blood.

If the index of free PSA is above 17%, it leans towards benign BPH.

If it is less than 17%, it strongly suggests prostate carcinoma.

Generally, an absolute PSA above 10 warrants a biopsy, and anything above 20 suggests the cancer has already escaped the prostate capsule.

Which perfectly explains case four.

We have a 67 -year -old man dealing with urinary hesitancy and lower back pain.

On exam, his prostate is large, firm, and craggy.

His PSA comes back at 28, his calcium is elevated at 2 .98, and his alkaline phosphatase, or ALP, is a massive 674.

The pieces fit together flawlessly.

The craggy prostate and the PSA of 28 flag a spreading carcinoma.

The lower back pain paired with the high calcium and the massively elevated ALP, which is an enzyme heavily concentrated in bone, tell you exactly where it spread.

The cancer has metastasized to his lumbar spine, creating osteoskelerotic bony secondaries.

Let's group the remaining tumor markers into logical clusters so they don't just feel like a random list to memorize.

Let's start with the markers tied to germ cell or liver tumors.

We have alpha -fetoprotein, or AFP, which is an on -calf -fetal protein heavily elevated in apatocellular carcinomas and teratomas.

We also look at ATG to monitor seminomas or choreocarcinomas.

And there's placental alkaline phosphatase for germ -felt tumors, with the bizarre caveat that simply being a smoker will cause false elevations.

Moving to the carbohydrate -antigen cluster, these are notoriously imperfect, but useful.

CA125 is tracked for ovarian carcinoma.

Remember that if it crosses the 35 -kilo unit threshold, the patient needs an ultrasound.

CA15 -3 tracks advanced breast carcinoma.

CA99 is monitored for pancreatic or colorectal carcinomas.

HE4 is a newer marker.

We also pair with CA125 for ovarian cancer.

Then we have the markers swimming in the blood and immune system.

Serum periprotein and urinary Benz -Jones protein are the classic hallmarks of multiple myeloma.

For hematological tumors like lymphomas, we track plasma lactate dehydrogenase, or LDH.

And finally, we have markers tied to specific tissue types.

Thyroglobulin monitors thyroid carcinomas, though you must be aware of autoantibodies ruining the assay.

Neuronal -specific NLAs acts as a marker for small -cell lung cancer and neuroblastoma.

Inhebin is used for specific ovarian and testicular tumors.

Squamous cell carcinoma antigen tracks cervical cancer.

Chromagranin A is released from neuroendocrine cells, mapping back to our pheochromocytomas and carcinoids.

And protein S100B helps monitor therapy in malignant melanoma.

The future of all this, of course, relies on genetic tracking, like BRCA gene mutations for breast cancer risk, and mapping circulating tumor DNA directly from a blood draw.

We really covered a lot of ground today.

This raises an important question.

We've talked so much today about cells de -repressing their ancient DNA, reverting to a fetal state to make ectopic hormones or uncoffinal proteins like AFP.

It forces you to wonder.

Exactly.

If these malignant factories are just reverting to a pluripotential embryonic state, what if future treatments could map that entire fetal state and figure out a way to reprogram the cell back into normal mature tissue, rather than just blasting it with toxic chemotherapy?

It is a fascinating biochemical puzzle to ponder.

It really is.

Yeah.

And that brings our tutoring session to a close.

Take a breath and look back at what we just accomplished.

We saw how mastering the normal biochemistry, like the synthesis of catecholamines and serotonin, gave you the exact blueprint to understand the pathophysiology of pheochromocytomas and carcinoid syndrome.

We saw how those pathways dictated the precise laboratory tests we ordered, and how the rogue factory concept explains ectopic hormones and tumor markers.

You have put in some serious work today, unpacking a highly complex chapter.

And if you keep connecting the concepts like this, you are going to absolutely crush your exams.

On behalf of the Last Minute Lecture Team, thank you so much for joining us, and we wish you the absolute best of luck with your clinical biochemistry studies.