Editing Cell growth (section)

== Cell growth regulation in animals ==
Many of the signal molecules that control of cellular growth are called [[growth factors]], many of which induce [[signal transduction]] via the [[PI3K/AKT/mTOR pathway]], which includes upstream lipid kinase [[Phosphoinositide 3-kinase|PI3K]] and the downstream serine/threonine protein [[kinase]] [[Akt/PKB signaling pathway|Akt]], which is able to activate another protein kinase [[mTOR|TOR]], which promotes [[Translation (biology)|translation]] and inhibits [[autophagy]] to drive cell growth.

Nutrient availability influences production of [[growth factors]] of the [[Insulin]]/[[IGF-1]] family, which circulate as hormones in animals to activate the [[PI3K/AKT/mTOR pathway]] in cells to promote [[mTOR|TOR]] activity so that when animals are well fed they will grow rapidly and when they are not able to receive sufficient nutrients they will reduce their growth rate. Recently it has been also demonstrated that cellular bicarbonate metabolism, which is responsible for cell growth, can be regulated by mTORC1 signaling.<ref>{{cite journal | vauthors = Ali E, Liponska A, O'Hara B, Amici D, Torno M, Gao P, Asara J, Yap M-N F, Mendillo M, Ben-Sahra I | title = The mTORC1-SLC4A7 axis stimulates bicarbonate import to enhance de novo nucleotide synthesis | journal = Molecular Cell | volume = 82 | issue = 1 | pages = 3284–3298.e7 | date = June 2022 | doi = 10.1016/j.molcel.2022.06.008 | pmid = 35772404 | pmc = 9444906 }}</ref>

In addition, the availability of [[amino acids]] to individual cells also directly promotes [[mTOR|TOR]] activity, although this mode of regulation is more important in single-celled organisms than in [[multicellular]] organisms such as animals that always maintain an abundance of [[amino acids]] in circulation.

One disputed theory proposes that many different mammalian cells undergo size-dependent transitions during the cell cycle. These transitions are controlled by the cyclin-dependent kinase Cdk1.<ref>{{cite book |doi=10.1016/S0074-7696(03)01004-0 |author=Mitchison JM |title=Growth During the Cell Cycle |journal=Int. Rev. Cytol. |volume=226 |pages=165–258 |year=2003 |pmid=12921238 |series=International Review of Cytology |isbn=978-0-12-364630-9 }}</ref> Though the proteins that control Cdk1 are well understood, their connection to mechanisms monitoring cell size remains elusive.

A postulated model for mammalian size control situates mass as the driving force of the cell cycle. A cell is unable to grow to an abnormally large size because at a certain cell size or cell mass, the S phase is initiated. The S phase starts the sequence of events leading to mitosis and cytokinesis. A cell is unable to get too small because the later cell cycle events, such as S, G2, and M, are delayed until mass increases sufficiently to begin S phase.<ref>{{cite journal|author=Cooper, Stephen|title= Control and maintenance of mammalian cell size|journal= BMC Cell Biology|volume= 5|issue= 1|pages= 35|doi=10.1186/1471-2121-5-35|pmid= 15456512|pmc= 524481|year= 2004|doi-access= free}}</ref>