Type 1 and type 2 diabetes both result from inadequate production of insulin by the beta-cells of the pancreatic islet. Accordingly, strategies that lead to increased pancreatic beta-cell mass, as well as retained or enhanced function of islets, would be desirable for the treatment of diabetes. Although pancreatic beta-cells have long been viewed as terminally differentiated and irreversibly arrested, evidence now indicates that beta-cells can and do replicate, that this replication can be enhanced by a variety of maneuvers, and that beta-cell replication plays a quantitatively significant role in maintaining pancreatic beta-cell mass and function. Because beta-cells have been viewed as being unable to proliferate, the science of beta-cell replication is undeveloped. In the past several years, however, this has begun to change at a rapid pace, and many laboratories are now focused on elucidating the molecular details of the control of cell cycle in the beta-cell. In this review, we review the molecular details of cell cycle control as they relate to the pancreatic beta-cell. Our hope is that this review can serve as a common basis and also a roadmap for those interested in developing novel strategies for enhancing beta-cell replication and improving insulin production in animal models as well as in human pancreatic beta-cells.

OBJECTIVESTo comprehensively inventory the proteins that control the G1/S cell cycle checkpoint in the human islet and compare them with those in the murine islet, to determine whether these might therapeutically enhance human β-cell replication, to determine whether human β-cell replication can be demonstrated in an in vivo model, and to enhance human β-cell function in vivo.RESEARCH DESIGN AND METHODSThirty-four G1/S regulatory proteins were examined in human islets. Effects of adenoviruses expressing cdk-6, cdk-4, and cyclin D1 on proliferation in human β-cells were studied in both invitro and in vivo models.RESULTSMultiple differences between murine and human islets occur, most strikingly the presence of cdk-6 in human β-cells versus its low abundance in the murine islet. Cdk-6 and cyclin D1 in vitro led to marked activation of retinoblastoma protein phosphorylation and cell cycle progression with no induction of cell death. Human islets transduced with cdk-6 and cyclin D1 were transplanted into diabetic NOD-SCID mice and markedly outperformed native human islets in vivo, maintaining glucose control for the entire 6 weeks of the study.CONCLUSIONSThe human G1/S proteome is described for the first time. Human islets are unlike their rodent counterparts in that they contain easily measurable cdk-6. Cdk-6 overexpression, alone or in combination with cyclin D1, strikingly stimulates human β-cell replication, both in vitro as well as in vivo, without inducing cell death or loss of function. Using this model, human β-cell replication can be induced and studied in vivo.

Abstract-Parathyroid hormone-related protein (PTHrP) is present in vascular smooth muscle (VSM), is markedly upregulated in response to arterial injury, is essential for normal VSM proliferation, and also markedly accentuates neointima formation following rat carotid angioplasty. PTHrP contains a nuclear localization signal (NLS) through which it enters the nucleus and leads to marked increases in retinoblastoma protein (pRb) phosphorylation and cell cycle progression. Our goal was to define key cell cycle molecules upstream of pRb that mediate cell cycle acceleration induced by PTHrP. The cyclin D/cdk-4,-6 system and its upstream regulators, the inhibitory kinases (INKs), are not appreciably influenced by PTHrP. In striking contrast, cyclin E/cdk-2 kinase activity is markedly increased by PTHrP, and this is a result of a specific, marked, PTHrP-induced proteasomal degradation of p27 kip1 . Adenoviral restoration of p27 kip1 fully reverses PTHrP-induced cell cycle progression, indicating that PTHrP mediates its cell cycle acceleration in VSM via p27 kip1 . In confirmation, adenoviral delivery of PTHrP to murine primary vascular smooth muscle cells (VSMCs) significantly decreases p27 kip1 expression and accelerates cell cycle progression. p27 kip1 is well known to be a central cell cycle regulatory molecule involved in both normal and pathological VSM proliferation and is a target of widely used drug-eluting stents. The current observations define a novel "PTHrP/p27 kip1 pathway" in the arterial wall and suggest that this pathway is important in normal arterial biology and a potential target for therapeutic manipulation of the arterial response to injury. Key Words: cell cycle progression Ⅲ p27Kip1/pRb Ⅲ proliferation Ⅲ PTHrP Ⅲ vascular smooth muscle cell proliferation P arathyroid hormone-related protein (PTHrP) was originally identified through the search for the humoral factor responsible for humoral hypercalcemia of malignancy.