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According to Campbell Biology,

A growth factor is a protein released by certain cells that stimulates other cells to divide.

and according to Wikipedia,

A mitogen is a chemical substance that encourages a cell to commence cell division, triggering mitosis.

What’s the difference between them? I suppose a mitogen specifically refers to mitosis, but if something is stimulating mitosis, isn’t it stimulating growth? Are mitogens all growth factors? Are all growth factors mitogens?

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There is a lot of confusion and conflicting / imprecise definitions of these terms. It's biology after all :)

A mitogen is an agent that causes a cell to enter mitosis. This definition is pretty clear, and there is a good consensus about it. (Well technically, mitosis is not the same as cell division, but we will gloss over this distinction.)

The term growth factor has at least two different definitions: (1) a factor that causes growth of tissues, organs or entire individuals; or (2) a factor that causes growth of cells (increase in cell size). These two versions are often mixed up, and this causes no end of confusion. Let's consider them both in turn.

Definition (1) is more common and probably older. Since growth of whole tissues usually (but not always) implies both cell growth and division, according to this definition, a growth factor is also a mitogen. But the reverse is not true: there are cases where cells divide without growing, for example the first few cell divisions of a fertilized egg. But note that, since this definition concerns the level of tissues or individuals, it's meaning is not entirely clear in terms of cell growth and division. Muscle growth in the adult stage usually does not involve cell division, for example.

Definition (2) is better suited to cell biology (in my opinion), because it actually concerns cells, not tissues. This definition is more common when studying cell growth and division; see for example this review. Cell growth is a separate phenomenon from cell division: cells can grow without dividing (fat cells, muscle fibers and neuron do this, for example), or divide without growing (as mentioned above). So with this definition, growth factor and mitogen are two completely unrelated concepts. Now, it is certainly true that many proteins can act both as growth factors and mitogens; Matej Pribis gives some nice examples in the other answer. But that is an empirical fact, not a question of definitions.

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  • $\begingroup$ What about PDGFs? Are they both mitogens and growth factors? $\endgroup$ – lightweaver Jan 31 '16 at 0:46
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    $\begingroup$ Yes, most of the classic proliferation-promoting proteins are both growth factors and mitogens, since in most cases cell growth and division go together. But to be precise, it depends on the target cell as well: a given protein may be a growth factor for one cell, a mitogen for another, and have no effect on a third. Not all cells respond to PDGF. Nerve growth factor is a good example of a "pure" growth factor (for neurons). $\endgroup$ – Roland Feb 6 '16 at 1:24
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You can find a great article about this here.

basically, these two groups are very similar in their effects, however they work through different pathways.

Mitogens directly promote cell to get through G1 checkpoint through highering the activity of cyclin D/cdk4 (the complex needed to get through the checkpoint.) and according to the article above, also the cyclin E/cdk2 complex (which helps cell to get into S phase). By activating the MAPK (mitogen-activated protein kinase) pathway leading to production of transcription factors (Elk-1, jun, fos), which then promote specific transcription of proteins needed to get through cell cycle. The mitogen pathway also causes phosphorylation of Rb (Retinoblastom) protein, which then dissociates from E2F transcription factor. This activates the E2F, which in turn helps synthesise proteins needed to promote S phase.

Growth factors usually stimulates mTOR receptor, which is a "serine/threonine protein kinase that regulates cell growth, cell proliferation, cell motility, cell survival, protein synthesis, autophagy, and transcription." Source

mTORC complex generally promotes translation. It plays role in "cell growth via the modulation of protein synthesis in yeast and mammals, it and its yeast homologs strongly stimulate transcription from all genes involved in ribosome biogenesis, transcription of rRNA genes by RNA polymerase I (Pol I), transcription of ribosomal protein genes by RNA polymerase II (Pol II), and transcription of tRNA and 5S genes by RNA polymerase III" and many other effects. Source.

Although all of this, in practice, I've seen many people use this categories interchangeably or referring to mitogens as a subgroup of growth factors.

I hope this helps.

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  • $\begingroup$ Nice with some examples, but I think you miss the point of the classification as growth factors or mitogens. These terms do not refer to individual proteins, it's a theoretical classification of function. And they are not "very similar in their effects", on the contrary they are completely unrelated, by definition --- cell growth and cell division are very different things. That some people use the two terms interchangably is just wrong. $\endgroup$ – Roland Jan 30 '16 at 9:33
  • $\begingroup$ Please note that mTOR is not a receptor, it is just a kinase, and also that it can signal through the MAPK pathway as well. $\endgroup$ – MattDMo Jan 30 '16 at 16:33
  • $\begingroup$ @Roland I definitely agree, and I might have misstated what I meant. These processes (cell growth and proliferation) are distinct, but I wouldn't say completely unrelated. They can affect each other e.g. the progression of cell cycle inhibited by too small cell size. Moreover, GFs (or rather the mTOR) have some effect not only on growth, but also on proliferation and cell cycle progression. According to this article $\endgroup$ – mpribis Jan 30 '16 at 16:33

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